footnotesize[table]font=footnotesize

2017 222 29 August

The ALICE Transition Radiation Detector

ALICE Collaboration††thanks: See Appendix A for the list of collaboration
members ALICE Collaboration

The Transition Radiation Detector (TRD) was designed and built to enhance the
capabilities of the ALICE detector at the Large Hadron Collider (LHC). While
aimed at providing electron identification and triggering, the TRD also
contributes significantly to the track reconstruction and calibration in the
central barrel of ALICE. In this paper the design, construction, operation,
and performance of this detector are discussed. A pion rejection factor of up
to 410 is achieved at a momentum of 1 GeV/$c$ in p–Pb collisions and the
resolution at high transverse momentum improves by about 40% when including
the TRD information in track reconstruction. The triggering capability is
demonstrated both for jet, light nuclei, and electron selection.

##  1 Introduction

A Large Ion Collider Experiment (ALICE) [1, 2] is the dedicated heavy-ion
experiment at the Large Hadron Collider (LHC) at CERN. In central high energy
nucleus–nucleus collisions a high-density deconfined state of strongly
interacting matter, known as quark–gluon plasma (QGP), is supposed to be
created [3, 4, 5]. ALICE is designed to measure a large set of observables in
order to study the properties of the QGP. Among the essential probes there are
several involving electrons, which originate, e.g. from open heavy-flavour
hadron decays, virtual photons, and Drell-Yan production as well as from
decays of the $\psi$ and $\rm\Upsilon$ families. The identification of these
rare probes requires excellent electron identification, also in the high
multiplicity environment of heavy-ion collisions. In addition, the rare probes
need to be enhanced with triggers, in order to accumulate the statistics
necessary for differential studies. The latter requirement concerns not only
probes involving the production of electrons, but also rare high transverse
momentum probes such as jets (collimated sprays of particles) with and without
heavy flavour. The ALICE Transition Radiation Detector (TRD) fulfils these two
tasks and thus extends the physics reach of ALICE.

Transition radiation (TR), predicted in 1946 by Ginzburg and Frank [6], occurs
when a particle crosses the boundary between two media with different
dielectric constants. For highly relativistic particles ($\gamma\gtrsim
1000$), the emitted radiation extends into the X-ray domain for a typical
choice of radiator [7, 8, 9]. The radiation is extremely forward peaked
relative to the particle direction [7]. As the TR photon yield per boundary
crossing is of the order of the fine structure constant ($\alpha=1/137$), many
boundaries are needed in detectors to increase the radiation yield [10]. The
absorption of the emitted X-ray photons in high-$Z$ gas detectors leads to a
large energy deposition compared to the specific energy loss by ionisation of
the traversing particle.

Since their development in the 1970s, transition radiation detectors have
proven to be powerful devices in cosmic-ray, astroparticle and accelerator
experiments [11, 12, 13, 14, 10, 15, 16, 17, 18, 19, 20]. The main purpose of
the transition radiation detectors in these experiments was the discrimination
of electrons from hadrons via, e.g. cluster counting or total charge/energy
analysis methods. In a few cases they provided charged-particle tracking. The
transition radiation photons are in most cases detected either by straw tubes
or by multiwire proportional chambers (MWPC). In some experiments [10, 13, 21,
16] and in test setups [22, 23, 24, 25], short drift chambers (usually about
$1\text{\,}\mathrm{cm}$) were employed for the detection. Detailed reviews on
the transition radiation phenomenon, detectors, and their application to
particle identification can be found in [10, 26, 27, 28].

The ALICE TRD, which covers the full azimuth and the pseudorapidity range
$-0.84<\eta<0.84$ (see next section), is part of the ALICE central barrel. The
TRD consists of 522 chambers arranged in 6 layers at a radial distance from
$2.90\text{\,}\mathrm{m}$ to $3.68\text{\,}\mathrm{m}$ from the beam axis.
Each chamber comprises a foam/fibre radiator followed by a Xe-CO2-filled MWPC
preceded by a drift region of $3\text{\,}\mathrm{cm}$. The extracted temporal
information represents the depth in the drift volume at which the ionisation
signal was produced and thus allows the contributions of the TR photon and the
specific ionisation energy loss of the charged particle
$\mathrm{d}E/\mathrm{d}x$ to be separated. The former is preferentially
absorbed at the entrance of the chamber and the latter distributed uniformly
along the track. Electrons can be distinguished from other charged particles
by producing TR and having a higher $\mathrm{d}E/\mathrm{d}x$ due to the
relativistic rise of the ionisation energy loss. The usage of the temporal
information further enhances the electron-hadron separation power. Due to the
fast read-out and online reconstruction of its signals, the TRD has also been
successfully used to trigger on electrons with high transverse momenta and
jets (3 or more high-$p_{\mathrm{T}}$ tracks). Last but not least, the TRD
improves the overall momentum resolution of the ALICE central barrel by
providing additional space points at large radii for tracking, and tracks
anchored by the TRD will be a key element to correct space charge distortions
expected in the ALICE TPC in LHC Run 3 [29]. A first version of the correction
algorithm is already in use for Run 2.

In this article the design, construction, operation, and performance of the
ALICE TRD is described. Section 2 gives an overview of the detector and its
construction. The gas system is detailed in Section 3. The services required
for the detector are outlined in Section 4. In Section 5 the read-out of the
detector is discussed and the Detector Control System (DCS) used for reliable
operation and monitoring of the detector is presented in Section 6. The
detector commissioning and its operation are discussed in Section 7. Tracking,
alignment, and calibration are described in detail in Sections 8, 9, and 10,
while various methods for charged hadron and electron identification are
presented in Section 11. The use of the TRD trigger system for jets,
electrons, heavy-nuclei, and cosmic-ray muons is described in Section 12.

##  2 Detector overview

Figure 1: Schematic cross-section of the ALICE detector perpendicular to the
LHC beam direction (status of the detector since the start of LHC Run 2). The
central barrel detectors cover the pseudorapidity range $|\eta|\lesssim 0.9$
and are located inside the solenoid magnet, which provides a magnetic field
with strength $B$ = $0.5\text{\,}\mathrm{T}$ along the beam direction.

A cross-section of the central part of the ALICE detector [1, 2], installed at
Interaction Point 2 (IP2) of the LHC, is shown in Fig. 1. The central barrel
detectors cover the pseudorapidity range $|\eta|\lesssim 0.9$ and are located
inside a solenoid magnet, which produces a magnetic field of $B$ =
$0.5\text{\,}\mathrm{T}$ along the beam direction. The Inner Tracking System
(ITS) [30], placed closest to the nominal interaction point, is employed for
low momentum tracking, particle identification (PID), and primary and
secondary vertexing. The Time Projection Chamber (TPC) [31], which is
surrounded by the TRD, is used for tracking and PID. The Time-Of-Flight
detector (TOF) [32] is placed outside the TRD and provides charged hadron
identification. The ElectroMagnetic Calorimeter (EMCal) [33], the PHOton
Spectrometer (PHOS) [34], and the High Momentum Particle Identification
Detector (HMPID) [35] are used for electron, jet, photon and hadron
identification. Their azimuthal coverage is shown in Fig. 1. Not visible in
the figure are the V0 and T0 detectors [36, 37], as well as the Zero Degree
Calorimeters (ZDC) [38], which are placed at small angles on both sides of the
interaction region. These detectors can be employed, e.g. to define a minimum-
bias trigger, to determine the event time, the centrality and event plane of a
collision [2, 39, 40]. Likewise, the muon spectrometer [41, 42] is outside the
view on one side of the experiment, only, covering $-$4 $<\eta<$ $-$2.5.

Figure 1 also shows the definition of the global ALICE coordinate system,
which is a Cartesian system with its point of origin at the nominal
interaction point ($x_{\rm lab}$, $y_{\rm lab}$, $z_{\rm lab}$ = 0); the
$x_{\rm lab}$-axis pointing inwards radially to the centre of the LHC ring and
the $z_{\rm lab}$-axis coinciding with the direction of one beam and pointing
in direction opposite to the muon spectrometer. According to the (anti-)clock-
wise beam directions, the muon spectrometer side is also called C-side, the
opposite side A-side.

Figure 2: Schematic cross-section of a TRD chamber in the $x$-$z$ plane
(perpendicular to the wires) with tracks of a pion and an electron to
illustrate the ionisation energy deposition and the TR contribution. The large
energy deposition due to the TR photon absorption is indicated by the large
red circle in the drift region. The drift lines (solid lines) are calculated
with Garfield [43] and correspond to the nominal voltage settings for chamber
operation. The radiator is not drawn to scale.

The design of the TRD is a result of the requirements and constraints
discussed in the Technical Design Report [44]. It has a modular structure and
its basic component is a multiwire proportional chamber (MWPC). Each chamber
is preceded by a drift region to allow for the reconstruction of a local track
segment, which is required for matching of TRD information with tracks
reconstructed with ITS and TPC at high multiplicities. TR photons are produced
in a radiator mounted in front of the drift section and then absorbed in a
xenon-based gas mixture. A schematic cross-section of a chamber and its
radiator is shown in Fig. 2. The shown local coordinate system is a right-
handed orthogonal Cartesian system, similar to the global coordinate system,
rotated such that the $x$-axis is perpendicular to the chamber. Six layers of
chambers are installed to enhance the pion rejection power. An eighteen-fold
segmentation in azimuth ($\varphi$), with each segment called ‘sector’, was
chosen to match that of the TPC read-out chambers. In the longitudinal
direction ($z_{\rm lab}$), i.e. along the beam direction, the coverage is
split into five stacks, resulting in a manageable chamber size. The five
stacks are numbered from 0 to 4, where stack 4 is at the C-side and stack 0 at
the A-side. Layer 0 is closest, layer 5 farthest away from the collision point
in the radial direction. In each sector, 30 read-out chambers (arranged in 6
layers and 5 stacks) are combined in a mechanical casing, called a
‘supermodule’ (see Fig. 3 and Section 2.3).

Figure 3: Cross-section (longitudinal view) of a supermodule.

In total the TRD can host 540 read-out chambers (18 sectors $\times$ 6 layers
$\times$ 5 stacks), however in order to minimise the material in front of the
PHOS detector in three sectors (sectors 13–15, for numbering see Fig. 1) the
chambers in the middle stack were not installed. This results in a system of
522 individual read-out chambers. The main parameters of the detector are
summarised in Table 1.

Parameter | Value
---|---
Pseudorapidity coverage |  $-$0.84 $<\eta<$ +0.84
Azimuthal coverage $\varphi$ | $None$
Radial position | $2.90\text{\,}\mathrm{m}3.68\text{\,}\mathrm{m}$
Length of a supermodule | $7.02\text{\,}\mathrm{m}$
Weight of a supermodule | $1.65\text{\,}\mathrm{t}$
Segmentation in $\varphi$ | 18 sectors
Segmentation in $z_{\rm lab}$ | 5 stacks
Segmentation in $r$ | 6 layers
Total number of read-out chambers | 522
Size of a read-out chamber (active area) |  $0.90\text{\,}\mathrm{m}$ $\times$ $1.06\text{\,}\mathrm{m}$ to $1.13\text{\,}\mathrm{m}$ $\times$ $1.43\text{\,}\mathrm{m}$
Radiator material | fibre/foam sandwich
Depth of radiator | $4.7\text{\,}\mathrm{cm}$
Depth of drift region | $3.0\text{\,}\mathrm{cm}$
Depth of amplification region | $0.7\text{\,}\mathrm{cm}$
Number of time bins ($100\text{\,}\mathrm{ns}$) | 30 (22–24)
Total number of read-out pads | $1\,150\,848$
Total active area | $673.4\text{\,}{\mathrm{m}}^{2}$
Detector gas | Xe-CO2 (85-15)
Gas volume | $27\text{\,}{\mathrm{m}}^{3}$
Drift voltage (nominal) |  ${\sim}$$2150\text{\,}\mathrm{V}$
Anode voltage (nominal) |  ${\sim}$$1520\text{\,}\mathrm{V}$
Gas gain (nominal) | ${\sim}3200$
Drift field |  ${\sim}$$700\text{\,}\mathrm{V}\mathrm{/}\mathrm{c}\mathrm{m}$
Drift velocity |  ${\sim}$$1.56\text{\,}\mathrm{c}\mathrm{m}\mathrm{/}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$
Avg. radiation length along $r$ $\langle X/X_{0}\rangle$ | 24.7%
Table 1: General parameters of the TRD. The indicated weight corresponds to a
supermodule with 30 read-out chambers; the length of the supermodule does not
include the connected services. At maximum 30 time bins can be read out,
typical values used in Run 1 and Run 2 are 22–24 (see Section 5.2).

At the start of the first LHC period (Run 1) in 2009 the TRD participated with
seven supermodules. Six further supermodules were built and integrated into
the experiment during short winter shutdown periods of the accelerator, three
in each winter shutdown period of 2010 and 2011. The TRD was completed during
the Long Shutdown 1 (LS) of the LHC in 2013–2014. With all 18 supermodules
installed, full coverage in azimuth was accomplished for the second LHC period
(Run 2) starting in 2015.

###  2.1 Read-out chambers

The size of the read-out chambers changes radially and along the beam
direction (see Fig. 3). The active area per chamber thus varies from
$0.90\text{\,}\mathrm{m}$ $\times$ $1.06\text{\,}\mathrm{m}$ to
$1.13\text{\,}\mathrm{m}$ $\times$ $1.43\text{\,}\mathrm{m}$ ($x\times z$).
The optimal design of a read-out chamber (see Fig. 2) was found considering
the requirements on precision and mechanical stability, and minimisation of
the amount of material.

The construction of the radiator, discussed in the following sub-section, is
essential for the mechanical stability of the chamber. The drift electrode, an
aluminised mylar foil ($25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thick), is
an integral part of the radiator. To ensure a uniform drift field throughout
the entire drift volume, a field cage with a voltage divider chain is employed
[44]. The current at nominal drift voltage is about
$170\text{\,}\mathrm{\SIUnitSymbolMicro A}$. The grounded cathode wires are
made of Cu-Be and have a diameter of $75\text{\,}\mathrm{\SIUnitSymbolMicro
m}$, while the anode wires are made of Au-plated tungsten with a diameter of
$20\text{\,}\mathrm{\SIUnitSymbolMicro m}$. The pitch for the cathode and
anode wires are $2.5\text{\,}\mathrm{mm}$ and $5\text{\,}\mathrm{mm}$,
respectively; the tensions at winding were $1\text{\,}\mathrm{N}$ and
$0.45\text{\,}\mathrm{N}$ [45]. The wire lengths vary from
$1.08\text{\,}\mathrm{m}$ to $1.45\text{\,}\mathrm{m}$. The maximum
deformation of the chamber frame was $150\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ under the wire tension indicated, leading to a maximum 10% loss in wire
tension. Even with an additional $1\text{\,}\mathrm{mbar}$ overpressure in the
gas volume (see Section 3), the deformation of the drift electrode can be kept
within the specification of less than $1\text{\,}\mathrm{mm}$. The segmented
cathode pad plane is manufactured from thin Printed Circuit Boards (PCB) and
glued on a light honeycomb and carbon fibre sandwich to ensure planarity and
mechanical stiffness. The design goal of having a maximum deviation from
planarity of $150\text{\,}\mathrm{\SIUnitSymbolMicro m}$ was achieved with
only a few chambers exceeding slightly this value. The PCBs of the pad plane
were produced in two or three pieces. The PCBs are segmented into 12 (stack 2)
or 16 pads along the $z$-direction, and 144 pads in the direction of the anode
wires ($r\varphi$). The pad area varies from $0.635\text{\,}\mathrm{cm}$
$\times$ $7.5\text{\,}\mathrm{cm}$ to $0.785\text{\,}\mathrm{cm}$ $\times$
$9\text{\,}\mathrm{cm}$ [45] to achieve a constant granularity with respect to
the distance from the interaction point. The pad width of
$0.635\text{\,}\mathrm{cm}$ to $0.785\text{\,}\mathrm{cm}$ in the $r\varphi$
direction was chosen so that charge sharing between adjacent pads (typically
three), which is quantified by the pad response function (PRF) [46], is
achieved. As a consequence, the position of the charge deposition can be
reconstructed in the $r\varphi$-direction with a spatial resolution of
$\lesssim$ $400\text{\,}\mathrm{\SIUnitSymbolMicro m}$ [46]. In the
longitudinal direction, the coarser segmentation is sufficient for the track
matching with the inner detectors. In addition, the pads are tilted by $\pm$
$None$ (sign alternating layer-by-layer) as shown in Fig. 4, which improves
the $z$-resolution during track reconstruction without compromising the
$r\varphi$ resolution. For clusters confined within one pad row, a $z$
position at the row centre is assumed, $z_{\rm cluster}=z_{0}$. The honeycomb
structure also acts as a support for the read-out boards. The pads are
connected to the read-out boards by short polyester ribbon cables via milled
holes in the honeycomb structure.

The original design of the TRD was conceived such that events with a
multiplicity of d$N_{\rm ch}$/d$\eta$ = $8000$ would have lead to an occupancy
of 34% in the detector [44]. The fast read-out and processing of such data on
$1.15\cdot 10^{6}$ read-out channels required the design and production of
fully customised front-end electronics (see Section 5).

Figure 4: Pad geometry of a TRD read-out chamber in layer 3 (not stack 2). The
pad tilt is $\pm$ $None$ with respect to the $z$-axis (along the beam
direction), with the sign alternating between layers.  Figure 5: Average pulse
height as a function of drift time for pions and electrons (with and without
radiator). The time axis is shown with an arbitrary offset of
$0.3\text{\,}\mathrm{\SIUnitSymbolMicro s}$. The measurements were performed
at the CERN PS with prototype read-out chambers that were smaller in overall
size (active area $25\text{\,}\mathrm{cm}$ $\times$ $32\text{\,}\mathrm{cm}$)
but otherwise similar in construction to that of the final detector. Figure
taken from [47].

The positive signal induced on the cathode pad plane is amplified using a
charge-sensitive PreAmplifier-ShAper (PASA) (see Section 5) and the signals on
the cathode pads are sampled in time bins of 100 ns inside the TRAcklet
Processor (TRAP, see Section 5). For LHC Run 1 and Run 2 running conditions
(see Section 7.2), the probability for pile-up events is small. The averaged
time evolution of the signal is shown in Fig. 5 for pions and electrons, with
and without radiator. In the amplification region (early times), the signal is
larger, because the ionisation from both sides of the anode wires contributes
to the same time interval. The contribution of TR is seen as an increase in
the measured average signal at times corresponding to the entrance of the
chamber (around $2.5\text{\,}\mathrm{\SIUnitSymbolMicro s}$ in Fig. 5), where
the TR photons are preferentially absorbed. At large times (beyond
$2.5\text{\,}\mathrm{\SIUnitSymbolMicro s}$), the effect of the slow ion
movement becomes visible as a tail. Various approximations of the time
response function, the convolution of the long tails with the shaping of the
PASA, were studied in order to optimally cancel the tails in data, see Section
8.

Figure 6: Distributions of the ionisation energy loss of pions and electrons
with momenta of 2 $\mathrm{GeV}\\!/c$. The symbols represent the measurements
obtained at the CERN PS with prototype read-out chambers that were smaller in
overall size (active area $25\text{\,}\mathrm{cm}$ $\times$
$32\text{\,}\mathrm{cm}$) but otherwise similar in construction to that of the
final detector. The lines are simulations accounting (continuous line) or not
(dashed line) for the long range of $\delta$-electrons as compared to the
chamber dimensions. Figure taken from [48].

The knowledge of the ionisation energy loss is important for the control of
the detector performance and for tuning the Monte Carlo simulations. A set of
measurements was performed with prototype read-out chambers with detachable
radiators for pions and electrons at various momenta [48]. An illustration of
the measured data is shown in Fig. 6 for pions and electrons with a momentum
of 2 $\mathrm{GeV}\\!/c$. The simulations describe the Landau distribution of
the total ionisation energy deposition, determined from the calibrated time-
integrated chamber signal. A compilation of such measurements over a broad
momentum range including data obtained with cosmic-ray muons and from
collisions recorded with ALICE is shown in Section 11, Fig. 37.

Measurements of the position resolution in the $r\varphi$-direction
($\sigma_{y}$) and angular resolution $\sigma_{\varphi}$, conducted with
prototype chambers, established that the required performance of the detector
and electronics ($\sigma_{y}$
$\lesssim$$400\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and $\sigma_{\varphi}$
$\leq$ $None$) is reached for signal-to-noise values of about 40, which
corresponds to a moderate gas gain of about 3500 [46].

The production of a chamber was performed in several steps [49] and completed
in one week on average. First, the aluminium walls of the chamber were aligned
on a precision table and glued to the radiator panel. The glueing table was
custom-built to ensure the required mechanical precision and time-efficient
handling of the components. For almost all junctions the two-component epoxy
glue Araldite® AW 116 with hardener HV 953BD was used. In a few places, where
a higher viscosity glue was needed, Araldite® AW 106 was applied. In a second
step, the cathode and anode wires were wound on a custom-made winding machine
and glued onto a robust aluminium frame in order to keep the wire tension.
This aluminium frame was subsequently placed on top of the chamber body, and
the cathode and anode wires were transferred to the G10 ledges glued to the
chamber body. After gluing of the anode and cathode wire planes, the tension
of each wire was checked by moving a needle valve with pressurised air across
the wires. The induced resonance frequency in each wire was determined by
measuring the reflected light of an LED [50]. Afterwards the pad plane and
honeycomb structure were placed on top of the chamber body. Following this
production process, each chamber was subjected to a series of quality control
tests with an Ar-CO2 (70-30) gas mixture. The tests were performed once before
the chamber was sealed with epoxy (closed with clamps) and repeated after
chamber validation and glueing. In the following the requirements are
described [51]. The anode leakage current was required not to exceed a value
of 10 nA. The gas leak rate was determined by flushing the chamber with the
Ar-CO2 gas mixture and measuring the O2 content of the outflowing gas. It was
required to be less than $1\text{\,}\mathrm{mbar}$ $\cdot$
$\mathrm{l}\mathrm{/}\mathrm{h}$. In addition, the leak conductance was
measured at an underpressure of 0.4–$0.5\text{\,}\mathrm{mbar}$ in the
chamber. The underpressure test was only introduced at a later stage of the
mass production after viscous leaks were found, see Section 3.4.1 for more
details. Comparisons of the anode current induced by a 109Cd source placed at
100 different positions across the active area allowed determinations of the
gain uniformity. The step size for this two-dimensional scan was about 10 cm
in both directions and the measured values were required to be within $\pm$
15% of the median. Electrically disconnected wires were detected by carrying
out a one-dimensional scan perpendicular to the wires with a step size of
$1\text{\,}\mathrm{cm}$. This scan clearly identified any individual wire that
was not connected due to the visible gas gain anomaly in the vicinity of this
wire, and allowed for repair. For one position the absolute gas gain was
determined by measuring the anode current and by counting the pulses of the
109Cd source. The long term stability was characterised by monitoring the gas
gain in intervals of 15 minutes over a period of 12 hours.

###  2.2 Radiator

Figure 7: Side (left) and top (right) view of the design of the TRD sandwich
radiator [44].

The design of the radiator is shown in Fig. 7. Polypropylene fibre mats of
$3.2\text{\,}\mathrm{cm}$ total thickness are sandwiched between two plates of
Rohacell® foam HF71, which are mechanically reinforced by lamination of carbon
fibre sheets of $100\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness.
Aluminised kapton foils are glued on top, to ensure gas tightness and to also
serve as the drift electrode. For mechanical reinforcement, cross-bars of
Rohacell® foam of $0.8\text{\,}\mathrm{cm}$ thickness are glued between the
two foam sheets of the sandwich, with a pitch of 20–$25\text{\,}\mathrm{cm}$
depending on the chamber size. After construction the transmission of the full
radiator was measured using the Kα line of Cu at $8.04\text{\,}\mathrm{keV}$
to ensure the homogeneity of the radiators [52]. This line was chosen as its
energy is close to the most probable value of the TR spectrum (see Fig. 8).

Measurements with prototypes [53] indicated that such a sandwich radiator
produces 30–40% less TR compared to a regularly spaced foil radiator. However,
constructing a large-area detector with radiators made out of 100 regularly
spaced foils each is infeasible. The impact of various radiators constructed
from fibres and/or foam on, e.g. particle identification is discussed in [53,
47]. Based on these measurements the fibre/foam sandwich radiator design was
chosen for the final detector.

Figure 8: Measured and simulated spectra of TR produced by electrons with a
momentum of 2 $\mathrm{GeV}\\!/c$ for the ALICE TRD sandwich radiator. Figure
adapted from [53].

The spectra of TR produced by electrons with a momentum of 2
$\mathrm{GeV}\\!/c$ as measured with the ALICE TRD sandwich radiator is shown
in Fig. 8. Such a measurement is important for the tuning of simulations in
the ALICE setup. As the production of TR is not included in GEANT3 [54], which
is used to propagate generated particles through the ALICE apparatus for
simulations, we have explicitly added it to our simulations in AliRoot [55],
the ALICE offline framework for simulation, reconstruction and analysis. An
effective parameterisation of the irregular radiator in terms of a regular
foil radiator is employed as an approximation. The simulations describe the
data satisfactorily including the momentum dependence [53].

###  2.3 Supermodule

The detector is installed in the spaceframe (the common support structure for
most of the central barrel detectors) in 18 supermodules, each of which can
host 30 read-out chambers arranged in 5 stacks and 6 layers (see Fig. 3). The
overall shape of the supermodule is a trapezoidal prism with a length of
$7.02\text{\,}\mathrm{m}$ ($8\text{\,}\mathrm{m}$ including services). Its
height is $0.78\text{\,}\mathrm{m}$ and the shorter (longer) base of the
trapezoid is $0.95\text{\,}\mathrm{m}$ ($1.22\text{\,}\mathrm{m}$). The weight
of a supermodule with 30 read-out chambers is about $1.65\text{\,}\mathrm{t}$.
Mechanical stability is provided by a hull of aluminium profiles and sheets,
connected with stainless steel screws. The materials were chosen to minimise
the interference with the magnetic field in the solenoid magnet. In front of
PHOS, where minimal radiation length is required, the aluminium sheets of the
short and long base of the trapezoid were replaced by carbon-fibre windows.

All service connections must be routed internally to the end-caps of the
supermodule. Those that require materials with large radiation length are
placed at the sidewalls, outside the active area of the TRD and most other
detectors in ALICE. This includes the low-voltage power distribution bus bars
as well as other copper wires for the Detector Control System (DCS) board
power, network and high-voltage (HV) connections between the fanout boxes and
read-out chambers, and the rectangular cooling pipes (see Section 4 for more
details).

Low-voltage (LV) power for the read-out boards is provided via copper power
bus bars (2 for each layer and voltage as described in Table 3) with a cross-
section of $6\text{\,}\mathrm{mm}$ $\times$ $6\text{\,}\mathrm{mm}$ (per
channel) running along the sidewalls of the supermodule. Each read-out board
is connected directly to the power bus bars. Heat generated by ohmic losses in
the power bus bars is partially transferred to the adjacent cooling pipes (see
Section 4.2). The power bus bars protrude about $30\text{\,}\mathrm{cm}$ from
each side of the supermodule hull, where they are equipped with capacitors for
voltage stabilisation. On one end-cap of the supermodule the power-bus bars
are connected via a low-voltage patch panel to the long supply lines to the
power supplies outside of the magnet.

Each read-out chamber is equipped with 6 or 8 read-out boards (see Section
2.1) and one DCS board (see Section 4.4). Power is provided and controlled
separately for each DCS board by a power distribution box. The DCS boards are
connected via twisted-pair cables to Ethernet patch panels at the end-caps and
the boards of two adjacent layers are connected via flat-ribbon cables in a
daisy chain loop to provide low-level Joint Test Action Group (JTAG) access to
neighbouring boards.

For each chamber, three optical fibres are routed to the end-cap on the
C-side. Two fibres connect the optical read-out interfaces to a patch panel,
where they are linked via the Global Tracking Unit (GTU) (see Section 5) to
the Data AcQuisition (DAQ) systems. One trigger fibre connects the DCS board
to the trigger distribution box (see Section 5.1), which receives the trigger
signals from the pretrigger system or its back-up system and splits them into
30 fibres (+ 2 spares).

The supermodules were constructed from 2006 to 2014. In the following, we
discuss the sequence of required steps. After the construction of the
supermodule hulls, the power bus bars and patch panels for the distribution of
low voltage for the read-out boards and the cooling bars for the water cooling
were mounted on the sidewalls. Next the power distribution box (DCS board
power), the box for trigger signal distribution, a patch panel for the optical
read-out fibres, and the high-voltage distribution boxes were installed at the
end-caps.

Before integrating the read-out chambers into a supermodule, they were
equipped with electronics (read-out boards, DCS boards) and cooling pipes.
After a series of tests were performed to ensure stable operation [56, 57],
the chambers were then inserted layer by layer. The first connection
established during the installation was the gas link between the chambers
(using polyether ether ketone connectors). The chambers were fixed to the hull
with three screws on each of the long sides after performing a manual physical
alignment. As demonstrated by later measurements (Section 9), the alignment in
$r\varphi$ between the chambers is of the order of
0.6–$0.7\text{\,}\mathrm{mm}$ (r.m.s.).

The cables to and from the read-out boards used for JTAG, low-voltage sensing,
Ethernet, and DCS power were routed along one side of the chambers. The cable
lengths in the active area on top of the chambers were minimised, avoiding
cables from the read-out pads to cross. On the other side of the chambers,
only the high voltage cables were routed. They were soldered at two separate
HV distribution boxes for anode and drift voltage at one end-cap of the
supermodule. Each read-out board (38 per layer) was connected to the power bus
bars (low voltage) using pre-mounted cables. The cooling pipes (4 per read-out
board) were connected by small Viton tubes. In the $z$-direction across the
read-out chambers, only optical fibres for the trigger distribution (1 per
chamber) and data read-out (2 per chamber) were routed.

In addition to layer-wise tests during installation, a final test was done
after completion. The test setup consisted of low-voltage and high-voltage
supplies, a cooling plant, a gas system [58], as well as a full trigger setup
and read-out equipment. Also a trigger for cosmic rays was built and installed
[59, 60]. It was used for first measurements of the gas gain and the chamber
alignment, and to also study the zero suppression during assembly [61, 62, 50,
63, 64, 65].

After transport to CERN pre-installation tests were performed (see Section 7.1
and [66]) and the supermodules were installed in the space frame with a
precision of $1\text{\,}\mathrm{cm}$ (r.m.s.) in $z_{\rm lab}$-direction. The
maximum tolerance in $\varphi$ is $2\text{\,}\mathrm{cm}$ due to constraints
given by the space frame.

In addition to the sequential assembly and installation, four supermodules
were completely disassembled again in 2008 and 2009. The initial tests were
not sensitive to viscous leaks of the read-out chambers and thus the
supermodules were rebuilt after improving the gas tightness (see Section
3.4.1). Furthermore, in 2013 during LS 1, one supermodule was disassembled in
order to improve the high-voltage stability of the read-out chambers (see
Section 7.3).

###  2.4 Material budget

A precise knowledge of the material budget of the detector is important to
obtain a precise description of the detector in the Monte Carlo simulations,
which are used, e.g. to compute the track reconstruction efficiencies.

Figure 9:  The radiation length map in units of $X/X_{0}$ in a zoomed-in part
of the active detector area as a function of the pseudorapidity and the
azimuthal angle, calculated from the geometry in AliRoot (the colour scale has
a suppressed zero). The positions of the MCMs and the cooling pipes are
visible as hot spots. The radiation length was calculated for particles
originating from the collision vertex. Therefore the cooling pipes of the six
layers overlap for small, but not large $\eta$.

The TRD geometry, as implemented in the simulation part of AliRoot, consists
of the read-out chambers, the services, and the supermodule frame. All these
parts are placed inside the space frame volume. The material of a read-out
chamber is obtained including several material components. A general overview
of the various components is given in Table 2.

Description |  $\textit{X}/\textit{X}_{0}$ (%)
---|---
Radiator | 0.69
Chamber gas and amplification region | 0.21
Pad plane | 0.77
Electronics (incl. honeycomb structure) | 1.18
Total | 2.85
Table 2: Parts of one read-out chamber, radiator, electronics, and their
average contribution to the radiation length in the active area for particles
with normal incidence.

The material budget in the simulation was adjusted to match the estimate based
on measurements during the construction phase of the final detector. The
supermodule frames consist of the aluminium sheets on the sides, top, and
bottom of a supermodule together with the traversing support structures, such
as the LV power bus bars and cooling arteries. Additional electronics
equipment is represented by aluminium boxes that contain the corresponding
copper layers to mimic the present material. The services are also introduced,
including, e.g. the gas distribution boxes, cooling pipes, power and read-out
cables, and power connection panels.

Figure 9 shows the resulting radiation length map, quantified in units of
radiation length ($X/X_{0}$), in a zoomed-in part of the active detector area.
It is clearly visible that the Multi-Chip Modules (MCM)s on the read-out
boards (see Section 5) and the cooling pipes introduce hot spots in $X/X_{0}$.
After averaging over the shown area, the mean value is found to be $\langle
X/X_{0}\rangle=$ 24.7% for a supermodule with aluminium profiles and sheets
and 30 read-out chambers (6 chambers per stack with the material budget as
indicated in Table 2). The reduced material budget of the supermodules in
front of the PHOS detector (carbon fibre inserts instead of aluminium sheets
and no read-out chambers in stack 2) is likewise modelled in the simulation.
In regions directly in front of PHOS $\langle X/X_{0}\rangle$ is only 1.9%.

The total weight of a single fully equipped TRD supermodule as described in
the AliRoot geometry, including all services, is $1595\text{\,}\mathrm{kg}$,
which is about 3.3% less than its real weight. This discrepancy can be
attributed to material of service components, such as the gas manifold (see
Section 3.3) and the patch panel, outside the active area, which were not
introduced in the AliRoot geometry.

##  3 Gas

At atmospheric pressure, a total of $27\text{\,}{\mathrm{m}}^{3}$ of a xenon-
based gas mixture must be circulated through the TRD detector. This expensive
gas cannot be flushed through, but rather has to be re-circulated in a closed
loop by using a compressor and independent pressure and flow regulation
systems. The gas system of the TRD follows a pattern in construction,
modularisation, control, and supervision which is common to all LHC gaseous
detectors, with emphasis on the regulation of a very small overpressure on the
read-out chambers and on the minimisation of leaks. The basic modules such as
mixer, purification, pump, exhaust, analysis, etc., are based on a set of
equal templates applied to the hardware and the software. A Programmable Logic
Controller (PLC) controls each system and the user interacts with it through a
supervision panel. Upon a global command, the PLC executes a sequence that
configures all elements of the gas system for a given operation mode and
continuously regulates the active elements of the system. In this manner the
modules and operational conditions can be customised to the specific
requirements of each detector, from the control of the stability of the
overpressure in the detectors, the circulation flow, and the gas purification,
recuperation and distillation, to the monitoring of the gas composition and
quality (Xe-CO2 (85-15), and as little O2, H2O and N2 as possible).

###  3.1 Gas choice

As well as being an array of tracking drift chambers, the TRD is an electron
identification device, achieved through the detection of TR photons. In order
to efficiently absorb these several keV photons, a high $Z$ gas is necessary.
Figure 10 shows, for three noble gases, the absorption length of photons of
energies in the range of typical TR production. At around
$10\text{\,}\mathrm{keV}$ the absorption length in Xe is less than a
$\mathrm{cm}$, whereas for Kr it is several cm. This argues for the choice of
Xe as noble gas for the operating mixture. CO2 is selected as the quenching
gas, since hydrocarbons are excluded for flammability and ageing reasons. The
choice of the exact composition is in this case rather flexible, since the
design of the wire chambers leaves enough freedom in the choice of the drift
field and anode potential. The best compromise for the CO2 concentration
corresponds to the mixture Xe-CO2 (85-15), which ensures a very good
efficiency of TR photon absorption by Xe and provides stability against
discharges to the detector.

Figure 10: Absorption length of X-rays in noble gases in the relevant energy
range of TR production.

Furthermore, this mixture exhibits a nice stability of the drift velocity, at
the nominal drift field, also with the inevitable contamination of small
amounts of N2 that accumulates in the gas through leaks (see Section 3.2).

Figure 11: Left: Drift velocity as a function of the drift field for the
nominal gas mixture Xe-CO2 and different admixtures of N2. Right: Gain as a
function of the anode voltage for the same gas mixtures.

The drift velocity of the Xe-CO2 (85-15) mixture, pure and with substantial
admixtures of N2, as a function of the drift field, is shown in Fig. 11
(left). The drift velocity does not depend on the N2 contamination at the
nominal drift field of $700\text{\,}\mathrm{V}\mathrm{/}\mathrm{c}\mathrm{m}$.
On the other hand, as illustrated in Fig. 11 (right), the anode voltage would
need a $50\text{\,}\mathrm{V}$ readjustment to keep the gain constant when
increasing the concentration of N2 by 10% in the mixture. It should be noted
that intakes of less than 5% N2 are typically observed in one year of
operation. After 2–3 years of operation, the N2 is cryogenically separated
from the Xe (see Section 3.3.9).

The operation of the chambers in a magnetic field of $0.5\text{\,}\mathrm{T}$,
perpendicular to the electric drift field
($700\text{\,}\mathrm{V}\mathrm{/}\mathrm{c}\mathrm{m}$), forces the drifting
electrons on a trajectory, which is inclined with respect to the electric
field. The so-called Lorentz angle is about
$9\text{\,}\mathrm{\SIUnitSymbolDegree}$ for this gas mixture (see Section
10).

For commissioning purposes, where TR detection is not necessary, the read-out
chambers are flushed with Ar-CO2 (82-18), which is available in a premixed
form at low cost.

###  3.2 Requirements and specifications

The TRD consists of read-out chambers with an area of about
$1\text{\,}{\mathrm{m}}^{2}$ which are built with low material budget. This
poses a severe restriction on the maximum overpressure that the detector can
hold. Therefore, while in operation, the pressure of each supermodule is
regulated by the gas system to a fraction of a mbar above atmospheric pressure
and the safety bubblers, installed close to the supermodules, are adjusted to
release gas at about $1.3\text{\,}\mathrm{mbar}$ overpressure. The detector
can hold an overpressure in excess of $5\text{\,}\mathrm{mbar}$.

Another tight constraint arises from the highly disadvantageous surface-to-
volume ratio of the detector, which enhances the challenge of keeping the gas
losses through leaks to a minimum. Cost considerations drive the criterion for
the maximum allowable leak rate of the system: a reasonable target is to lose
less than 10% of the total gas volume through leaks in one year. This
translates into a total leak conductance of
$1\text{\,}\mathrm{m}\mathrm{l}\mathrm{/}\mathrm{h}$ per supermodule at
$0.1\text{\,}\mathrm{mbar}$ overpressure. As a result, unlike in other gas
systems, gas is not continuously vented out to the atmosphere. Furthermore,
the filling and emptying of the system must be performed with marginal losses
of xenon. Adequate gas separation and cryogenic distillation techniques are
therefore implemented. Furthermore, any pulse-height measuring detector must
be operated with a gas free of electronegative substances, such as O2, which
is continuously removed from the gas stream. Precautions are taken by
chromatographic analyses of both the supply xenon and of the air inside the
volume of the solenoid magnet to avoid any SF6 contamination of the gas
through gas supply cylinders or from neighbouring detectors.

###  3.3 Description of the gas system

The TRD gas system follows the general architecture of all closed loop systems
of the LHC detectors, but is customised to meet the requirements specified
above. The various modules of the gas system are distributed, as shown
schematically in Fig. 12, on the surface, in a location halfway down the
cavern shaft, and in the cavern. The gas is circulated by compressors that
suck the gas from the detector and compresses it to a high pressure value.
This pumping action is regulated to keep the desired overpressure at the
detector. In the high-pressure part of the system, at the surface, gas
purification, mixing, and other operations are carried out. On its way to the
cavern, the gas is distributed to individual supermodules using pressure
regulators. The gas circulates through the detector and at the outlet of each
sector a gas manifold is used to return the gas through a single line and to
hold the pressure regulation hardware. Halfway to the surface, a set of
pneumatic valves is used to regulate the flow from each supermodule in order
to keep the desired overpressure. The gas is then compressed into a high
pressure buffer prior to circulation back to the surface.

Figure 12: Schematic view of the TRD gas system. The gas circulates in a
closed loop pushed by a compressor. The flow for each supermodule is
determined by the pressure set at individual pressure reducers in the inlet
distribution modules. The overpressure is regulated with individual pneumatic
valves at the return modules. The gas is purified at the surface and, when
needed, supply gas is mixed and added to the loop. For the filling and the
removing of the expensive xenon, semipermeable membranes are used to separate
it from the CO2. The recovered xenon can be treated in a cryogenic plant in
order to remove accumulated N2, prior to storage.

####  3.3.1 Distribution

Xenon is a heavy gas; its standard condition density at ambient conditions is
$5.76\text{\,}\mathrm{k}\mathrm{g}\mathrm{/}\mathrm{{}^{3}}\mathrm{m}$, 4.7
times that of air. This means that over the $7\text{\,}\mathrm{m}$ height-span
of the TRD in the experiment, the total hydrostatic pressure difference
between the top and the bottom supermodules would be about
$2.8\text{\,}\mathrm{mbar}$. In order to overcome this, gas is circulated
separately through each supermodule (except the top three and the bottom
three, which are installed at similar heights) and the pressure is thus
individually regulated to equal values everywhere. In addition, due to the
different heights of the supermodules, the gas, supplied from the surface,
would flow unevenly through the different supermodules, the lower ones being
favoured over the higher ones. This second inconvenience is overcome by
supplying the gas to each supermodule from the distribution area (half way
down the cavern shaft) through $4\text{\,}\mathrm{mm}$ thin lines over a
length of about $100\text{\,}\mathrm{m}$. The pressure drop of the circulating
gas in these lines, of several tens of mbar, is much larger than the
difference in hydrostatic pressure between supermodules, and therefore nearly
equal flow, at equal overpressure, is assured in all supermodules.

The six layers of the supermodules are supplied from one side (A-side) with
three inlet lines, each of them serving two consecutive layers. Small bypass
bellows connect two consecutive layers on the opposite side. In the A-side, a
manifold arrangement is used to connect the gas outlets and a common safety
bubbler, pressure sensors and back-up gas. The return outlets in each
supermodule are connected together into one line which returns to the pump
module. The three top and three bottom supermodules are connected to one
single return line each. This arrangement results into 14 independently
regulated circulation loops. Each supermodule has its own two-way bubbler,
which provides the ultimate safety against over- or underpressure.

####  3.3.2 Pump

In the distribution area, the flow through each return line is regulated by a
pneumatic valve per loop driven by the pressure sensors located at the
detector. In this area, the gas is kept at a pressure slightly below
atmospheric pressure, and it is stored in a $0.8\text{\,}{\mathrm{m}}^{3}$
buffer container before it is compressed by two pumps which operate at a
constant frequency. The compressor module drives a bypass valve in order to
maintain a calculated pressure set point at its inlet. In this manner, a dual
regulation concept is used to handle the 14 loops. The role of the inlet
buffer is to act as a damper of possible regulation oscillations. This
pressure regulation system keeps the overpressure in the supermodule stable at
$0.1\text{\,}\mathrm{mbar}$ above atmospheric pressure (set point) within
$0.03\text{\,}\mathrm{mbar}$.

A $0.93\text{\,}{\mathrm{m}}^{3}$ high pressure buffer at the compressor
outlet is used as a storage volume. Its content varies according to the
atmospheric pressure, either by providing gas to the detectors, or by
receiving it from them. The overpressure in this buffer typically ranges
between 0.8 and $2\text{\,}\mathrm{bar}$. Knowledge of all the system volumes
allows the pressure in the buffer to be predicted for any atmospheric pressure
value. Gas leaks ultimately result in a reduction of this pressure, in that
case the dynamic regulation of the high pressure triggers the injection of
fresh gas from the mixer until the high pressure is restored. From this
buffer, the pressurised gas is circulated up to the gas building at the
surface.

####  3.3.3 Purifier

The purifier module consists of two 3 litre cartridges each filled with a
copper catalyser which is efficient in chemically removing oxygen by oxidising
the copper, and mechanically removing water by absorption. Upon saturation,
the PLC switches between cartridges at the pre-defined frequency, and launches
an automatic regeneration cycle where CuO2 is reduced at high temperatures
with a flow of H2 diluted in argon. As the detector is rather gas tight, the
O2 intake through leaks is moderate, and the purifier keeps it between 0 and 3
ppm. However, H2O diffusion, probably through the aluminised Mylar foil which
constitutes the drift electrode of every read-out chamber, makes it necessary
to switch between purifiers about every 3.5 days, in order to keep the H2O
content below a few hundred ppm.

####  3.3.4 Recirculation

The surface module is used to recirculate the gas at high enough pressure to
the distribution modules in the cavern shaft area. It also contains provisions
for extracting gas samples for analysis, and a bypass loop to allow for the
installation of containers such as a krypton source for gain calibration (see
Section 10).

####  3.3.5 Mixer

Under normal operation and since the gas is only exhausted through leaks, gas
injection into the system happens only if the pressure in the high pressure
buffer falls below a dynamic threshold, as explained above. On such occasions,
the mixer is activated and injects the nominal gas mixture at a rate of a few
tens of $\mathrm{l}\mathrm{/}\mathrm{h}$ until the high pressure buffer is
replenished. The amount of gas injected by the mixer during a given period
provides a direct measurement of the leak rate.

In addition, a second set of mass flow controllers provides flows in the
$\mathrm{{}^{3}}\mathrm{m}\mathrm{/}\mathrm{h}$ range and is used for filling
and emptying the detector.

####  3.3.6 Backup system

When the gas system is in stop mode, e.g. when there is a power failure, the
safety bubbler installed on each supermodule ensures that the detector
pressure always remains within about $\pm$$1.3\text{\,}\mathrm{mbar}$ relative
to atmospheric pressure. In order to avoid that air, i.e. oxygen, enters the
detector, the external side of the bubbler is connected to a continuous flow
of neutral gas, in this case N2, that flows through the bubbler in case of a
large detector underpressure. The choice of N2 is driven by the small
influence on the gas properties that this admixture has (see Fig. 11). The
full TRD is served by three independent backup lines, each with connections to
six supermodule bubblers, and arranged such that the flow points downwards. In
this way, if the xenon mixture is exhausted through the bubblers, it falls
down the back-up line, relieving its high hydrostatic pressure. A differential
pressure transmitter measures the pressure difference between the detector and
the backup gas.

####  3.3.7 Analysis

The control of the gas quality is perhaps the most demanding aspect of running
detectors where both signal amplitude and drift time information are
important. This control is even more crucial for the ALICE TRD, where accurate
and uniform drift velocity and gain values are needed for triggers based on
online tracking and particle identification. Thus, in addition to effective
tightness of the system and continuous removal of O2 and H2O, constant
monitoring of the gas composition and in particular of the N2 is necessary.
Although for a large volume system such as that of the TRD the changes in
composition are obviously slow, the precision and stability requirement of the
measuring instruments are quite challenging. Furthermore, constantly measuring
analysers, such as O2, H2O and CO2 sensors, must be installed in the gas loop,
since xenon must not be exhausted. Therefore they must be free of outgassing
of contaminants into the gas.

The analysis module samples the return gas from individual supermodules in a
bypass mode, before it is compressed. For this, a fraction of the gas is
pushed through the analysis chain by a small pump, and returned to the loop at
the compressor inlet. Usually, the PLC is programmed to continuously sample
one supermodule after the other, for about 10 minutes each.

An external gas chromatograph is used to periodically measure the gas
composition. This device is not in the gas loop; rather, the gas is exhausted
while purging and sampling a small stream for a few seconds every few hours.

####  3.3.8 Membranes

One system volume of xenon is injected for operation and, typically every two
or three years, removed for cleaning and storage. This means that it must be
possible to separate CO2 from Xe. This separation is achieved with a set of
two semipermeable membrane cartridges. Each cartridge consists of a bundle of
capillary polyimide tubes through which the mixture flows. The bundle is in
turn enclosed in the cartridge case. While the CO2 permeates through the
polyimide walls, most of the xenon is contained and continues to flow into the
loop. The permeating gas can be circulated through the second membrane
cartridge to further separate and recover most of the Xe.

During the filling, the detector is first flushed with CO2 and then, in
closed-loop circulation, the xenon is injected as the CO2 is removed through
the membranes. The reverse process is used for the recuperation of the xenon
into a cryogenic plant.

####  3.3.9 Recuperation

N2 inevitably builds up in the gas through small leaks and cannot be removed
by the purifier cartridges. Therefore, after each long period (2–3 years) of
operation, the N2 is cryogenically separated from the Xe. A cryogenic buffer
is filled with xenon after separating it from CO2. At the same time, CO2 is
injected into the gas system in order to replace the removed gas.

The cryogenically isolated buffer is surrounded by a serpentine pipe with a
regulated flow of liquid nitrogen (LN2) in order to keep its temperature at
$-170\text{\,}\mathrm{\SIUnitSymbolCelsius}$, just above the N2 boiling point
($-195.8\text{\,}\mathrm{\SIUnitSymbolCelsius}$). At this temperature Xe (and
CO2) freezes whereas N2 stays in the gaseous phase. Once the buffer is full,
the stored gas is pumped away. After this, the buffer is heated up in a
regulated way, and the evaporating Xe is compressed into normal gas cylinders.
The resulting Xe has typically a N2 contamination of <1%, and the total Xe
loss (due to the efficiency of the membranes and the cryogenic recovery
process) is about $1\text{\,}{\mathrm{m}}^{3}$ for a full recovery operation.

###  3.4 Operational challenges

The gas system has been operating reliably over several years in several
modes, but mainly in so-called run mode. Aside from minor incidents, a number
of important leaks have been dealt with, which deserve a brief description.

####  3.4.1 Viscous leaks

As part of the standard quality assurance procedure, a leak test was performed
on each chamber prior to installation in the supermodule. The leak test
consisted of flushing the chamber with gas and measuring the O2 contamination
at the exhaust, where the overpressure was typically about
$1\text{\,}\mathrm{mbar}$. It was found, however, that a supermodule would
lose gas even if the O2 content was very low. The reason turned out to be the
particular construction of the pad planes, which are glued to a reinforcement
honeycomb panel with a carbon fibre sheet. Viscous leaks would develop between
the glued surfaces and gas would find its way out through the cut-outs for the
signal connections machined in the honeycomb sandwich. The impedance of this
kind of leak is large enough that gas can escape the detector with no intake
of air through back-diffusion. The concerned read-out chambers were then
extracted and repaired, and the leak tests on subsequent chambers were
modified such that the O2 was measured both at over- and underpressure in the
read-out chamber, resulting in a tight system.

####  3.4.2 Argon contamination

At one point, the routine gas analysis with the gas chromatograph showed
increasing levels of Ar in the Xe-CO2 mixture. This elusive leak came from a
faulty pressure regulator which was pressurised with argon on the atmospheric
side. Occasionally, depending on the pressure, the membrane of the regulator
would leak and let Ar enter the gas volume. A total of 1% Ar accumulated in
the mixture and was removed by cryogenic distillation, together with N2.

####  3.4.3 Leak in pipe

The last major leak in the system was detected when suddenly the pressure at
the high pressure buffer started to steadily decrease. Any leak of the system
would appear, while running, as a decrease in the high pressure buffer,
because the system always ensures the right overpressure at the read-out
chambers. By stopping the system and isolating all of its modules, it was
found that the source of the leak was a long, stainless steel pipe which
connected the compressor module, half way down the cavern shaft, to the
surface, where the gas, still at high pressure, is cleaned and recirculated.
It was not possible to find the exact location of the leak. This was solved by
replacing the pipe by a spare.

##  4 Services

The supermodules installed in the space frame require service infrastructure
for their operation. To reduce the weight, the connections (low and high
voltage, cooling, gas, read-out, and control lines) are routed via dedicated
frames on the A- and C-side, respectively. Both frames are
$2\text{\,}\mathrm{m}$ extensions of the space frame with similar geometry,
but mechanically independent except for the flexible services. Most of the
equipment, such as the low-voltage power supplies, is placed in the cavern
underground and thus inaccessible during beam operation. Some devices are
situated in counting rooms in the cavern shaft, which are supervised radiation
areas but accessible.

###  4.1 Low voltage

The low voltage system supplies power to various components of the TRD. The
largest consumer is the Front-End Electronics (FEE), i.e. the electronics of
the Read-Out Boards (ROB) mounted on the chamber (see Section 5). To minimise
noise, separate (floating) voltage rails are used for analogue and digital
components. The power supply channels for analogue $1.8\text{\,}\mathrm{V}$,
analogue $3.3\text{\,}\mathrm{V}$, and digital $1.8\text{\,}\mathrm{V}$ are
grouped such that one power supply channel supplies two layers of a
supermodule. For the digital $3.3\text{\,}\mathrm{V}$ there is one channel per
supermodule. For each supermodule, this results in the supply channels listed
in Table 3. The DCS boards (see Section 4.4) are powered by a power
distribution box (PDB), two of which (in two adjacent supermodules) are
supplied by a dedicated channel. The PDBs are controlled by Power Control
Units (PCU) over a redundant serial interface.

Channel |  $\mathbf{U_{\mathrm{nom}}(\mathrm{V})}$ |  $\mathbf{I_{\mathrm{typ}}(\mathrm{A})}$
---|---|---
3 x Analogue $1.8\text{\,}\mathrm{V}$ | 2.5 | 125
3 x Analogue $3.3\text{\,}\mathrm{V}$ | 4.0 | 107
3 x Digital $1.8\text{\,}\mathrm{V}$ | 2.5 | 95–150
Digital $3.3\text{\,}\mathrm{V}$ | 4.0 | 110
DCS boards | 4.0 | 2 $\times$ 30
Table 3: Number of low voltage channels, nominal voltages and typical currents
for the electronics on the chamber-mounted read-out boards of one supermodule.
The current for the TRAP cores (digital 1.8 V) increases with the trigger
rates. The current for the DCS boards is 2 $\times$ $30\text{\,}\mathrm{A}$
for two adjacent sectors.

Because of the high currents, the intrinsic resistances of the cables and
connections are critical and are constantly monitored by measuring the voltage
drop between the power supply unit (terminal voltage) and the patch panel at
each supermodule (sense voltage). Typical values are
6–$8\text{\,}\mathrm{m\SIUnitSymbolOhm}$, depending on the cable length. In
addition, the voltages at the end of each power bus bar are monitored.

The Global Tracking Unit (GTU) (see Section 5.3) uses additional power
supplies which are shared with the PCUs. The pretrigger system (see Section
5.1) is powered by separate power supplies, laid out in a fail-safe redundant
architecture.

Different customizations of the Wiener PL512 power supply units are used. The
power supplies feeding the FEE are connected to a PLC-based interlock based on
the status of the cooling. Power is automatically cut in case of a cooling
failure.

During the Run 1 operation, several low-voltage connections on the
supermodules showed increased resistivity resulting in excessive heat
dissipation, which in some cases required to switch off part of the detector
until the problem could be fixed during an access. Later, during LS 1, the
affected supermodules were pulled out of the experiment and the connections
were reworked in the cavern. The supermodules were re-inserted and re-
commissioned immediately after the rework. The complete procedure took about
one day per supermodule.

###  4.2 Cooling

The complexity of the cooling system, whose cooling medium is deionised water,
is driven by the large amount of heat sources (more than $100\,000\text{\,}$)
distributed over the complete active area of the detector. Heat is produced by
the MCMs and the Voltage Regulators (VR) on the read-out boards, the DCS
boards, and the power bars. The total heat dissipation in a supermodule
amounts to about $3.3\text{\,}\mathrm{kW}$, of which about
$2.6\text{\,}\mathrm{kW}$ are produced in the FEE, the remaining
$700\text{\,}\mathrm{W}$ originate from the voltage regulators and the bus
bars. The DCS boards contribute with about $130\text{\,}\mathrm{W}$ per
supermodule. Overall, the rate of heat to be carried away during detector
operation amounts to $55\text{\,}\mathrm{kW}$ and $70\text{\,}\mathrm{kW}$ in
Pb–Pb and pp collisions, respectively, due to different read-out rates. Apart
from the power bus bars, the heat sources are positioned on top of the read-
out boards.

In the cooling system the pressure is kept below atmospheric pressure. Thus a
leak leads to air entering in the system but no water is spilled onto the
detector. The cooling plant [67] consists of a $1500\text{\,}\mathrm{l}$
storage tank positioned at the lowest point outside the solenoid magnet, which
is able to contain all the water of the installation, the circulation pump,
the 18 individual circuits that supply cooling water to the 6 layers of each
supermodule, and the heat exchanger connected to the CERN chilled water
network. The reservoir is kept at 300–$350\text{\,}\mathrm{mbar}$ below
atmospheric pressure by means of a vacuum pump that also removes any air
collected through small leaks. In addition, the pressure of the circulation
pump ($1.8\text{\,}\mathrm{bar}$) and the diameter of all pipes are chosen
such that a sub-atmospheric pressure is maintained in all places of the
detector, despite a difference in height of about $7\text{\,}\mathrm{m}$
between the lowest and the highest supermodule. Each circuit is equipped with
individual heaters and balancing valves in order to control the temperature
and the flow in each loop separately. The heaters are regulated by a
proportional-integral-derivative controller. A temperature stability in the
cooling water of $\pm$$0.2\text{\,}\mathrm{\SIUnitSymbolCelsius}$ is achieved.
The typical water flow is about $1300\text{\,}\mathrm{l}\mathrm{/}\mathrm{h}$
per supermodule. To avoid corrosion a fraction of the total water flow is
passed by a deioniser to keep the water conductivity low. As the water is in
contact with similar materials (stainless steel and aluminium), the TRD
cooling system also supplies the water to the cooling panels of the thermal
screening between TPC and TRD [31].

The loop regulations and cooling plant control is done by a PLC. Warnings and
alarms are issued by the PLC if the parameters are outside the allowed
intervals and read out by the Detector Control System (see Section 6). Two
independent security levels were implemented in each loop. The first
continuously monitors the pressure of each loop and stops the water
circulation of the cooling plant if any value reaches atmospheric pressure.
Secondly, large safety valves were installed at the entrance to each
supermodule. They will open in case an overpressure of
$50\text{\,}\mathrm{mbar}$ is reached, providing a low resistance path for the
water evacuation in case of emergency.

The cold water is supplied in the lowest point of each supermodule and the
warm water is collected on the highest point in order to have more homogeneous
water flow in all pipes. A water manifold at one end-cap of the supermodule
distributes the water in parallel to the 6 layers inside each supermodule, and
on the opposite side a similar manifold collects the warm water. In each
layer, two rectangular pipes along the $z$-direction (65 $\times$ 8 $\times$
$7500\text{\,}{\mathrm{}}^{3}\text{\,}\mathrm{mm}$) supply (collect) water to
(from) the meanders, 76 individual cylindrical aluminium pipes
($3\text{\,}\mathrm{mm}$ in diameter) running across the $y$-direction where
the heat sources are. A total of 17 meander types were designed for the
system. To bring the water from the rectangular pipes to the individual
meanders, the rectangular pipe has small stainless steel pipes
($3\text{\,}\mathrm{mm}$ diameter and $5\text{\,}\mathrm{cm}$ length) soldered
at the proper position for each MCM row. A Viton tube of about
$2\text{\,}\mathrm{cm}$ length is used to connect the small stainless steel
pipes and the meanders as well as for the connections between the two meanders
(one per ROB) in $y$-direction. A total of about 25000 Viton tube connectors
were used in the system. This kind of connector was previously used in the
CERES/NA45 leakless cooling system [68] because of its low price and
reliability.

The cooling pad mounted on top of the heat source consists of an
$0.4\text{\,}\mathrm{mm}$ thick aluminium plate. The meander is glued on top
of the pad by aluminium-filled epoxy (aluminium powder: Araldite® 130:100 by
weight) to increase the thermal conductivity. In order to maximise the heat
transfer, the longest possible path was chosen. The choice of aluminium was
driven by the necessity of keeping the material budget as low as possible in
the active area of the detector.

###  4.3 High voltage

The high voltage distribution for the drift field and the anode-wire plane is
made separately for each chamber, reducing the affected area to one chamber in
case of failure. The power supplies for the drift channels and anode-wires
were purchased from ISEG [69] (variants of the model EDS 20025). Each module
has 32 channels, which are grouped in independent 16-channel boards. Each
channel is independently controllable in terms of the voltage setting and
current limit as well as monitoring of current and voltage. Eight modules are
placed into each crate and remotely controlled via CANbus (Controller Area
Network) from DCS (see Section 6). The HV crates are placed in one of the
counting rooms in the cavern shaft, which allows access even during beam
operation.

For each of the 30 read-out chambers in a supermodule one power supply is
needed for the drift field and one for the anode-wire plane. A multiwire HV
cable connects the 32 channel HV module with a 30 channel HV fanout box (patch
box) located at one end of the supermodule, where the output is redistributed
to single wire HV cables (see Section 2.3). The individual HV cables are then
connected to a HV filter box, mounted along the side of the read-out chamber.
The HV filter box supplies the HV to the 6 anode segments and the drift
cathode of the read-out chamber, and in addition it allows connection of the
HV ground to the chamber ground. It consists of a network of a resistor and
capacitors ($2.2\text{\,}\mathrm{nF}$ and $4.7\text{\,}\mathrm{nF}$) to
suppress load-induced fluctuations of the voltages in the chamber.

The HV crates are equipped with an Uninterruptible Power Supply (UPS) and a
battery to bridge short term power failures. In case of a longer power failure
(> $10\text{\,}\mathrm{s}$) a controlled ramp-down is initiated, i.e. the HV
of the individual drift and anode-wire channels is slowly ramped down. Details
on maximum applied voltages, channel equalisation, ramp speed as well as high-
voltage instability observed during data taking are discussed in Sections 6
and 7.3.

###  4.4 Slow control network

The slow control of the TRD is based on Detector Control Systemboards [70].
They communicate with the DCS (see Section 6) by a
$10\text{\,}\mathrm{M}\mathrm{bit}\mathrm{/}\mathrm{s}$ Ethernet interface,
mostly using Distributed Information Management (DIM) as protocol for
information exchange. The use of Ethernet allows the use of standard network
equipment, but a dedicated network restricted to the ALICE site is used. The
DCS boards are used as end points for the DCS to interact with subsystems of
the detector. Later sections will discuss how the DCS boards are used as
interface to the various components, e.g. the front-end electronics or the
GTU.

The DCS boards were specifically designed for the control of the detector
components and are used by several detectors in ALICE. At the core, the board
hosts an Altera Excalibur EPXA1 (ARMv4 core + FPGA), which hosts a Linux
operating system on the processor and user logic in the FPGA fabric depending
on the specific usage of the board. The DCS board also contains the Trigger
and Timing Control receiver (TTCrx) for clock recovery and trigger reception.
The Ethernet interface is implemented with a hardware PHY (physical layer) and
a soft-Media Access Controller (MAC) in the FPGA fabric. In case of the boards
mounted on the detector chambers, the FPGA also contains the Slow Control
Serial Network (SCSN) master used to configure the front-end electronics.
Further general purpose I/O lines are, e.g. used for JTAG and I2C
communication.

Since the Ethernet connections are used for configuration and monitoring of
the detector components, reliable operation is crucial. All DCS boards are
connected to standard Ethernet switches installed in the experimental cavern
outside of the solenoid magnet. Because of the stray magnetic field and the
special Ethernet interface of the DCS board (no inductive coupling), there are
limitations on the usable switches. Since the failure of an individual switch
would result in the loss of connectivity to a large number of DCS boards, a
custom-designed Ethernet multiplexer was installed in front of the switches in
the second half of Run 1. This allows the connection of each DCS board to be
remotely switched between two different switches with separate uplinks to the
DCS network. The multiplexers themselves are implemented with fully redundant
power supplies and control interfaces.

##  5 Read-out

The read-out chain transfers both raw data and condensed information for the
level-1 trigger. While the former requires sufficient bandwidth to minimise
dead time, the latter depends on a low latency, i.e. a short delay of the
transmission. The data from the detector are processed in a highly
parallelised read-out tree. Figure 13 provides an overview and relates
entities of the read-out system to detector components. In the detector-
mounted front-end electronics, the data are processed in Multi-Chip Modules
grouped on Read-Out Boards (ROB) and eventually merged per half-chamber. Then,
they are transmitted optically to the Track Matching Units (TMU) as the first
stage of the Global Tracking Unit (GTU). The data from all stacks of a
supermodule are combined on the SuperModule Unit (SMU) and eventually sent to
the Data AcQuisition system (DAQ) through one Detector Data Link (DDL) per
supermodule.

Figure 13: Detector structures and corresponding read-out stages [71]. The top
row of the figure represents the detector and the bottom row the GTU
components. The dimensions are not to scale.

The read-out of the detector is controlled by trigger signals distributed to
both the FEE and the GTU. The ALICE trigger system is based on three hardware-
level triggers (level-0, 1, 2) and a High Level Trigger (HLT) [72] implemented
as a computing farm. In addition to these levels, the FEE requires a dedicated
wake-up signal as described in the next subsection.

###  5.1 Pretrigger and LM system

Both FEE and GTU must receive clock and trigger signals, which are provided by
the Central Trigger Processor (CTP) [73] using the Trigger and Timing Control
(TTC) protocol over optical fibres. While the GTU only needs the level-0/1/2
and is directly connected to the CTP, the FEE requires a more complicated
setup. To reduce power consumption, it remains in a sleep mode when idle and
requires a fast wake-up signal before the reception of a level-0 trigger to
start the processing. During Run 1, an intermediate pretrigger system was
installed within the solenoid magnet [74, 75]. Besides passing on the clock
and triggers received from the CTP, it generated the wake-up signal from
copies of the analogue V0 and T0 signals (reproducing the level-0 condition)
and distributed it to the front-end electronics. In addition, the signals from
TOF were used to generate a pretrigger and level-0 trigger on cosmic rays.
Because of limitations of this setup, the latencies of the contributing
trigger detectors at the CTP were reduced for Run 2 (also by relocating the
respective detector electronics) such that the functionality of the pretrigger
system could be integrated into the CTP. The latter now issues an LM (level
minus 1) trigger for the TRD before the level-0 trigger. An interface unit
(LTU-T) was developed for protocol conversion [76] in order to meet the
requirements of the TRD front-end electronics. A comparison of the two designs
is shown in Fig. 14. The new system has been used since the beginning of
collision data taking in Run 2.

Figure 14: In Run 1, the wake-up signal required for the front-end electronics
was generated by a dedicated pretrigger system. In Run 2, the functionality
was implemented in the central trigger processor and the LTU-T serves as an
interface to the TRD FEE.

###  5.2 Front-end electronics

The FEE is mounted on the back-side of the read-out chamber. It consists of
MCMs which are connected to the pads of the cathode plane with flexible flat
cables. An MCM comprises two ASICs, a PASA and a TRAP, which feature a large
number of configuration settings to adapt to changing operating conditions.
The signals from 18 pads are connected to the charge-sensitive inputs of the
PASA on one MCM. An overview of the connections is shown in Fig. 15.

Figure 15: Connections in one MCM [66].

The very small charges induced on the read-out pads (typically
$7\text{\,}\mathrm{\SIUnitSymbolMicro A}$ during $1\text{\,}\mathrm{ns}$) are
not amenable to direct signal processing. Therefore, the signal is first
integrated and amplified by a Charge Sensitive Amplifier (CSA). Its output is
a voltage signal with an amplitude proportional to the total charge. The CSA
has a relatively long decay time, which makes it vulnerable to pile-up. A
differentiator stage removes the low frequency part of the pulse. The
exponential decay of the CSA feedback network, in combination with the
differentiator network, leads to an undershoot at the shaper output with the
same time constant as the CSA feedback network. A Pole-Zero network is used to
suppress the undershoot. A shaper network is required to limit the bandwidth
of the output signal and avoid aliasing in the subsequent digitisation
process. At the same time the overall signal-to-noise ratio must be optimised.
These objectives are achieved by a semi-Gaussian shaper, implemented with two
low-pass filter stages. Each stage consists of two second-order bridged-T
filters connected in cascade. The second shaper consists of a fully
differential amplifier with a folded cascode configuration and a common-mode
feedback circuit. This circuit network was implemented to prevent the output
of the fully differential amplifier from drifting to either of the two supply
voltages. It establishes a stable common-mode voltage. The last stage in the
chain comprises a pseudo-differential amplifier with a gain of 2. This stage
adapts the DC voltage level of the PASA output to the input DC-level of the
TRAP ADC [77].

Parameter | Value
---|---
PASA gain | $12\text{\,}\mathrm{m}\mathrm{V}\mathrm{/}\mathrm{f}\mathrm{C}$
PASA power | $15\text{\,}\mathrm{m}\mathrm{W}\mathrm{/}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{n}\mathrm{e}\mathrm{l}$
PASA pulse width (FWHM) | $116\text{\,}\mathrm{ns}$
PASA noise (equivalent charge) | $1000~{}\mathrm{e}$
TRAP power | $12.5\text{\,}\mathrm{m}\mathrm{W}\mathrm{/}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{n}\mathrm{e}\mathrm{l}$
TRAP ADC depth | 10 bit
TRAP sampling frequency | $10\text{\,}\mathrm{MHz}$
Table 4: Achieved PASA and TRAP characteristics.

The differential PASA outputs are fed into the ADCs of the TRAP, the second
ASIC on the same MCM. The PASA and TRAP parameters are listed in Table 4. The
TRAP is a custom-designed digital chip produced in the UMC
$0.18\text{\,}\mathrm{\SIUnitSymbolMicro m}$ process. The TRAP comprises
cycling 10-bit ADCs for 21 channels, a digital filter chain, a hardware
preprocessor, four two-stage pipelined CPUs with individual single-port,
Hamming-protected instruction memories (IMEM, 4k x 24 bit), about 400
configuration registers usable by the hardware components, a quad-port
Hamming-protected data memory (DMEM, 1k x 32 bit), and an arbitrated Hamming-
protected data bank (DBANK, 256 x 32 bit) [78]. Three excess ADC channels are
fed with the amplified analogue signal from the two adjacent MCMs to avoid
tracking inefficiencies at the MCM boundaries. The signals of all 21 channels
are sampled and processed in time bins of $100\text{\,}\mathrm{ns}$. The
number of time bins to be read out, can be configured in the FEE. At the
beginning of Run 1 24 time bins were conservatively read out. At a later stage
the number of time bins was reduced to 22 in order to reduce the readout time
and the data volume.

The first step in the TRAP is the digitisation of the incoming analogue
signals. In order to avoid rounding effects, the ADC outputs are extended by
two binary digits and fed into the digital filter chain. First, the pedestal
of the signal is equilibrated to a configurable value. Then, a gain filter is
used to correct for local variations of the gain, arising either from detector
imperfections or the electronics themselves. A tail cancellation filter can be
used to suppress the ion tails. The filtered data are fed into a pre-processor
which contains hardware units for the cluster finding. The four CPUs (MIMD
architecture) are used for the further processing. The local tracking
procedure is discussed in detail in Section 12.1.

The MCMs are mounted on the ROB. On each board, 16 chips are used to sample
and process the detector signals. A full detector chamber is covered by 8 ROBs
(6 for chambers in stack 2). The read-out is organised in a multi-level tree.
First, the data from four chips are collected by so-called column merger
chips. The latter, in addition to processing the data from their own inputs,
receive the data from three more MCMs. The data are merged and forwarded to
the board merger, which combines the data from all chips of one ROB. One ROB
per half-chamber carries an additional MCM which acts as half-chamber merger
(without processing data of its own). It forwards the data to the Optical
Read-out Interface (ORI) from where it is transmitted through an optical link
(DDL) to the GTU. The link is operated at 2.5 Gbit/s and is implemented for
uni-directional transmission without handshaking, i.e. the receiving side must
be able to handle the incoming data for a complete event as it arrives. As the
FEE does not provide multi-event buffering, the detector is busy until the
transmission from the FEE is finished. The slowest half-chamber determines the
contribution to the dead time of the full detector.

###  5.3 Global Tracking Unit

The GTU receives data via 1044 links from the FEE. The aggregate net bandwidth
amounts to $261\text{\,}\mathrm{G}\mathrm{B}\mathrm{/}\mathrm{s}$. The two
main tasks of the GTU are the calculation of level-1 trigger contributions
from a large number of track properties in about
$2\text{\,}\mathrm{\SIUnitSymbolMicro s}$ and the preparation of the event
data for read-out. Accordingly, the data processing on the GTU features a
trigger path, which is optimised for low latency, and a data path, which
equips the detector with the capability to buffer up to 4 events (multi-event
buffering, MEB). The derandomisation of the incoming data rate fluctuations
with multiple event buffers minimises the read-out related dead time. The data
transfer from the GTU to the DAQ contributes to the dead time only when the
read-out rate approaches the rate which saturates the output bandwidth as
shown in Fig. 16.

Figure 16: Simulation of the dead time as function of the read-out rate in
Pb–Pb collisions for the effective DDL bandwidth in Run 1 (DDL) and Run 2
(DDL2). The simulation assumes a 5% L1/L0 accept ratio and no L2 rejects. The
scenarios central and mix correspond to an event size of 470 kB and 310 kB per
supermodule, respectively, mimicking thus different event multiplicities.

The GTU consists of three types of FPGA-based processing nodes organised in a
three-layer hierarchy (see Fig. 17). The central component of all nodes is a
Virtex-4© FX100 FPGA, supplemented by a $4\text{\,}\mathrm{MB}$ source-
synchronous DDR-SRAM, $64\text{\,}\mathrm{MB}$ DDR2-SDRAM and optical
transceivers. Depending on the type, the nodes are equipped with different
optical parts and supplementary modules. 90 TMUs and 18 SMUs are organised in
18 segments of 5+1 nodes (corresponding to the 18 sectors). The TMUs and SMU
of a segment are interconnected using a custom LVDS backplane, which is
optimised for high-bandwidth transmissions at low latency. A single top-level
Trigger Unit (TGU) is connected to the SMUs of the individual segments via
LVDS transmission lines.

Figure 17:  Major design blocks of the TMU and SMU stages of the GTU and data
flow. The busy and trigger logic information are combined on the TGU before
transmission to the CTP.

The data from one stack is received by the corresponding TMU. Each TMU
implements the global online tracking, which combines pre-processed track
segments to tracks traversing the corresponding detector stack, as first stage
of the trigger processing (see Section 12). The TMUs furthermore implement the
initial handling and buffering of incoming events as a pipelined data push
architecture. Input shaper units monitor the structural integrity of the
incoming data and potentially restore it to a form that allows for stable
operation of all downstream entities. Dual-port, dual-clock BRAMs in the FPGA
are utilised to compactify data of the 12 incoming link data streams to dense,
wide lines suitable for storage in the SRAM. The SRAM provides buffer space
for multiple events and its controller implements the required write-over-read
prioritisation to ensure that data can be handled at full receiver bandwidth.
On the read side, a convenient interface is provided to read out or discard
stored events in accordance to the control signals generated by the segment
control on the SMU.

Via its DCS board the SMU receives relayed trigger data issued by the CTP to
synchronise the operation of the experiment. The trigger sequences are
decoded, and converted to suitable control signals and time frames to steer
the operation of the segment. The segment control on the SMU supports
operation with multiple, interlaced trigger sequences in order to support the
concurrent handling and buffering of multiple events. Upon reception of a
level-2 trigger, the SMU requests the corresponding event data from the event
buffers and initiates the building of the event fragment for read-out. The
built fragment contains, in addition to the data originating from the
detector, intermediate and final results from tracking and triggering relevant
for offline verification, as well as checksums to quickly assess its
integrity. The SMU implements the read-out interface to the DAQ/HLT with one
DDL. The endpoint of the DDL is a Source Interface Unit (SIU), which in Run 1
was a dedicated add-on card mounted on the SMU backside that operates at a
line rate of $2.125\text{\,}\mathrm{G}\mathrm{bit}\mathrm{/}\mathrm{s}$. The
read-out upgrade for Run 2 integrates the functionality of the SIU into the
SMU FPGA and employs a previously unused transceiver on the SMU at a line rate
of $4\text{\,}\mathrm{G}\mathrm{bit}\mathrm{/}\mathrm{s}$. The elimination of
the interface between SMU and SIU add-on card, the higher line rate as well as
data path optimisations resulted in an increase of the effective DDL output
bandwidth from $189\text{\,}\mathrm{M}\mathrm{B}\mathrm{/}\mathrm{s}$ to
$370\text{\,}\mathrm{M}\mathrm{B}\mathrm{/}\mathrm{s}$ in Run 2. Figure 16
illustrates the performance improvement for the assumed data taking scenarios.
With the upgrade the read-out-related dead time can be kept at an acceptable
level. The almost linear increase at low rates is due to the dead time
associated with the L0–L1 interval and the FEE-GTU transmission. The typical
aggregate output bandwidth for all 18 supermodules is
$126\text{\,}\mathrm{M}\mathrm{B}\mathrm{/}\mathrm{s}$,
$202\text{\,}\mathrm{M}\mathrm{B}\mathrm{/}\mathrm{s}$, and
$1260\text{\,}\mathrm{M}\mathrm{B}\mathrm{/}\mathrm{s}$ in pp, p–Pb, and Pb–Pb
collisions (see also Section 7.3).

The top-level TGU consolidates the status of the segments, which operate
independently in terms of read-out, as well as the segment-level contributions
of the triggers. It constitutes the interface to the CTP, to which it
communicates the detector busy status and the TRD-global trigger contributes
for various signatures (see Section 12).

##  6 Detector Control System

The purpose of the DCS is to ensure safe detector conditions, to allow fail-
safe, reliable and consistent monitoring and control of the detector, and to
provide calibration data for offline reconstruction. In addition it provides
detailed information on subsystem conditions and full functionality for expert
monitoring and detector operation. Tools were implemented to reduce the
operational complexity and the information on detector conditions to a level
that allows operators to monitor and handle the detector in an intuitive and
safe way. The TRD DCS is integrated with the rest of the ALICE detector
control systems into one system which is operated by one operator.

###  6.1 Architecture

Figure 18: Overview of the DCS software architecture. The tree structure
consists of device units (boxes) and logical control units (ellipses). The
abbreviations PT, DR and AN correspond to the pretrigger, the drift and anode
channels, respectively.

The hardware architecture of the DCS can be divided into three functional
layers. The field layer contains the actual hardware to be controlled (power
supplies, FEE, etc). The control layer consists of devices which collect and
process information from the field layer and make it available to the
supervisory layer. Finally, the devices of the control layer receive and
process commands from the supervisory layer and distribute them to the field
layer.

The software on the supervisory layer is distributed over 11 server computers.
It is based on the commercial Supervisory Control and Data Acquisition (SCADA)
system PVSS II from the company ETM [79], now called Symatic WinCC [80]. The
implementation uses the CERN JCOP control framework [81], shared by all major
LHC experiments. This framework provides high flexibility and allows for easy
integration of separately developed components in combination with dedicated
software developed for the TRD, including Linux-based processes.

The software architecture is a tree structure that represents (sub-)systems of
the detector and its devices, as shown in Fig. 18. The entities at the bottom
of the hierarchy represent the devices (device units), logical entities are
represented by control units. The DCS system monitors and controls 89 low
voltage (LV) power supplies with more than 200 channels, and 1044 high voltage
channels. The system also monitors the electronics configuration of more than
one million read-out channels, the GTU, and the cooling and gas systems.

###  6.2 Detector safety

To ensure the safety of the equipment, nominal operating conditions are
maintained by a hierarchical structure of alerts and interlocks. Whenever
applicable, internal mechanisms of devices (e.g. power supply trip) are used
to guarantee the highest level of reliability and security. Thresholds and
status of the interlocks are controlled by the system, but the functioning of
the device is independent of the communication between hardware and software.
The possible range of applied settings (e.g. anode channel high voltage) is
limited to a nominal range to prevent potential damage due to operator errors.

In addition, the system employs a three-level alert system, which is used to
warn operators and detector experts of any unusual detector condition.

On the control and supervisory layer, cross system interlocks protect the
devices and ensure consistent detector operation. These are a few examples:

  * •

In case of a failure of the cooling plant for the FEE, a PLC-based interlock
disables the LV power supplies.

  * •

The temperature of the FEE is monitored at the control and supervisory level
and interlocked with the PCU to switch off the devices in case of overheating
or loss of communication to the SCADA system.

  * •

In case of a single LV channel trip, the corresponding FEE channels are
consistently switched off.

  * •

Unstable LHC beam conditions, e.g. during injection or adjustment of the beam
optics, pose a potential danger to gas-filled detectors. Therefore the HV
settings are adapted to the LHC status (see Section 7.2). At injection, the
anode voltages are decreased automatically to an intermediate level to reduce
the chamber gain. Restoring the nominal gain is inhibited until the LHC
operators declare stable beams via a data interchange protocol.

###  6.3 High voltage

The HV system comprises 36 HV modules in 5 crates. The 1044 HV channels, 1 of
each polarity providing anode and drift voltage to each chamber, are
controlled via a 250 kbit/s CAN bus through a dedicated Linux-based DIM server
[82]. The published DIM services, commands and remote procedure calls (RPC)
resemble the logical structure of items used in commercial process control
servers: the command to change a setting is confirmed by the server via a read
back setting. In addition, the actual measured value from the device is
published. Update rates for different services can be adjusted independently.

The HV gain and drift velocity are equilibrated for each chamber individually
to compensate for small differences in the chamber geometry. Changes of
environmental conditions (atmospheric pressure and temperature) as well as
small variations of the gas composition cause changes in gas gain and drift
velocity. To ensure stable conditions for the level-1 trigger (see Section
12), these dynamic variations are compensated by automatic adjustments of the
anode and drift voltages which are performed in between runs. These and other
automatic actions on the HV are described in Section 7.2.

###  6.4 Detector operation

The DCS employs a dedicated Graphical User Interface (GUI) and a Finite State
Machine (FSM). The FSM allows experts and operators intuitive monitoring and
operation of the detector. The FSM hierarchy reflects the structure of
subsystems and devices shown in Fig. 18. Detector conditions are mapped to FSM
states, and these are propagated from the device level upwards to the FSM top
node. Standard operational procedures (configuration of read-out and trigger
electronics, ramping voltages etc.) are carried out via FSM commands which
propagate down to the devices and cause a transition to a different state.

The GUI for detailed monitoring and expert operation comprises a dedicated
panel for each node in the FSM tree. An example is shown in Fig. 19. Detector
subsystem ‘ownership’, i.e. the right to execute FSM commands and change the
detector state, is only granted to a single operator at a time, and is
represented by symbolic ‘locks’. Operators can work on-site or access the DCS
system remotely through appropriate gateways.

Figure 19: Graphical User Interface: example panel (FSM top node) representing
the status in the year 2010 (i.e. with 10 installed TRD sectors). The FSM
state of the main systems of each sector is represented by the corresponding
colour; run status and alarm summary are displayed. The panel gives quick
access to emergency actions and detailed monitoring panels via single mouse
clicks.

The monitoring data acquired by the DCS system are stored in dedicated
databases. Dedicated trending GUIs allow the experts to visualise the time
dependence of the detector conditions. During data taking, the monitoring data
needed for detector calibration is queried and made available for offline
analysis (see Section 7.3).

##  7 Operation

In this section, first the commissioning steps for the detector and the
required infrastructure and then the operation and performance for different
collision systems are described.

###  7.1 Commissioning

The service connections in the cavern were prepared and tested in parallel to
the construction of the supermodules. The low-voltage connections were tested
with dummy loads and the leak tightness of the cooling loops was verified. The
Ethernet connections were checked using both cable testers and stand-alone DCS
boards. The optical fibres for the read-out were controlled for connectivity
and mapping. These tests were crucial in order to identify connection problems
prior to the detector installation when all connections were still well
accessible.

The supermodules were installed in different installation blocks as described
in Section 2. Prior to the installation the supermodules were tested at the
surface site. They were rotated along the $z_{\rm lab}$-axis to the
orientation corresponding to their foreseen installation position (e.g.
relevant for cooling). A test setup provided all relevant services (low/high
voltage, cooling, Ethernet, read-out, …) to allow a full system test of each
supermodule. The testing procedure included basic functionality tests, such as
water and gas tightness, front-end electronic stress tests, read-out tests as
well as checks of the noise level [66].

After successful surface testing, the supermodules were installed into the
space frame in the cavern (see Section 2.3). Subsequently, the services were
connected and the basic tests described above repeated to verify operation in
the final setup. At this stage, also the full read-out of the detector with
the experiment-wide trigger and data acquisition systems was commissioned. To
check the data integrity of the read-out chain, test pattern data, generated
either in the FEE or in the GTU, were used. Errors observed during those
tests, e.g. bitflips on individual connections on a read-out board, were cured
by switching to spare lines or by masking channels from the read-out if a
correction was not possible. After establishing the read-out, pedestal runs
(without zero suppression) were recorded to determine the baseline and noise
of each channel. If needed, further data were recorded to perform a Fourier
analysis in order to identify and fix noise sources, e.g. caused by missing
ground connections. In addition, these runs were used to identify inactive
channels which cannot be read out.

After each installation block of new supermodules, a dedicated calibration run
was performed before the actual data taking. The detector is read out with
radioactive 83mKr distributed through the gas system (see Section 10.2). Since
this was usually the first high-rate data taking after the end-of-year
shutdown (and installation), these runs and the preparations for them were an
important step to get ready for the real data taking.

Before each physics production run, periods of cosmic-ray data taking were
scheduled to study the performance of the detector system, to align individual
detector components (see Section 9.1) and to provide reference spectra for
particle identification (see Section 11). Data were obtained with and without
magnetic field. A two-level trigger condition was used to ensure sufficient
statistics in the detector acceptance, even when only the first supermodules
were installed in the horizontal plane (see Section 12.3).

###  7.2 High voltage operation

To avoid HV trips during the critical phases of beam injection (e.g. a
possible kicker failure), the anode voltages are reduced to values with very
low gain. After the injection is completed, the anode voltages are ramped up
from ${\sim}$$1030\text{\,}\mathrm{V}$ (gain of about a factor ${\sim}100$
lower than nominal) to an intermediate voltage of
${\sim}$$1230\text{\,}\mathrm{V}$ (gain ${\sim}6.5\%$ of nominal). The ramp
speed is $6\text{\,}\mathrm{V}\mathrm{/}\mathrm{s}$. After the declaration of
stable beams, the anode voltages are ramped to the nominal voltages
($U_{\mathrm{anode}}\simeq$$1520\text{\,}\mathrm{V}$) for data taking. The
drift voltages always remain at nominal settings.

To equalise the gain and drift velocity of all chambers, the results from the
calibration (see Section 10) are used. The nominal voltages and r.m.s.
variations for drift and anode voltages are 2150$\pm$$22\text{\,}\mathrm{V}$
and 1520$\pm$$14\text{\,}\mathrm{V}$, respectively.

Based on measurements in pp, p–Pb and Pb–Pb collisions in Run 1, it has been
estimated that the chambers had a time averaged current of about 200 nA. This
led to a total accumulated charge of less than $0.2\text{\,}\mathrm{mC}$ per
$\mathrm{cm}$ of wire for Run 1. As the chambers were validated for charges
above $10\text{\,}\mathrm{m}\mathrm{C}\mathrm{/}\mathrm{c}\mathrm{m}$, it is
expected that no ageing effect occurs during the time the TRD is going to be
operated. Up to now, in fact no deterioration in the performance of tracking,
track matching and energy resolution was observed.

The average anode current as a function of the interaction rate as measured by
the T0 detectors used for the ALICE luminosity measurement has a linear
dependence with a slope of
$1\text{/}200\text{\,}\mathrm{n}\mathrm{A}\mathrm{/}\mathrm{H}\mathrm{z}$ for
p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. The slope parameter
was obtained from different LHC fills ranging from minimum-bias data taking up
to high rate interaction running, where the LHC background conditions can be
different. Under the vacuum conditions in Run 1, about 1/3 of the current was
due to the background rate, which is nearly negligible in Run 2.

The expected dependence of the measured current on detector occupancy was
found. The probability for pile-up events in, e.g. p–Pb collisions at
$\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV at $200\text{\,}\mathrm{kHz}$ interaction
rate is about 14% when averaged over time, with a maximum of ${\sim}$24% as
calculated from the bunch spacing and the number of bunch crossings in the LHC
filling scheme [2, 83] as well as the integration time of the read-out chamber
(drift length/drift velocity).

For the level-1 trigger it is crucial to reduce the time dependence of the
drift velocity and the gain to a minimum. The former impacts the track
matching, the latter the electron identification. To ensure the required
stability, the anode and drift voltages are adjusted to compensate for
pressure changes (the temperature is sufficiently stable). The parameters for
the correction were obtained by correlating the calibration constants with
pressure (see Section 10). A relative pressure change d$p/p$ results in a
change of gain of d$G/G=-6.76\pm 0.04$ and drift velocity of
d$v_{\mathrm{d}}$/$v_{\mathrm{d}}$ = $-1.41\pm 0.01$ [84]. In addition, the
dependences of the gain and drift velocity on the anode and drift voltage,
respectively, as obtained from test beam measurements [85] were used (from Run
2 onwards the dependence of gain on voltage was taken from the krypton
calibration runs). This results in voltage changes of about
$0.83\text{\,}\mathrm{V}$ and $1.4\text{\,}\mathrm{V}$ for a pressure change
of $1\text{\,}\mathrm{mbar}$. During Run 1 the gain and the drift velocity
could be kept constant within about 2.5% and 1%, respectively. These values
include the precision of the determination of the calibration constants (see
Section 10). The variations can be further reduced by measuring and correcting
for the gas composition using a gas chromatograph installed during LS 1.

During Run 1, 10% of the anode and 5.5% of the drift channels turned out to be
problematic (see Fig. 35). The respective channels had to either be reduced in
anode voltage or switched off. As the detector is segmented into 5 stacks
along the beam direction and 6 layers in radial direction, the loss of a
single chamber in a stack is tolerable and excellent performance is still
achieved for tracking and particle identification (see Sections 8 and 11).
Most of the problematic chambers showed strange current behaviours (trending
vs time). The de-installation of a supermodule and disassembly of the
individual read-out chambers followed by detailed tests revealed that the
inspected problematic anode and drift channels had broken filter capacitors
($4.7\text{\,}\mathrm{nF}$/$3\text{\,}\mathrm{kV}$). Thus, the
$4.7\text{\,}\mathrm{nF}$ capacitors (see Section 4.3) were removed from the
resistor chain in the last supermodules built and installed during the LS 1 (5
supermodules).

###  7.3 In-beam performance

After commissioning with cosmic-ray tracks and krypton calibration runs in
2009, the detector went into operation and worked reliably during the first
collisions at the LHC on December 6th 2009\. Since then, the detector has
participated in data taking for all collision systems and energies provided by
the LHC [2]:

  * •

pp collisions from $\sqrt{s}$ = 0.9 to 13 TeV at low interaction rates
(minimum-bias data taking) and high intensities (minimum-bias data taking and
rare triggering) with a maximum interaction rate of
200–$500\text{\,}\mathrm{kHz}$. During the rare trigger periods, the detector
contributed level-1 triggers on high-$p_{\mathrm{T}}$ electrons and jets (see
Section 12).

  * •

p–Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV and 8 TeV with
interaction rates at the level of $10\text{\,}\mathrm{kHz}$ (minimum-bias data
taking) and at maximum $200\text{\,}\mathrm{kHz}$ (rare triggering). The
detector contributed the same triggers as in the pp running scenario.

  * •

Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV and 5.02 TeV with
maximum interaction rates of up to $8\text{\,}\mathrm{kHz}$ (minimum-bias and
rare triggering).

At the beginning of a fill, once all detectors within ALICE are ready for data
taking, a global physics run is started. A run is defined in ALICE as an
uninterrupted period of data taking, during which the conditions (trigger
setup, participating detectors, etc.) do not change. A run can last from a few
minutes to several hours until either the experimental setup or conditions
have to be changed or the beam is dumped. An additional end-of-run (EOR)
reason is given by the occurrence of a problem related to a given detector or
system. The detector parameters measured during a run, such as the voltages
and currents of the anode and drift channels as well as temperatures of the
FEE, are dumped at the EOR to the Offline Conditions Database (OCDB) via the
Shuttle framework [86, 87]. The relevant parameters can then be used in the
offline reconstruction and analysis.

In order to ensure sufficiently stable conditions during a run, any change,
such as the failure of a part of the detector, e.g. due to a LV/HV trip,
triggers the ending of the run. In order to avoid too frequent interruptions,
the failure of a single chamber within a stack is ignored. Technically, this
is realised using the so-called Majority Unit within DCS.

All subcomponents of the TRD detector (infrastructure and gas system) are
monitored via DCS (see Section 6). In case any entity deviates from nominal
running conditions by pre-defined thresholds a warning is issued. The single
entity is either recovered by the DCS operator in the ALICE Run Control Centre
or by an expert intervention. During Run 1 data taking, most interventions
were related to the recovery of single event upsets (SEU) and HV trips of
problematic channels by re-configuration of the FEE or ramping up of the
anode/drift channels. For Run 2 an automatic recovery of the FEE and HV was
put in place.

####  7.3.1 Read-out performance

The event size depends on the charged-particle multiplicity. It is therefore
influenced by the collision system and the background conditions of the LHC.
The event size vs. charged-particle multiplicity is shown for various
collision systems for one supermodule in Fig. 20. For the most central Pb–Pb
collisions an event size of 800 kB per supermodule is found.

The dead time per event is composed of the front-end processing and
transmission time to the GTU and a potential contribution from the shipping to
DAQ. On average the former scales approximately linearly with the event size
and rate, the latter is suppressed by the MEB as long as the read-out data
rate stays sufficiently below the effective link bandwidth. The typical event
sizes of $7\text{\,}\mathrm{kB}$, $14\text{\,}\mathrm{kB}$,
$200\text{\,}\mathrm{kB}$ in minimum-bias data taking for pp, p–Pb, and Pb–Pb
collisions result in front-end contributions of
$20\text{\,}\mathrm{\SIUnitSymbolMicro s}$,
$25\text{\,}\mathrm{\SIUnitSymbolMicro s}$,
$50\text{\,}\mathrm{\SIUnitSymbolMicro s}$, respectively. This does not
include the read-out induced part. However, as illustrated by the Pb–Pb case
shown in Fig. 16, the detector is typically operated in the linear range of
the curve, indicating that input rate fluctuations are absorbed by the MEB and
that the read-out does not contribute significantly to the dead time.

The read-out rate during Run 1 and until now in Run 2 ranged from about
$100\text{\,}\mathrm{Hz}$ in rare trigger periods to about
$850\text{\,}\mathrm{Hz}$ in minimum-bias data taking in pp and p–Pb
collisions. In Pb–Pb collisions, the read-out rate was about
$100\text{\,}\mathrm{Hz}$ and $350\text{\,}\mathrm{Hz}$ for minimum-bias data
taking in Pb–Pb collisions in Run 1 and up to now in Run 2, respectively.

Figure 20: Event size vs charged-particle multiplicity for various collision
systems for one supermodule. To obtain the charged-particle multiplicity,
global tracks (see Section 8) fulfilling minimum tracking quality criteria
were counted on an event-by-event basis.

####  7.3.2 Radiation effects

The radiation on the TRD was for Run 1 and Run 2 (until the end of 2016)
rather low both in terms of flux and dose. The following radiation
calculations for the inner radius of the TRD are based on simulations obtained
using the FLUKA transport code [88] and taking into account the measured
multiplicities of Pb–Pb, p–Pb and pp collisions [89, 90, 91, 92, 93, 94] as
well as the running scenarios (luminosities, running time, and interaction
rate). For the indicated time range the Total Ionisation Dose (TID) and the
Non-Ionising Energy Loss (NIEL), quoted in 1-MeV-neq fluence, were $7\cdot
10^{-3}$ krad and $2\cdot 10^{9}\rm{cm^{-2}}$, respectively. The flux of
hadrons is highest in Pb–Pb collisions, because it is proportional to the
product of the interaction rate and the particle multiplicity. For Pb–Pb
collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, the flux of hadrons with
$>20$ keV energy and charged particles is about $3.8\cdot
10^{-2}\rm{kHz/cm^{2}}$ and $2.5\cdot 10^{-2}\rm{kHz/cm^{2}}$, respectively.
The radiation load in terms of flux and dose are far below the values, for
which the experiment was designed for [1].

In the radiation environment described above, very few SEUs are observed in
the electronics. The most affected device is the DCS board, for which SEUs
result in occasional reboots (a few DCS boards per LHC fill). The DCS board is
needed for control and monitoring but is not part of the read-out chain
meaning that the reboots do not affect the data taking. The external RAM on
the DCS board can be monitored for SEUs by writing and verifying known
patterns in unused areas of the $\sim$13 MB memory per chamber. During 2.5
months of pp data taking at LHC luminosities of about $5\cdot
10^{30}\rm{cm^{-2}s^{-1}}$, 20 SEUs as shown in Fig. 21 were observed in the
external RAM, i.e. a negligible amount compared to the occasional reboots of a
few DCS boards.

Figure 21: Monitored external DCS memory and occurrences of SEUs as a function
of time. The periods of stable beam are indicated as well.

The memories of the TRAPs are Hamming-protected and, thus, resilient to SEUs.
However, the configuration registers are not protected and can be affected by
radiation. Therefore, the configuration is compressed and written to a
Hamming-protected memory area. In this way, the registers can be checked (and
corrected) against the compressed configuration.

####  7.3.3 Data quality assurance

The Data Quality Monitoring framework (DQM) provides online feedback on the
data and allows problems to be quickly spotted and identified during data
taking. The Automatic MOnitoRing Environment (AMORE) was developed for ALICE
[95] and allows run-based, detector-specific analyses on the raw data. The
results are visualised in a dedicated user interface. The monitored
observables, such as noise level, event size per supermodule, trigger timing,
FEE not sending data, are compared with reference values or diagrams
(depending on the data taking scenario). Deviations from the references
indicate a problem to the operator. Based on the information obtained from the
online DQM all runs are directly marked with a quality flag, both globally and
for the individual ALICE subdetectors. For the offline physics analyses, lists
of runs are selected based on these flags according to the physics case under
study.

####  7.3.4 Pretrigger performance

A dedicated wake-up signal is required for the FEE (see Section 5.1). It
should reflect the level-0 trigger condition as closely as possible. However,
as it needs to be generated before the actual level-0 trigger, it cannot use
the same information. This introduces some inefficiency into the TRD read-out.
In the early Run 1 LHC filling schemes (e.g. during the LHC ramp-up in 2009)
with only a few colliding bunches per orbit, it was possible to send a wake-up
signal for all of the bunch crossings with potential interactions. This
resulted in a fully efficient operation [71]. During this time, the pretrigger
system was commissioned to use the V0 and T0 signals as inputs. They could
then also be used for filling schemes with many bunches. The trigger condition
was configured as closely as possible to the ALICE level-0 interaction
trigger, i.e. a coincidence of either the V0 or the T0 detectors (simultaneous
signals in the A- and C-side, see Section 2). The efficiency of the V0- and
T0-derived wake-up signals depends on the discrimination thresholds used for
those detectors and on the inherent dead time between pretrigger and the abort
or end of the read-out (see Section 5). The latter is particularly important
when subsequent collisions are close in time, e.g. in LHC filling schemes that
have bunch trains with 25 or 50 ns bunch spacing [96]. For runs taken at low
interaction rates the pretrigger efficiency is above 97%; for higher rates the
efficiency depends on the colliding bunch structure of the filling scheme and
reaches average values down to about 83% in Run 1 [71]. These inefficiencies
were avoided with the LM system used in Run 2 (see Section 5.1).

The analysis of electrons from heavy-flavour hadron decays in p–Pb collisions
at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV in events satisfying the pretrigger
condition showed no bias compared to results from events triggered with the
ALICE level-0 minimum-bias interaction trigger [97].

##  8 Tracking

The charged particle tracking in the ALICE central barrel is based on a Kalman
filtering [98]. Track finding and fitting are performed simultaneously [2].
The algorithm operates on clusters of track hits from the individual
detectors. The clusters carry position information and, depending on the
detector, the amount of charge from the ionisation signal. The cluster
parameters are calculated locally from the raw data, implying that the cluster
finding can be parallelised.

The global tracking starts from seed clusters at the outer radius of the TPC
(see Fig. 1). During the first inward propagation of the tracks previously
unassigned TPC clusters are attached while updating the track parameterisation
at the same time. If possible, the track is further propagated to the ITS.
Subsequently, an outward propagation adds information from TRD, TOF, and
HMPID. A second inward propagation is used to obtain the final track
parameters, which are stored at a few important detector positions, most
importantly at the primary vertex.

The TRD contributes to the tracking in various ways. First, it adds roughly
$70\text{\,}\mathrm{cm}$ to the lever arm, which improves significantly the
momentum resolution for high-$p_{\mathrm{T}}$ tracks. Second, it increases the
precision and efficiency of assigning clusters from the detectors at larger
radii, in particular the TOF, to propagated tracks. In addition, the TRD is
used as reference to obtain correction maps for distortions in the TPC, which
arise from the build up of space charge at high interaction rates. For this
the TRD and ITS track segments are reconstructed using as seeds the TPC tracks
(with relaxed tolerances accounting for potential distortions). Then, the
estimate of the real track position is built as a weighted average of the ITS
and TRD refitted tracks (without TPC information). The TPC distortions are
deconvoluted from the residuals between these interpolations and the measured
TPC cluster positions.

The tracking in the TRD can be subdivided into the formation of tracklets
(track segments within one read-out chamber) from clusters and the updating of
the global tracks based on the tracklets. These steps are performed layer-by-
layer. The chambers within a layer can be treated in parallel. For each layer,
a seed track is prepared by propagation from the TPC and used to calculate the
intersection with a chamber. Based on this information a tracklet is formed
from the clusters in the vicinity of this intersection and then the track
parameterisation is updated accordingly. In the following, details of the
individual steps will be given.

###  8.1 Clusterisation

Primary ionisation in the detector gas leads to a signal that spreads over
several pads. Because of the slower ion drift, the charge carries over into
subsequent time bins, resulting in a correlation between time bins (see
Section 2.1). The cluster algorithm combines the data from adjacent pads in
the same time bin, producing clusters with information on position and total
charge. The former is calculated from the weighted mean of the charge shared
between adjacent pads (up to 3). Look-Up Tables (LUT) are used to relate the
measured charge distribution to the actual position. These LUTs are the result
of calculations for the different pad width sizes, based on measurements in a
test beam [46]. The cluster position can deviate from the LUT values because
of detector parameters which are subject to calibration (see Section 10), most
importantly the drift velocity $v_{\mathrm{d}}$ and the time offset $t_{0}$
(time corresponding to the position of the anode wires, see Fig. 30). In
addition, a correction for the $E\times B$ effect is applied. The complete
position characterisation also includes the estimated uncertainty, which
determines the weight for updating the global track. The uncertainties are
derived from differential analyses of Monte Carlo simulations. Cluster
properties such as the deposited energy, time bin, and reconstructed position
relative to the pad with the maximum charge are taken into account as well as
particle level characteristics such as electrical charge and incident angle. A
linear model relates all uncertainties with parameters being defined by all
conditions determining a cluster.

Figure 22: Signal produced by a positively charged particle
($p_{\mathrm{T}}=0.5~{}\mathrm{GeV}\\!/c$). Left: Total charge per time bin
used for particle identification. Right: Ionisation signal vs. pad number and
time bin. The cluster positions are shown as reconstructed from the charge
distribution (raw clusters) and after correction for the $E\times B$ effect
(Lorentz-corr. clusters).

###  8.2 Track reconstruction

For the preparation of the TPC-based track seed used to match with the TRD
clusters, the Kalman parameterisation (at the outer radius of the TPC) is
propagated to the radial position of the anode wires of a given chamber. At
this radius the position is least affected by variations in calibration
parameters. If a chamber is rotated with respect to the tracking frame, the
radial position of the anode wires depends on the intersection point of the
track in the $y$-$z$ plane. As this is only known after the propagation, the
preparation of the track seed is an iterative process.

The clusters that are assigned to the seed track in a given layer are combined
into tracklets. A straight line fit is sufficient for their description since
the negligible sagitta of the trajectory is only of the order of tens of
microns.

Since in the read-out chamber the electrons drift in the radial direction,
that is approximately parallel to the track, and due to the long ion tails,
the signals pile up. The measured charges, sampled in time intervals of
$100\text{\,}\mathrm{ns}$, are therefore correlated between different time
samples. Since such correlations degrade the angular resolution, a tail
cancellation correction is applied [46]. It subtracts an exponential tail
proportional to the current signal from the subsequent samples for each read-
out pad.

The number of pads on the the read-out plane onto which a track is projected
depends on the track incident angle. For decreasing transverse momentum, more
pads will carry a signal. The Lorentz angle also affects this spread. For
negatively (positively) charged particles the Lorentz drift is along (opposite
to) the track inclination, independent of the polarity of the magnetic field.
On average, negatively charged particles are thus spread over fewer pads than
positively charged ones. In the right panel of Fig. 22, an example of a
positively charged particle of $p_{\mathrm{T}}=0.5~{}\mathrm{GeV}\\!/c$ (worst
case) is shown. Its projection spans over 6 pads.

The procedure to find candidates for seeds involves a preliminary stage in
which clusters are searched in the neighbourhood of the propagated seed. In
Fig. 23 the mean and width of the residuals are shown for the arising
tracklets in $\Delta y$ in layer 0 as a function of the seed $p_{\mathrm{T}}$.
The imperfect tail cancellation results in different position biases for
tracklets from positive and negative tracklets, the signal spreading over more
pads for the former.

Figure 23: Residuals in $\Delta y$ of tracklets with respect to global tracks
as a function of $p_{\mathrm{T}}$ in pp collisions at
$\sqrt{s}=13~{}\mathrm{TeV}$. For every bin the mean (marker) and r.m.s. width
(error bar) of the distribution are shown.

###  8.3 Performance

The relative frequencies of the number of tracklets assigned to a track are
shown in Fig. 24 for pp collisions at $\sqrt{s}=13~{}\mathrm{TeV}$. Tracks
consisting of 6 layers account for more than 50% (60%) for
$p_{\mathrm{T}}<$1\text{\,}\mathrm{G}\mathrm{eV}\mathrm{/}\mathrm{c}$$
($p_{\mathrm{T}}>$1\text{\,}\mathrm{G}\mathrm{eV}\mathrm{/}\mathrm{c}$$).
Tracks with 4 and 5 layers are mainly produced by particles crossing dead
areas of the detector.

Figure 24: Fraction of tracks, originating from the primary vertex, consisting
of a given number of layers in pp collisions at $\sqrt{s}=13~{}\mathrm{TeV}$.

A crucial figure of merit for the tracking is the fraction of global tracks
matched to the TRD. This includes acceptance effects, between the TPC and the
TRD as well as the TRD and the TOF detector. The momentum dependence is shown
in Fig. 25 for tracks with at least 4 layers (about 75% of all tracks). For
positively charged particles, the Lorentz drift of the electrons is opposite
to the track inclination, which (together with the tail cancellation) results
in a slightly higher efficiency.

Figure 25: Fraction of tracks matched between the TPC and the TRD (TPC-TRD)
and further the TOF detector (TRD-TOF) as a function of transverse momentum in
pp collisions at $\sqrt{s}=13~{}\mathrm{TeV}$.

A systematic analysis of the position resolution in the bending plane
($r\varphi$) is presented in Fig. 26. The resolution ($\sigma_{\Delta y}$) is
expressed as the width of a Gaussian fit to the difference between the
position reconstructed via tracklets and different references ($\Delta y$). It
is shown as a function of the inverse transverse momentum scaled with the
particle charge ($q/p_{\mathrm{T}}$). First, the ideal position resolution is
derived from Monte Carlo simulations by comparing the reconstructed tracklet
position with the true particle position at the reference radial point (anode
wire plane of the read-out chamber). This is shown as the red curve in Fig.
26, calculated in local chamber coordinates to decouple residual misalignment
effects from the result. A parabolic best fit is performed for which the
parameters show the best position resolution of close to
$200\text{\,}\mathrm{\SIUnitSymbolMicro m}$ at $p_{\mathrm{T}}$ = 1.8
$\mathrm{GeV}\\!/c$. The best performance is achieved for tracks where the
inclination angle cancels the $E\times B$ effect. In the case of real data,
the comparison can be performed only against a measured estimator, i.e.
against the reconstructed global (ITS + TPC) track. The black curve shows the
distribution for pp collisions at $\sqrt{s}=8~{}\mathrm{TeV}$. The combined
position resolution of the TRD and global tracks is around
$700\text{\,}\mathrm{\SIUnitSymbolMicro m}$ at very large transverse momentum.
In order to bridge the two results, observables at the level of reconstruction
and simulation are compared. The blue curve shows the position resolution of
the global tracks as reconstructed against the true position from the Monte
Carlo simulation. The green line represents the theoretical value for the
combined resolution for TRD and global tracks, given by the quadratic sum of
the dependencies described by the red and the blue distributions. These tracks
from simulation yield a slightly worse resolution because the theoretical
limit does not consider the pad tilting. It is worth noting that the simulated
position resolution describes the measured dependency reasonably well. Effects
of remaining miscalibration and misalignment of all central barrel detectors
lead to a degradation of about $500\text{\,}\mathrm{\SIUnitSymbolMicro m}$ for
the resolution in the TRD.

Figure 26: Dependence of the position resolution on charge over transverse
momentum for simulated tracks in the TRD (red) and in the TPC (blue),
reconstructed global tracks from simulation (gray) and from pp collisions at
$\sqrt{s}=13~{}\mathrm{TeV}$ (black). The label TRD-TPC indicates global
tracks reconstructed with the ITS and TPC that were extrapolated to the TRD.
The green line represents the theoretical value for the combined resolution of
TRD and global tracks. The red line shows a parabolic fit to the corresponding
points.

The good position resolution capabilities demonstrated by the TRD detector can
be used in the central barrel tracking of ALICE to improve the transverse
momentum resolution of reconstructed particles. Figure 27 shows the
$q/$$p_{\mathrm{T}}$ resolution of the combined ITS-TPC tracking with and
without the TRD for various running scenarios. In all considered cases the TRD
was also used as reference to obtain the correction maps for the distortions
in the TPC. The inclusion of the TRD in tracking in addition improves the
resolution by about 40% at high transverse momentum for pp collisions recorded
at both low (12 kHz) and high interaction (230 kHz) rates. For example in the
low interaction scenario of pp collisions, the achieved $p_{\mathrm{T}}$
resolution is 3% at 40 GeV. In addition the inclusion of the TRD in the track
reconstruction improves the impact parameter resolution and the reconstruction
of tracks that pass at the edges of the TPC sectors, i.e. increasing the
acceptance of the experiment.

Figure 27: Improvement of the $q/$$p_{\mathrm{T}}$ resolution in data when TRD
information is included as compared with the performance of tracking without
TRD information for various running scenarios. The labels low and high IR
indicate interaction rates of 12 and 230 kHz, respectively.

##  9 Alignment

The physical alignment of the detectors during installation (see Section 2.3)
has a finite precision of the order of $1\text{\,}\mathrm{mm}$ for chambers
within a supermodule and of $1\text{\,}\mathrm{cm}$ for supermodules in the
spaceframe. The subsequent software alignment, i.e. accounting for the actual
positions of supermodules and chambers in the reconstruction and simulation
software, is the subject of this section. The alignment parameters (three
shifts and three rotation parameters per alignable volume) are deduced from
optical survey data and/or from reconstructed tracks. In the latter case, the
obtained values have to be added to those already used during the
reconstruction. The obtained alignment sets are stored in the OCDB and used in
the subsequent reconstructions.

The different alignment steps are described in the following subsections. The
alignment is checked and, if necessary, redone after shutdown periods and/or
interventions that may affect the detector positions, e.g. installations of
new supermodules.

###  9.1 Internal alignment of chambers with cosmic-ray tracks

The internal detector alignment, i.e. the relative alignment of the read-out
chambers within one stack, is performed with cosmic-ray tracks recorded
without magnetic field (Fig. 28, left).

Figure 28: Left: Cosmic-ray tracks with at least 100 TPC clusters and 5 TRD
layers, recorded without magnetic field, used for the relative $r\varphi$
alignment of the TRD chambers within stacks (internal alignment). Right:
Charged-particle tracks with at least 100 TPC clusters and 4 TRD layers from