# Discharge mitigation methods in MPGD-based detectors

P. Gasik<EMAIL_ADDRESS>GSI Helmholtzzentrum für Schwerionenforschung GmbH
(GSI), Darmstadt, Germany  Facility for Antiproton and Ion Research in Europe
GmbH (FAIR), Darmstadt, Germany

###### Abstract

This work reviews various methods used to minimize the probability of
discharge occurrence in MPGDs or to mitigate the effect of spark discharges on
the detector system. These include techniques that can reduce the probability
that the fundamental discharge limits of MPGD structures are reached during
detector operation, methods based on the HV scheme optimisation, or the
implementation of resistive electrodes to enhance local discharge quenching
capabilities in an amplification structure. In addition, design optimization
and quality control methods to ensure stable, long-term operation of an MPGD-
based detector, are considered.

###### keywords:

MPGD, GEM, THGEM, discharge, streamer

††journal: NIM A

##  1 Introduction

The requirements of a new generation of experiments are driving factors for
developing new detectors. Novel devices must handle steadily increasing
particle rates, providing high sensitivity in a wide dynamic range for
detecting minimum ionizing particles and highly ionizing nuclear fragments.
Outstanding particle identification capabilities of time projection chambers
(TPC), or large-area high-rate trackers are still the domain of the gaseous
detector technology providing cost-effective solutions characterized by great
performance figures. Among many new innovative technologies, Micro-Pattern
Gaseous Detectors (MPGD) have become widely used in nuclear physics
experiments and are foreseen at future facilities.

Despite many years of experience in the production and successful operation of
MPGDs, the high demands of modern experiments, which, in particular, concern
the substantial increase in active detector area, require further developments
and improvements. Constantly increasing luminosity demands operation in a
harsh, high-rate environment posing stringent requirements on the performance
of gaseous detectors including, depending on the use case, high gain and high
efficiency, spatial, time and energy resolutions, or ion backflow
capabilities. The key parameters for long-term operation are radiation
hardness, aging resistance, and stability against electrical discharges. The
latter in particular poses a threat to the integrity of the detector and its
readout electronics, as they may cause irreversible damage to the
amplification structure, ranging from enhanced leakage currents to permanent
electric short circuits that render the detector effectively blind and lead to
a loss in its acceptance. It is, therefore, of the highest importance to
further study and optimize this particular aspect of detector operation. By
quantitatively understanding underlying mechanisms, the probability of the
development of discharge events can be substantially reduced allowing for a
stable operation under extreme conditions. In the following, we present an
overview of well-established mitigation strategies for the safe operation of
MPGD-based detectors.

##  2 Discharges in Micro-Pattern Gaseous Detectors

The main mechanism of discharge formation in MPGD structures (see recent
review in [1]) is the streamer mechanism [2, 3, 4, 5], and the development of
a spark discharge. Indeed, it is observed that a streamer develops in a micro-
pattern detector when the total number of charges in an avalanche reaches a
critical charge limit $Q_{\mathrm{crit}}$ of
${10}^{6}{10}^{8}\text{\,}\mathrm{e}$ [6, 7, 8, 9]. However, it should be
noted, that in none of these references (and many others, not mentioned here)
a universal $Q_{\mathrm{crit}}$ value can be specified for a given MPGD
structure. Reported critical charge values and discharge probabilities depend
not only on the geometry of the structure, or the size of its amplification
gap, but also on the type of radiation source (number of primary electrons)
[6] and its inclination [10], or the exact composition of the gas mixture [7,
11]. In the latter, a clear correlation between the discharge probability and
the average atomic number $\langle Z\rangle$ of the gas mixture is observed
pointing to low-$Z$ gases being more stable concerning spark discharge
development. It is therefore clear, that the local charge densities
approaching single amplification cells (e. g. a GEM hole) play a key role in
the discharge development and the long-term stability of an MPGD structure.

The latter has been recently studied measuring discharge stability of GEMs,
THGEMs, and Micromegas in Ne- and Ar-based gas mixtures [12, 13, 14]. In all
cases, a clear gas dependence is observed pointing to low-$Z$ mixtures as more
stable ones. Surprisingly, it is also observed that increasing the content of
a quencher gas does not always improve the stability which, again, can be
explained by the average atomic number of the mixture and the local charge
densities obtained therein [13]. A simulation framework was developed that
allows estimating critical charge limits from the comparison to the measured
data. The values of the $Q_{\mathrm{crit}}$ parameter have been extracted for
all the gas mixtures used in the measurements with GEMs and THGEMs and are
summarised in Table LABEL:tab:Qcrit.

Table 1: Critical charge values extracted from the simulation fits to the experimental data for GEMs [12] and THGEMs [13]. Gas |  $Q_{\mathrm{crit}}$ THGEM |  $Q_{\mathrm{crit}}$ GEM
---|---|---
$[\times 10^{6}\,e]$ | $[\times 10^{6}\,e]$
Ne-CO2 (90-10) | 7.1 $\pm$ 2.2 | 7.3 $\pm$ 0.9
Ar-CO2 (90-10) | 4.3 $\pm$ 1.5 | 4.7 $\pm$ 0.6
Ar-CO2 (70-30) | 2.5 $\pm$ 0.9 | –

The results confirm the observation of the $Q_{\mathrm{crit}}$ gas dependency.
Moreover, the values obtained for both amplification structures nicely agree
with each other, despite the clear geometrical differences and different
electric field configurations inside GEM and THGEM holes. The primary charge
limits shall be therefore considered per single hole and not normalized to the
hole volume. This would support the hypothesis that the effective volume of
streamer formation is similar in both cases [13].

Both GEM and THGEM measurements support the conclusion that the primary charge
density, arriving at the single amplification cell, is a key factor
influencing the stability of a GEM-like structure against a spark discharge.
In the recently launched Micromegas studies, we show that similar conclusions
can be drawn in the case of mesh-based structures, and the mesh cells can be
considered as independent amplification units [14].

##  3 Discharge mitigation methods

There is no single way to minimize the probability of discharge occurrence or
to mitigate the effect of spark discharges on the detector system. Depending
on the type of detector, its purpose, design details, and experimental
conditions, there may be different ways to avoid or mitigate electrical
discharges. There are, however, several groups of methods one can define to
describe different approaches. While the fundamental, physics-driven stability
limits of an MPGD cannot be easily changed, the system optimization and
quality control are continuously being improved and with more than 25 years of
experience become more and more effective. Methods presented in the following
are based on well-established principles and have been further worked out in
the course of the recent large-scale detector developments [15, 16, 17].

###  3.1 Choice of gas and operational conditions

A gas mixture is a basic ingredient of any gaseous detector. Its final choice
is based on thorough consideration of many requirements, drift and diffusion
coefficients, amplification, attachment, or aging properties being only a few
of them. Together with the optimized HV settings, they define the detector
performance such as gain, position and energy resolutions, ion backflow, etc.
The necessity of stable operation in a high-radiation environment requires
often contradictory sets of parameters to be considered, which may compromise
the detector’s performance. On the other hand, long-term operation in a
hermetic detector setup is a must for high-efficiency running.

In general, gas optimization is strongly related to the intrinsic stability
limits of the amplification structure and the gas itself. One can summarise
their optimization concerning the spark discharge occurrence as follows.

  * 1.

Choice of light gases: since a spark discharge is the most common in MPGDs,
light noble gases are preferable for operation.

  * 2.

Care should be taken while choosing the quencher type and content to optimize
primary charge density and electron transport properties. Note, that poor
quenching capabilities of CO2 imply photon and ion feedback issues, especially
at high gains, which can be minimised by employing heavier quenchers (e. g.
iso-butane). In this case, however, primary electron densities may need to be
again considered.

  * 3.

Gain reduction: a trivial, however the most efficient, method to minimize the
discharge probability, is to lower gain as much as allowed by the signal-to-
noise ratio requirements.

  * 4.

Electric field optimization above/below the MPGD structure: if allowed by the
measurement requirements, the minimum diffusion coefficient and maximum drift
velocity regions shall be avoided. Electron collection and extraction
efficiencies shall be also considered as they directly influence the total
number of charges entering a single amplification cell.

  * 5.

Absolute voltage values: taking into account possible defects, residual
contamination of MPGD structures, high absolute voltages, and high fields
shall be avoided. In gases like neon (preferable from the spark development
point of view) a glow discharge may develop at relatively low voltage values
causing a constant current flow and a trip of the power supply, or damage to
the detector. For this reason, high-quality production is mandatory.

###  3.2 MPGD stacks

The most common way to enhance the gain capabilities of an MPGD-based detector
is to build stacks. Among others, GEM-like structures are obvious candidates
for this mechanical solution, due to their construction principles, including
GEMs stretched on individual frames or THGEMs made of a thick, self-supporting
PCB material. Structures that do not allow for easy stacking, such as
Micromegas111Although double-mesh structures can be built [18, 19] their
dimensions are still not sufficient for large area systems., or cannot be
stacked being an integral part of the readout electrode (e. g. MSGC [20] or
WELL-like structures [21, 22, 23, 24]), can be used in hybrid constructions
where they play a role of the main amplification stage with a pre-
amplification by GEM or a THGEM mounted above.

The aim of stacking several structures is to share the charge cloud between
many individual amplification cells (e. g. GEM holes) utilizing the charge
diffusion and stage the gain process in subsequent detector layers. This way,
individual stages can be operated at lower gains, and the probability of
reaching critical charge limits in a single amplification unit can be
substantially reduced. This allows obtaining much higher effective gains,
although the fundamental $Q_{\mathrm{crit}}$ value for a given structure does
not change.

A stack operation requires, however, a thorough optimization of voltages,
taking into account the non-trivial interplay between amplification fields,
transfer fields between two MPGD layers, a drift field above the stack, and an
induction field between the last amplification stage and a readout electrode.
They all influence the electron and ion transport properties as well as
collection and extraction efficiencies which, together with the multiplication
of an individual MPGD, results in an effective gain of the entire structure
(see for example [25] or more recent [26, 27]). In addition, there are
mechanical constraints to be considered while building an MPGD stack. Also in
this case the reduction of discharge probability is a matter of finding a
compromise between the overall performance and the long-term stability of a
detector.

####  3.2.1 Multi-GEM

In multi-GEM systems, the influence of transfer fields and charge sharing
between GEM foils may seriously alter the discharge probability. The maximum
gain reached in a triple GEM configuration supersedes the one of a single GEM
by more than two orders of magnitude, which is a remarkable improvement in the
detector performance [10].

Conventional triple GEM systems have been operated reliably in high-rate
experiments [28, 29, 30] with the so-called standard HV configuration, highly
optimised for the minimum discharge probability. In this configuration, the
gain of subsequent GEMs is decreasing towards the bottom of a stack, and thus
the probability of exceeding the maximum charge limits is significantly
reduced [10]. Further improvement of stability can be achieved by stacking
more than three GEMs. For example, quintuple GEM stacks, considered for RICH
detectors at the future electron-ion collider (eIC), are reported to operate
reliably in the beam at gains exceeding $10^{5}$ [31].

However, the operation of a detector in a so-called low ion backflow (low-IBF)
mode requires the application of HV settings which violate the rules of stable
operation discussed above. This mode is envisaged for cases where the number
of ions drifting back from the amplifications stage towards the drift volume
needs to be maximally reduced. This is the case for Time Projection Chambers,
where the back drifting ions build up space-charge, distorting the drift
field, or photo-detectors, where the quality of a photo-sensitive cathode
deteriorates upon bombardment of positive ions. In the low-IBF mode, the order
of amplification within the stack is reversed and the fields are substantially
altered with respect to the standard configuration (see [15, 32], for
example).

In [33] we show that these modifications result in a noticeable decrease in
detector stability, which underlines the strong dependence of the latter on
the HV settings. Discharge probability measured with the low-IBF configuration
exceeds values obtained for the standard configuration by more than three
orders of magnitude. The main reason for the stability deterioration in the
low-IBF mode is the strong amplification in the last foil which, in
combination with the pre-amplification of charges in the upper GEM foils,
results in higher local charge densities in the last foil compared to those
present in the standard configuration. In addition, the peculiar HV settings
necessary to further minimize IBF require transfer fields above the last GEM
in the amplification stack. This in addition alters the electric field
configuration and the electron transport properties and may focus the charges
into a lower number of individual GEM holes.

The stability of a GEM stack operated in low-IBF mode can be, however,
restored by adding a fourth GEM. In [33] we show that the discharge
probability measured with a quadruple GEM system improves significantly and
the quadruple GEM configurations are more stable than a triple GEM operated in
the low-IBW mode. It also shows that tuning the HV settings to obtain the
lowest possible value of the IBF seriously affects the stability of the
system.

####  3.2.2 Multi-Micromegas

There is not much experience with the multi-Micromegas stacks to date. First
attempts performed in Saclay [18] focused on the ion backflow optimization
showing the great potential of a double mesh configuration operated with
highly optimized HV settings. Similar studies were launched recently at the
University of Science and Technology of China, where a low ion backflow figure
of $<0.1\text{\,}\mathrm{\char 37\relax}$ at the gain of $\sim$2000 has been
achieved with double Micromegas setups making them an attractive candidate for
applications in gaseous photomultiplier tubes, RICH photoelectric readout, or
high-rate TPCs [19, 34]. The results, however, were obtained with small
25$\times$25 mm2 prototypes and the feasibility of building large-area,
uniform structures, is still to be presented.

In addition, the first attempt to quantify the discharge probability of a
double Micromegas detector, based on 500 LPI meshes of $\sim
40\text{\,}\mathrm{\char 37\relax}$ transparency each, is reported in [19].
Preliminary results were obtained with an 8 keV x-ray source For a direct
comparison with the available literature on MPGD stability, dedicated
measurements with alpha particles and hadron beams shall be considered. The
possibility of employing meshes with different optical transparency in double
Micromegas should also be studied. Although it will require a thorough
optimization of the energy resolution and ion-stopping capabilities, the
employment of low-transparency meshes may strongly improve the suitability of
a Micromegas stack for operation in a harsh radiation environment. A scenario
where a high-transparency mesh is used on top of the stack for high collection
efficiency of primary electrons, and a low-transparency mesh is installed at
the bottom for improved charge sharing between Micromegas cells, can be
considered as a starting point for such an optimization.

####  3.2.3 Hybrid stacks

Another category of stacks are hybrids of different MPGD technologies, with a
GEM-like structure playing a role of the pre-amplification stage for the main
amplification structure, usually a Micromegas, mounted below. The goal is to
further suppress the ion backflow fraction and improve the discharge stability
of the Micromegas. The former is reported to reach values down to
$0.1\text{\,}\mathrm{\char 37\relax}$ [35, 36, 37]. Introducing one or more
GEM foils allows the operation of the Micromegas at a lower gain and enforces
the spread of the charge cloud lowering the local charge densities arriving at
the Micromegas stage. Discharge probability of the latter can be therefore
substantially decreased by a factor of $10100\text{\,}$ which has been
confirmed with several measurements and simulations [38, 39, 40]. The hybrid
structures are successfully employed in a number of experiments, for example
in the recent COMPASS RICH-1 upgrade [41, 42], where THGEM + Micromegas
structures are operated at high gains of $\sim$3$\times$104 with moderate and
acceptable spark rates [43].

However, in the case of a very demanding low-IBF operation, the stability of a
hybrid stack is again compromised, as the gain of the Micromegas needs to be
further increased. In the studies reported in [35] a 2GEM + MMG hybrid is
compared to a quadruple GEM structure in the same conditions in a test-beam
campaign at the CERN SPS fixed target facility. Full-size prototypes of both
types were exposed to hadron showers from a thick iron target irradiated with
a high-intensity secondary pion beam. Discharge rates measured with the 2GEM +
MMG detector are three orders of magnitude higher than those recorded with the
quadruple GEM solution. The results show the necessity for further
investigation of the hybrid detectors in terms of their performance and
stability optimization. Following the discussion above, low-transparency
meshes could be combined with standard or small-pitch GEMs, keeping the energy
resolution at, presumably, a satisfactory level while reducing the discharge
probability of the Micromegas. This or similar configurations should be
considered for further optimization in view of their application in future
TPCs, for example at the planned Circular Electron Positron Collider [44].

###  3.3 HV circuit optimisation

Several recommendations for the safe operation of MPGD-based detectors have
been worked out already in the early phase of their application (see for
example [28, 45, 30]). These include protection resistors of
$\mathcal{O}(1-10\,\mathrm{M\Omega})$, connected in series between the MPGD
electrodes and their HV sources. In GEMs, where top electrodes are usually
divided into $\sim$100 cm2 segments, each segment is protected individually.
Although a spark discharge can develop, independently on the protection
resistor, when the critical charge limit is reached, protection resistors
decouple the HV voltage circuitry from the detector and quench the development
of self-sustained discharges. In case of high ionization rates, currents
induced on GEM electrodes will cause a voltage drop across the protection
resistors which in turn will cause the gain drop and subsequently damp the
possible discharge development. This, of course, needs to be balanced with the
high-rate capability requirements of the detector (see also discussion on the
resistive layers in Section 3.4 below). A similar effect can be achieved with
Micromegas detectors where, apart from a moderate decoupling resistor
connected to the non-segmented mesh, resistors of $\mathcal{O}(10)$ M$\Omega$
can be connected to the individual readout strips causing only local voltage
drops and reducing part of the active area being affected in case of a
discharge [45]. It should be noted, however, that Micromegas-based detectors
evolved considerably in recent years and new techniques are being used to
optimize their efficiency at high-ionization rates (see also Section 3.4 and
Section 3.5).

In addition to discharge quenching capabilities, a protection resistor reduces
current flow through the structure in case of an electrical short development.
In case the MPGD electrodes are divided into segments, each biased via its own
protection resistor, a short in one or more segments can be tolerated without
affecting the healthy part of the structure. The choice of resistor values is
usually a compromise between gain fluctuations, caused by the voltage drop
across the protection resistors, and hardware limitations to supply excess
currents in case of a short in one or several MPGD segments. The effect of
current flowing through a short on the overall potential difference across
unaffected segments can be compensated by regulating the voltage source
accordingly (see below).

The proper choice of resistive elements may also effectively quench the
development of the so-called secondary discharges, developing in a gap between
two GEMs or between a GEM and the readout electrode [46], not discussed in
detail in this manuscript. Studies revealed that careful optimization of
protection resistors on both sides of a GEM and reduction of parasitic
capacitance, parallel to the transfer and induction gaps, shall be seriously
considered while designing a multi-layer GEM detector [46, 47]. A list of
clear recommendations for the HV scheme optimization can be found in [47].

###  3.4 Resistive layers

Resistive electrodes are commonly employed in all types of MPGD-based
detectors to improve spatial resolution and reduce the number of readout
channels thanks to the charge-sharing capabilities but also as a method to
provide spark protection to electronics and to reduce discharge probability.
The recent review of the resistive gaseous detector can be found in [48].
Employed in an MPGD as an HV electrode, or a resistive layer capacitively
coupled to the readout electrode (strips, pads), it creates a self-quenching
mechanism for discharge development. It forces field reduction, by delaying
charge evacuation, which results in reducing the discharge current, and thus
discharge in the gas cannot be sustained. Moreover, the field reduction
effects occur locally, in the region of the avalanche/streamer, and do not
affect the entire detector area. The locality of the effect can be enhanced by
segmented resistive electrodes.

With the gas discharge quenching capabilities, on the other hand, gain
reduction comes into play at higher rates. In one of the pioneering studies
with resistive Micromegas [49, 50] it was shown that the signal amplitude
slowly decreases with increasing rate, reaching a drop of $\sim
20\text{\,}\mathrm{\char 37\relax}$ at the rate of
$10\text{\,}\mathrm{kHz}\text{\,}{\mathrm{cm}}^{-2}$. Similar rate
capabilities were reported in the first tests of THGEMs with resistive
electrodes [51].

Further improvements in resistive-electrode technology continue. A diamond-
like carbon (DLC), a class of meta-stable amorphous carbon material that
contains both diamond-structure and graphite-structure [52], has recently
attracted much attention in the MPGD community and is exploited to make
resistive electrodes to suppress discharges occurring in MPGDs [53, 54]). A
promising development in this sector is represented by the µ-RWELL structures
[24], a single-sided GEM coupled to the readout anode through the material of
high surface resistivity. A clear improvement of the stability in heavily
irradiated environments was presented by comparing discharge probability
values obtained with µ-RWELL and a standard 3-GEM setup, where both detectors
showed similar performance [55]. In addition, it was shown that high-rate
capabilities of the resistive structure can be restored by the proper
grounding of the DLC layers assuring improved charge evacuation. The obtained
performance figures together with simple construction (easy technology
transfer to industry) make µ-RWELL a very attractive option for the
application in, for example, high-rate, large-area trackers.

The DLC coating can also reduce gain variations versus time and minimize
charging-up effects, as shown in a recent study with standard THGEM foil,
produced by an industrial print-circuit board fabrication procedure, coated
with DLC films using magnetron sputtering technique [56].

###  3.5 Detector design optimisation

Specific know-how was developed within the gaseous detector community
providing a number of good practices to follow while designing a new detector.
It is based on many years of experience gained by the MPGD developers and
experimental groups using them. It is not possible to collect all
recommendations in a single chapter, as many of the design considerations are
also detector- and experiment-specific. Still, an approach to compile a list
of the most necessary design considerations can be made, based on the
available literature. The following overview focuses on GEM-like detectors but
in most of the cases, the discussed recommendations can be applied to any kind
of an MPGD.

####  3.5.1 Segmentation of electrodes

The most common design rule first proposed in [10] and applied thereafter to
most of the GEM-based detectors is to subdivide their top electrodes into
individual HV segments of $\sim 100\text{\,}{\mathrm{cm}}^{2}$ area each. This
way one can reduce the energy stored in a single-segment capacitor and limit
the amount of charge released in case of an electrical discharge. Lower energy
liberated in a discharge reduces the possibility of burning GEM foil
constituents and developing an ohmic connection between two GEM electrodes. In
case a segment does develop a short, the affected area of the detector is
minimized, and the rest of the detector can be operated normally. Similar
design can be introduced in other MPGD types (THGEMs, µ-RWELL, etc.) where
electrodes can be conveniently segmented. In the case of Micromegas, where a
standard mesh electrode cannot be easily divided (unless produced in a
microbulk technology [57]), segmentation of the readout anode into strips or
pads and their electrical connection play a similar role as individual GEM
segments. In work [58] a so-called floating-strip Micromegas is proposed to
minimize the effect of electrical discharges on the operation of a Micromegas
detector. By grounding the mesh and applying HV to the resistive strips,
capacitively coupled to the copper readout strips, one can effectively
minimize the affected area of a discharge and thus the dead-time of the
detector which reaches large values in case full Micromegas area discharges to
the readout anode as in the case of normally operated bulk Micromegas
detectors.

Segmentation of a GEM electrode reduces also the probability of secondary
discharges, which scales with an energy of the primary discharge[10]. Based on
studies presented in [47] the following points should be taken into account in
the GEM detector design, to minimize the probability of secondary discharge
occurrence.

  * 1.

Capacitance of the transfer/induction gaps shall be reduced by limiting the
single detector area, segmenting the bottom electrode of GEM foils, or
increasing the gap size.

  * 2.

Transfer and induction gaps in the detector should be kept uniform to avoid
electric field enhancement and thus increased secondary discharge probability
or even gas amplification in the gap. Using support structures is recommended,
when possible.

It should be noted that the configuration with a segmented bottom electrode
was usually avoided in the GEM community due to insufficient results and
understanding of the secondary discharge formation process. With the newest
results, this option is now being considered for future large-scale projects
such as the CMS muon spectrometer upgrade with new GE2/1 and ME0 triple-GEM
stations [59, 60].

####  3.5.2 Number of amplification cells

Following the results discussed in Section 2, optimization of a (TH)GEM
geometry by reducing the pitch between holes can significantly increase the
stability of the structure. This is related to the number of primary charges
entering a single hole (an amplification cell). As the primary charge density
is considered a driving factor of discharge formation, the larger number of
holes in a structure directly influences the discharge stability of the
latter. Reducing the pitch can be a highly beneficial design option, on the
other hand, it needs to be balanced with the production capabilities and
quality of a small-pitch structure.

The same argumentation can be presumably considered for Micromegas detectors,
as pointed out in Section 2. Although further studies are still necessary to
quantify the effect, a correlation between the size of a mesh cell and
discharge probability has been shown [14]. Thus, a single Micromegas cell can
be considered an independent amplification unit such as a GEM hole. This way,
by varying the optical transparency of a mesh, one can adjust the discharge
probability to a tolerable level, while keeping the ion backflow and energy
resolution performance as required by the experiment. Further studies of this
effect may provide valuable input for the performance optimization of
Micromegas stacks or hybrid structures (see also the discussion in Section
3.2.2 and 3.2.3).

####  3.5.3 Elimination of electric field hotspots

Although the discharge formation in a high-radiation environment is driven by
charge densities approaching amplification cells, the elimination of high
electric fields can minimize the occurrence of spurious discharges affecting
the stable operation of an MPGD structure. Further design optimization should
follow general recommendations for any kind of a gaseous detector, namely:
avoid electric field enhancement around electrode edges, avoid sharp tips and
corners of metallic parts, and maximize the distance between adjacent
electrodes operated at different potentials or any HV electrode and the
detector ground (this, of course, needs to be balanced with the requirement on
minimum dead area). These recommendations highly depend on achievable
production quality and can change in the future together with the improvement
of micropattern technology. In order to avoid high peak field values in the
case of Micromegas, also fundamental mesh parameters need to be considered.
The size of wires, their density, and bending as well as the type of mesh
influence the electric field values. Studies presented in [61] show the peak-
to-average field ratio can be reduced from three in the case of woven meshes
to two for calendared ones.

###  3.6 Production optimisation

The most efficient way to assure long-term stability and integrity of MPGD-
based detectors, and to avoid instabilities caused by production defects and
contamination, or design and assembly flaws, is a proper quality assurance
(QA) and quality control (QC) of the produced MPGDs. Similarly to the design
recommendations discussed above, there is no pre-defined list of tests to be
performed with newly produced GEMs, Micromegas, or other types of structures.
To large extent, it is driven by the requirements of the experiment but also
the experience of users. However, recent developments for the ALICE TPC [15],
CMS GE1/1 muon [16] and ATLAS NSW [17] upgrades allowed to collect a high-
statistics sample of MPGD structures which were produced and quality
controlled using dedicated protocols, thoroughly worked out in the course of
the projects.

##  4 Conclusions

The constantly increasing requirements of a new generation of experiments in
nuclear and particle physics are driving factors for the development of new
detectors. Among many innovative technologies, Micro Pattern Gaseous Detectors
have become widely used in high-rate physics experiments and are foreseen at
future facilities. The key parameter for the long-term operation of MPGDs is
stability against electrical discharges. Despite several open questions on
primary and secondary discharge development, various results available in the
literature and their interpretation contribute to the list of recommendations
and good practices to be considered while designing new experimental
apparatus. The stability limits of MPGD structures can serve as a reference
for the optimization of such detectors. Detailed studies of discharge
mechanisms in MPGD detectors result in a number of mitigation strategies that
have been shown to efficiently minimize the probability of discharge
occurrence. Many of them have been successfully implemented in recent large-
scale projects. It should be noted that for each detector design, and
operational conditions, careful optimization should be carried out to take
into account project-specific constraints. The constant development of gaseous
detector technology is essential for the realization of cutting-edge detectors
in future nuclear and particle physics experiments.

## Acknowledgements

The publication is funded by the Open Access Publishing Fund of GSI
Helmholtzzentrum fuer Schwerionenforschung. This work was partially supported
by the Deutsche Forschungsgemeinschaft - Sachbeihilfe [DFG FA 898/5-1].

## References

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