# Field emission mitigation studies in LCLS-II cavities via in situ plasma
processing

Bianca Giaccone<EMAIL_ADDRESS>Physics Department, Illinois Institute of
Technology, 10 W 35th St, Chicago, IL 60616 USA  Martina Martinello  Paolo
Berrutti  Oleksandr Melnychuk  Dmitri A. Sergatskov  Anna Grassellino  Fermi
National Accelerator Laboratory, Kirk Rd and
Pine St, Batavia, IL 60510 USA  Dan Gonnella  Marc Ross  SLAC National
Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, CA 94025 USA  Marc
Doleans  Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN
37830 USA  John F. Zasadzinski  Physics Department, Illinois Institute of
Technology, 10 W 35th St, Chicago, IL 60616 USA

###### Abstract

Field emission is one of the factors that can limit the performance of
superconducting radio frequency cavities. In order to reduce possible field
emission in LCLS-II (Linac Coherent Light Source II), we are developing plasma
processing for $1.3\text{\,}\mathrm{GHz}$ 9-cell cavities. Plasma processing
can be applied in situ in the cryomodule to mitigate field emission related to
hydrocarbon contamination present on the cavity surface.
In this paper, plasma cleaning was applied to single cell and 9-cell cavities,
both clean and contaminated; the cavities were cold tested before and after
plasma processing in order to compare their performance. It was proved that
plasma cleaning does not negatively affect the nitrogen doping surface
treatment; on the contrary, it preserves the high quality factor and quench
field. Plasma processing was also applied to cavities with natural field
emission or artificially contaminated. It was found that this technique
successfully removes carbon-based contamination from the cavity iris and that
it is able to remove field emission in a naturally field emitting cavity.
Vacuum failure experiments were simulated on four cavities, and in some cases
plasma processing was able to achieve an increase in performance.

††preprint: AIP/123-QED

##  I Introduction

A collaboration among Fermi National Accelerator Laboratory (FNAL), SLAC
National Accelerator Laboratory and Oak Ridge National Laboratory (ORNL) is
working to develop plasma processing for LCLS-II [1, 2]
$1.3\text{\,}\mathrm{GHz}$ nitrogen doped [3, 4, 5, 6] cavities. LCLS-II
(Linac Coherent Light Source II) is the LCLS XFEL (X-ray Free Electron Laser
[7, 8]) upgrade, and will utilize a superconducting linear accelerator, along
with other cutting-edge components, to produce an X-ray laser beam 1E4 times
brighter than LCLS [9, 10].

The scope of plasma processing is to be applied in situ in LCLS-II cryomodules
and help mitigate hydrocarbon-related field emission in 9-cell cavities.

Field emission (FE) is a phenomenon that limits the accelerating gradient at
which a cavity can operate [11]; it consists of electron emission from regions
of the cavity surface with intense applied electric field [12]. The emitted
electrons are accelerated by the electric field and impact on the cavity walls
depositing heat and creating bremsstrahlung X-rays. These electrons can also
interact with and disrupt the beam passing through the cavity. The X-rays
produced by the field emission can cause radiation damage in the cryomodule’s
components, decreasing their lifetime. Once FE is activated, it limits the
cavity’s accelerating field and it causes a degradation in quality factor, due
to the additional dissipation introduced by the emitted electrons. If field
emission is severe it can cause the cavity’s thermal breakdown and it can also
activate the beamline, causing induced radioactivity in the cavity.

Sources of field emission are contaminants (dust or metal particles) or cavity
surface defects that cause a local enhancement of the FE current. In addition,
the presence of few monolayers of hydrocarbons, or other adsorbate gases, on
the cavity surface can further decrease the Nb work function [13],
facilitating field emission. The origin of hydrocarbon contamination on the
cavity inner surface is not completely understood; however, its presence has
been reported in literature in multiple cases [14, 15, 16] and carbon has been
observed both in the form of adventitious and as a local contamination on the
Nb surface. Doleans et al report in [14] that evidence of volatile hydrocarbon
has been found through residual gas analysis on thermally cycled SNS
(Spallation Neutron Source [17]) cryomodules; they explain that these signals
must originate from the released gases that were previously condensed on the
cavity walls at cryogenic temperature or from species produced during
accelerator operation by the interaction of electrons with the cavity surface
contaminants.

Plasma processing can be used on superconducting radio frequency (SRF)
cavities in situ in the cryomodules to remove the hydrocarbon contamination
and restore the niobium work function obtaining a decrease in field emission
and a corresponding increase in the accelerating gradient. This technique was
first applied to SRF cavities at ORNL, where Doleans et al developed plasma
cleaning for SNS high beta $805\text{\,}\mathrm{MHz}$ cavities [18]. Plasma
processing has been applied to multiple SNS cryomodules, both offline and
online, showing improvement in the accelerating gradient [19]. Starting from
SNS experience, a new method of plasma ignition for LCLS-II
$1.3\text{\,}\mathrm{GHz}$ TESLA-shaped [20] cavities has been developed by
Berrutti et al at FNAL [21, 22, 23]. Studies on plasma ignition are being
conducted also at the Institute of Modern Physics, CAS (Chinese Academy of
Sciences) on half wave resonators (HWR). Wu et al used an experimental setup
that replicates the cavity assembled in the cryomodule to study plasma
ignition with Ar/O_2 gas mixture in HWR cavities [24] and studied the effect
of plasma processing on a HWR cavity contaminated with methane gas [25]. Huang
et al [26] applied helium and plasma processing to low beta HWR.

In this paper we present the results of plasma processing applied to multiple
$1.3\text{\,}\mathrm{GHz}$ cavities. The cavities were cold tested before and
after plasma processing in order to compare their performance in terms of
quality factor ($\mathrm{Q_{0}}$) and radiation versus accelerating field
($\mathrm{E_{acc}}$). A N-doped [3] single cell was used for the first plasma
processing test in order to study the possible effect of the plasma on the
surface treatment. Afterwards, plasma cleaning was applied on two 9-cell
cavities cavities with natural field emission (meaning FE of unknown source,
not caused by a contamination intentionally introduced in the cavity). Given
the results of these tests, it was decided to investigate the efficacy of
plasma processing on cavities artificially contaminated with carbon-related
sources or through vacuum failure simulations.

##  II  Experimental System and Plasma parameters

Plasma cleaning uses a glow discharge [27, 28] ignited inside the cavity
volume to remove the hydrocarbons from the niobium surface, restoring the Nb
work function and causing a decrease in field emission [29]. An inert gas
(neon) is injected into the cavity to ignite and sustain the plasma and a
small percentage of O_2 is added to the Ne. The oxygen molecules are
dissociated in the plasma and the reactive oxygen binds with the hydrocarbons
on the surface, creating volatile by-products that are easily pumped out of
the cavity.

The glow discharge is ignited inside RF volume, cell by cell, using the
cavity’s resonant modes. Scope of plasma processing is to be used in situ in
the cryomodule, so this technique relies only on the hardware present in the
cryomodule cavity’s assembly. The SNS method for plasma ignition is the dual
tone excitation [14] and uses the fundamental pass-band. For LCLS-II
$1.3\text{\,}\mathrm{GHz}$ cavities, it is not possible to ignite the glow
discharge using the fundamental power coupler (FPC). As Berrutti et al explain
in [21], the cavity quality factor $\mathrm{Q_{0}}$ and the FPC
$\mathrm{Q_{ext}}$ are highly mismatched at room temperature [30]. Therefore,
the approach taken here for LCLS-II cavities, is a new plasma ignition method,
developed by Berrutti et al, using the higher order modes (HOMs) and the HOM
couplers [22]. Modes belonging to two dipole pass-bands are used to ignite the
glow discharge in the central cell and to plasma process the entire cavity.
The procedure is composed of two identical rounds; during each round all the
cavity cells are plasma processed. Using the newly developed HOM ignition
method, the glow discharge is ignited in the central cell and then immediately
transferred to cell number 9, passing through adjacent cells. Once arrived in
the desired cell, the RF driving frequency is tuned in order to increase the
resonant peak’s frequency shift and to maximize the plasma density [22, 31,
32]. The cell is processed for $50\text{\,}\mathrm{min}$, then the plasma
density is decreased and the glow discharge is transferred to the adjacent
cell using a combination of resonant dipole modes. The procedure is repeated
until all the cavity cells, from $\\#$ 9 to $\\#$ 1, have been plasma
processed. Once the first round is completed, the second identical round is
performed.

Each cell is processed for a total of $100\text{\,}\mathrm{min}$. The duration
of each round has been decided using the data collected by the Residual Gas
Analyzer (RGA) assembled on the pumping system: it has been observed that
peaks in the carbon-related signals are often present when the plasma is
ignited in (or transferred to) a new cell. These peaks usually decrease to the
background level in approximately $30\text{\,}\mathrm{min}$ or less. During
the second round of plasma processing, there is usually no increase in the
C-related signals. The RGA is used during the entire plasma cleaning procedure
to monitor the concentration of oxygen in the mixture and the byproducts of
the reaction between O2 and the hydrocarbons on the cavity surface.

Figure 1: Experimental setup used for plasma processing: RF rack on the left
side, gas and vacuum system on the right, and cavity in the center. A portable
cleanroom, not shown in the picture, has been added to the setup and placed
around the cavity. All the connections between the cavity and the gas/vacuum
system take place inside the cleanroom.

Plasma processing on LCLS-II cavities is performed at room temperature, using
a mixture of neon with approximately 1-1.5$\%$ oxygen, for a total gas
pressure around $75\,$mTorr; the experimental setup in use at FNAL is shown in
Fig. 1. The set of parameters currently used has been developed during the
first plasma processing experiment on a 9-cell cavity, however it has not been
optimized yet. Studies to identify the set of parameters (pressure, duration,
oxygen percentage, plasma density/frequency shift) that maximizes the plasma
efficiency are currently ongoing.

###  II.1  Removal study of C-based contamination from cavity iris

After the plasma ignition studies on $1.3\text{\,}\mathrm{GHz}$ cavities [22],
the first plasma processing test was performed on a 9-cell cavity assembled
with viewports on the beam tubes. A permanent marker was used to introduce a
carbon-based contamination on the iris of one end cell. Permanent marker ink
is composed of hydrocarbon chains, as shown by the EDS (Energy Dispersive
X-ray spectroscopy) analysis in Fig. 2, and it was previously used for plasma
processing studies at SNS [31, 29]. Two permanent markers (black and red ink)
were used to draw 8 dots on the cavity iris. We applied plasma processing to
the contaminated cell for $19\text{\,}\mathrm{h}$: Fig. 4 shows the initial
and final state of the cavity, while Fig. 4 offers a close up of the initial,
intermediate and final state.

Figure 2: Permanent marker ink analyzed with Scanning Electron Microscopy
(SEM) on the left and Energy Dispersive X-ray spectroscopy (EDS) on the right.
A Nb sample is used as substrate.

Figure 3: Glow discharge ignited in the end cell contaminated with permanent
marker dots. (a) initial state, (b) final state after 19 hours of plasma
processing.

Figure 4: Close up of the contamination showed in Fig. 4. Initial state of the
contamination is shown in (a), final state in (c); (b) shows the progress
after 5 hours of plasma processing.

With this test, we confirmed that plasma processing is able to remove a
C-based contamination from the cavity iris, the area most problematic for FE.
The experiment has also allowed us to develop a first plasma recipe in terms
of duration of the process, O2 percentage, pressure, and plasma density. This
set of parameters was later used to apply plasma processing on multiple LCLS-
II cavities, both single cell and 9-cell cavities.

##  III  Plasma processing and RF tests on $1.3\text{\,}\mathrm{GHz}$ Cavities

Plasma processing was applied to LCLS-II cavities, both single cell and
9-cells, and the effectiveness of plasma cleaning was measured in terms of
$\mathrm{Q_{0}}$ versus $\mathrm{E_{acc}}$ and radiation versus
$\mathrm{E_{acc}}$ curves. All the cavities were cold tested before and after
plasma processing in order to compare their performance.
The first test was carried out on a clean nitrogen doped cavity, the following
two on naturally field emitting 9-cell N-doped cavities. The subsequent
studies were conducted on cavities artificially contaminated in order to
investigate the efficacy of plasma processing under different circumstances.
Table 1 summarizes the tests conducted on LCLS-II $1.3\text{\,}\mathrm{GHz}$
cavities.

Table 1: Summary of the $1.3\text{\,}\mathrm{GHz}$ single cell and 9-cell TESLA-shaped cavities cold tested for plasma processing studies. The terminology ’n/m’ for the nitrogen doping is to be interpreted as $n$ minutes in 25 mTorr of nitrogen and $m$ minutes of annealing in vacuum. Cavity | Surface Treatment | Contamination | Test Scope
---|---|---|---
Single cell | ’2/6’ N-doped |  | Plasma Processing Effect on N-doping
9-cell | ’3/60’ N-doped | Natural FE | Removal of Natural FE
9-cell | ’3/60’ N-doped | Natural FE | Removal of Natural FE
Single cell | ’2/6’ N-doped | Aquadag® | Removal of C-contamination
9-cell | EP | Vacuum Failure Simulated Inside Cleanroom | FE Mitigation
Single cell | ’2/6’ N-doped | Vacuum Failure Simulated Outside Cleanroom | FE Mitigation
9-cell | ’2/6’ N-doped | Vacuum Failure Simulated Outside Cleanroom | FE Mitigation
9-cell | ’2/6’ N-doped | Vacuum Failure Simulated Outside Cleanroom | FE Mitigation

All the RF cold tests were done in the vertical test stand (VTS) facility at
FNAL, following the measurement method explained in [33]. The cryogenic dewars
are equipped with two radiation detectors [34] positioned on the top and on
the bottom of the dewar where the cavity is placed for the cold test. In order
to reliably compare the results of the RF tests done before and after plasma
processing, we have attempted to test each cavity always in the same VTS dewar
and, when possible, in the same position inside the dewar. In some cases it
was not possible to test the cavity in the same dewar and it is indicated in
the text.

All the plots in this paper use the following symbols: solid symbols for
$\mathrm{Q_{0}}$ versus $\mathrm{E_{acc}}$ curves, empty and half filled
symbols for the radiation versus $\mathrm{E_{acc}}$ curves, with empty symbols
used for the radiation detector located on top of the cryogenic dewar,
vertically half filled symbols for the bottom radiation detector.

###  III.1  Baseline test on N-doped Cavity

N-doping is a surface treatment developed at FNAL that has allowed to increase
the cavity $\mathrm{Q_{0}}$ by a factor of 3 [3, 5]. The recipe used to N-dope
LCLS-II cavities is called ’2/6’, which consists of baking the cavity in
vacuum (p $<$1E-6 Torr) at $800\text{\,}\mathrm{\SIUnitSymbolCelsius}$, once
the temperature is stable nitrogen is injected in the furnace at a pressure of
25 mTorr for $2\text{\,}\mathrm{min}$. Afterwards the vacuum is restored and
the cavity undergoes $6\text{\,}\mathrm{min}$ of annealing. After the doping,
$5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ are removed from the inner cavity
surface through electropolishing (EP) in order to eliminate possible nitrides.
After $5\text{\,}\mathrm{\SIUnitSymbolMicro m}$ of EP, only the interstitial
nitrogen remains in the niobium with a concentration in the order of 100 ppm.
N_2 atoms are absorbed as interstitial impurities in the niobium lattice and
cause a reduction of the Mattis-Bardeen surface resistance $\mathrm{R_{BCS}}$
with the accelerating field [35].

Since all $1.3\text{\,}\mathrm{GHz}$ LCLS-II cavites are nitrogen doped [4,
6], we have started our studies applying plasma cleaning to a N-doped single
cell cavity in order to understand if this processing can affect the surface
treatment.
The single cell used for the test was built by welding together two end-cells
of a 9-cell cavity. The result is a single cell cavity with HOMs couplers on
the beamtubes (sew panel (a) in Fig. 5). This characteristic makes it suitable
for plasma processing, as it allows to ignite the glow discharge using the
HOMs and, with only one cell to process, drastically reduces the duration of
the cleaning.

Figure 5: In (a) single cell cavity with HOM couplers. Figure (b) shows the
results of the RF tests measured at $2\text{\,}\mathrm{K}$ before (green) and
after (red) plasma processing. The baseline RF test has been intentionally
limited at 28 MV/m at $2\text{\,}\mathrm{K}$ to avoid quench. The quench field
was reached at $\mathrm{E_{acc}}=33.5$ MV/m during the
$1.4\text{\,}\mathrm{K}$ baseline test and the vertical dotted line indicates
the quench field value. As explained in section III solid symbols are used to
plot the $\mathrm{Q_{0}}$ vs $\mathrm{E_{acc}}$ curve, empty symbols for the
radiation detected from the top detector, half filled symbols for the bottom
radiation detector.

The results of the RF cold tests obtained on the single cell before and after
plasma processing are shown in panel (b) Fig. 5. We intentionally stopped the
baseline test before quench in order to measure the cavity $\mathrm{Q_{0}}$ at
$1.4\text{\,}\mathrm{K}$, where it reached a quench field equal to 33.5 MV/m.
After the first vertical test, the cavity was connected to the vacuum/gas and
RF system used for plasma cleaning and processed for $16\text{\,}\mathrm{h}$
with Ne$/$O2 plasma.
Comparing the RF tests measured before and after plasma processing, it is
clear that plasma cleaning does not negatively affect the performance of the
nitrogen doped single cell; on the contrary, it preserves the high quality
factor and quench field characteristic of N-doped cavities.

###  III.2  Naturally Field Emitting N-doped Cavities

Figure 6: Figure (a) shows the results of the first naturally field emitting
cavity: in blue $\mathrm{Q_{0}}$ and radiation level vs $\mathrm{E_{acc}}$
performed before plasma processing. The X-ray onset is at 16 MV/m before
plasma processing; after plasma processing the FE was completely removed: the
RF test measured after the treatment shows no X-rays. Figure (b) shows the RF
tests of the second cavity. The test was stopped before quench due to intense
X-rays. The red curves show the performance after plasma processing; no
increase in performance has been registered after the cleaning.

We used two cavities with natural field emission to test the efficacy of
plasma processing on FE with unknown source, not caused by an artificial
contamination. The two 9-cell cavities showed X-rays during the first vertical
tests performed at FNAL. Both cavities were assembled with a second valve to
allow the flow of gas for plasma processing and RF tested again after valve
assembly. Afterwards the two cavities were plasma processed and cold tested
again.

The top plot in figure 6 shows the results of the cold tests of the first
9-cell; the curves registered before plasma show that the cavity quenched at
$\mathrm{E_{acc}}\,=\,18.5\,$MV/m with X-rays onset at 16 MV/m. After plasma
processing, the cavity reached $\mathrm{E_{acc}}\,=\,18\,$MV/m showing no
X-rays. In this case plasma processing completely removed field emission. The
fact that no change in quench field has been registered suggests that is a
hard quench, not due to FE.

The bottom plot in figure 6 shows the performance of the second cavity: the
radiation onset before plasma processing was registered at 7 MV/m; the test
was stopped before the quench field due to intense radiation levels (1.1E4
mR/h at 16.5 MV/m) and final FE onset was measured at 7.8 MV/m. The cold test
conducted after plasma processing showed the X-ray onset decreased to 7 MV/m
and the quality factor degraded. Also in this case, the RF test was
interrupted due to intense FE. The $\mathrm{Q_{0}}$ degradation could be due
to higher ambient magnetic field during cooldown, which results in increased
trapped magnetic flux in the cavity. The two RF tests measured on this cavity
were carried out in different cryogenic dewar, this could also explain the
difference in trapped flux.

The fact that plasma processing was effective on one cavity with natural field
emission but not on the second indicates that the FE may originate from
different sources. The evidence suggests that in the first cavity the FE was
caused by a hydrocarbon contamination and that plasma processing was effective
in removing it thanks to the reactive oxygen, present in the glow discharge,
that binds with the H_xC_y and creates volatile byproducts. In the second
cavity instead the field emission may not be caused by a hydrocarbons but it
could be due to surface defects or metal flakes on the cavity surface. If that
is the case, it is expected that the plasma cleaning would have little effect
in mitigating FE since no volatile byproducts are generated in the reaction.

####  III.2.1 Residual Gas Analyzer spectrum

A Residual Gas Analyzer is used to monitor the composition of the gas pumped
out of the cavities for the duration of the entire plasma cleaning. Figure 7
shows the RGA data acquired during the first round of plasma processing
applied to the first naturally field emitting cavity and it represents a
typical example of the RGA spectrum registered during plasma processing.
During the first round of plasma cleaning, the spectrum often shows peaks in
C, CO, CO2 in correspondence with the moment when the plasma is ignited in, or
transferred to, a new cell. The increase in the C-related signals shows that
the oxygen is reacting with the H_xC_y on the cavity surface.

The RGA can record masses from 1 to $300\text{\,}\mathrm{u}$, however in this
plot are shown only the elements of interest. In figure 7 the C-related peaks
are clearly visible in correspondence with the plasma being initially ignited
in cell $\\#$ 5, then transferred to, and tuned, in cell $\\#$ 9, and
transferred and tuned in cell $\\#$ 4, up to cell $\\#$ 1\. Cells from $\\#$ 8
to $\\#$ 5 don’t show prominent peaks in the C-related signals. This could be
due to the fact that, at the beginning of the procedure, the glow discharge is
ignited in the central cell (first peak on the left in figure 7) and then
moved from cell $\\#$ 5 up to cell $\\#$ 9, going through all the intermediate
cells.

Figure 7: Example of RGA spectrum acquired during the first day of plasma
processing on a 9-cell cavity. Cell $\\#$4 and cell $\\#$1 exhibit a double
peak due to the plasma being accidentally turned off while reaching the
desired plasma density.

###  III.3  Carbon-based contamination

Figure 8: Scanning Electron Microscope images of pure (a) and diluted Aquadag®
on Nb substrate. The dilution factor has been calculated as the ratio between
H2O mass and Aquadag mass. Figure (b) and (c) show respectively Aquadag
diluted by a factor of 100 and 2E4.

Figure 9: Quality factor versus accelerating field curves measured on the
contaminated cavity and after plasma cleaning. The contaminated cavity (blue
triangle) shows a degradation in quench field and $\mathrm{Q_{0}}$. After
plasma processing it shows an increase in quality factor ($\mathrm{Q_{0}}=$
2E10 at 16.4 MV/m) and a complete recovery in $\mathrm{E_{acc}}$ (quench field
is 33.5 MV/m). In green is shown the baseline performance of the cavity before
contamination.

We contaminated the single cell cavity, previously used for the N-doping test,
using a carbon-based paint in order to study the effectiveness of plasma
processing on an artificial carbon contamination. Aquadag® [36], a conductive
paint made of graphite and ultra pure water, was used to contaminate the
cavity. A small drop of highly diluted paint was deposited on the cavity iris.
Figure 9 shows images of pure and diluted Aquadag acquired with the Scanning
Electron Microscope (SEM). The Aquadag used to contaminate the single cell
iris was diluted by a factor 2E4, where the dilution was calculated as the
ratio between the H_2O and the Aquadag mass.

For this study it was not possible to cold test the cavity in the same
cryogenic dewar before and after plasma processing.

Figure 9 shows the results of the RF tests. The contaminated single cell was
plasma processed for $17\text{\,}\mathrm{h}$ between the two cold tests. In
blue is the curve measured on the contaminated cavity; comparing it with the
baseline test (here in green, same curve shown in red in Fig. 5), it can be
seen that the cavity shows a degradation in the quality factor and quench
field: $Q_{0}\,=$ 1.7E10 at $\mathrm{E_{acc}}\,=$ 16.2 MV/m, quench field is
registered at 18.8 MV/m. The radiation detector positioned at the bottom of
the cryogenic dewar was not working correctly during this cold test, however
the top radiation detector was connected and no X-rays were registered. After
$17\text{\,}\mathrm{h}$ of plasma processing, the cavity exhibits an increase
in quality factor ($\mathrm{Q_{0}}\,=$ 2E10 at $\mathrm{E_{acc}}\,=$ 16.4
MV/m). Plasma processing increased the quench field by almost 15 MV/m,
restoring the initial quench field at $\mathrm{E_{acc}}\,=$ 33.5 MV/m.

###  III.4  Vacuum Failure Experiments

A possible cause of cavity contamination is a vacuum leak or a complete vacuum
loss. Multiple experiments were conducted on cavities exposed to air in order
to understand if plasma processing can be effective in mitigating field
emission in these scenarios. The tests were carried out under different
conditions, on both 9-cell and single cell cavities. We refer to these tests
as vacuum failure experiments (or simulations).

####  III.4.1  Vacuum Failure Experiment Inside the Cleanroom

We conducted the first test inside a cleanroom environment, in order to
introduce a controlled amount of particulate. High pressure rinsing (HPR) was
used to clean the cavity and, after drying, it was slowly evacuated to high
vacuum. To simulate the vacuum failure, the mini right angle valve (RAV) was
opened while the cavity was in a class 100 cleanroom. The cavity quickly
reached atmospheric pressure and, after sitting at this pressure for a few
minutes, it was slowly evacuated to reach a pressure in the low E-6 - high E-7
Torr range.

Plasma processing was applied twice to this cavity, each time using the
standard parameters and duration (approximately $1\text{\,}\mathrm{h}$
$40\text{\,}\mathrm{min}$ per cell). After each plasma processing, the cavity
was RF tested at $2\text{\,}\mathrm{K}$. The RF tests on the contaminated
cavity and after the second plasma processing were conducted in the same
cryogenic dewar, while the cold test after the first plasma cleaning was
carried out in a different dewar.

Panel (a) of figure 10 summarizes the performance of the cavity during the
cold tests. In blue the curves registered before plasma processing (on the
contaminated cavity): the 9-cell reached a first quench at 7.5 MV/m, it was
then possible to increase the power and measure the $\mathrm{Q_{0}}$ versus
$\mathrm{E_{acc}}$ curve up to 23 MV/m (with intermediate quenches at 20.5 and
22 MV/m), when the cavity reached the final quench. The X-ray onset was
registered at 18.5 MV/m for the bottom radiation detector, at 20 MV/m for the
top detector. The cavity was tested also at $1.4\text{\,}\mathrm{K}$, and the
purple curves show the radiation level registered during the final power rise
(intentionally stopped before quench): the maximum radiation level switched
from the bottom to the top detector, the onset was registered at 16 MV/m and
19 MV/m respectively for bottom and top detector. This, along with the change
in radiation intensity, suggests that some field emission processing has
occurred during the RF test, however the cavity continued showing significant
FE. The RF test measured after the first plasma processing is shown in red:
the cavity quenched at 23 MV/m and no X-ray activity was registered during the
test, indicating that the field emission was completely removed. The quality
factor instead showed degradation: from 2.4E10 at 16.3 MV/m registered before
plasma to 2.2E10 at 16.4 MV/m after plasma processing.
We applied a second round of plasma processing to the 9-cell cavity to
investigate if the quality factor degradation was caused by the plasma
treatment or if it was due to different amount of trapped flux since the two
RF tests (after the contamination and after the first plasma processing) were
measured in different dewars. After an additional $2\text{\,}\mathrm{h}$ of
plasma cleaning per cell, we cold tested the cavity. The curves are plotted in
black in panel (a) figure 10: the test confirms that the quality factor is
actually preserved ($\mathrm{Q_{0}}$=2.5E10 at 16.4 MV/m) and both radiation
detectors show no X-ray activity, confirming that the field emission was
eliminated.

Figure 10: Vacuum failure experiments simulated on four cavities. Figure (a)
shows the curves of the 9-cell cavity vented in cleanroom; the results of the
cold test perfored after the cavity was contaminated is plotted in blue; the
final radiation curves were acquired during this test at
$1.4\text{\,}\mathrm{K}$, they are plotted with a solid line and purple
triangles (empty triangles for the top radiation detector, half filled for the
bottom detector). This cavity was plasma processed twice: the cold test
conducted after the first round of plasma cleaning is plotted red, in black is
the cold test performed after the second plasma round. Figure (b) contains the
RF tests of the single cell cavity vented outside the cleanroom through the
RAV (right angle valve), both RF tests (after the contamination and the
subsequent after plasma test) were stopped before quench at 10 MV/m; Fig. (c)
shows the results of the 9-cell cavity quickly vented through the mini RAV;
plot (d) contains the curves of the 9-cell cavity slowly vented through the
mini RAV.
For comparison all the plots also contain the baseline curves of the cavities,
measured before they were exposed to the vacuum failure simulations.

####  III.4.2  Vacuum Failure Experiments Outside the Cleanroom

Following the experiment in cleanroom, additional vacuum failures were
simulated outside the cleanroom. The procedure was repeated with small
variations on three cavities: one single cell and two 9-cells. After the
contamination, the single cell cavity was plasma processed for
$20\text{\,}\mathrm{h}$, and the 9-cell cavities for approximately
$1\text{\,}\mathrm{h}$ $40\text{\,}\mathrm{min}$ per cell. After the venting,
the cavities were evacuated to the low E-6 - high E-7 Torr range; RF tests
were performed before and after plasma processing. The plots in Fig. 10 show
the $\mathrm{Q_{0}}$ and radiation versus $\mathrm{E_{acc}}$ curves.

The single cell cavity was quickly vented through the mini RAV from high
vacuum to atmospheric pressure. Panel (b) in figure 10 contains the RF tests
of the single cell cavity. The green curve shows the baseline before the
vacuum failure experiment (red curve in Fig. 9, $\mathrm{Q_{0}}$=1.9E10 at 5
MV/m). The contaminated cavity (blue curves) exhibits a degradation in quality
factor ($\mathrm{Q_{0}}$=9.4E9 at 5 MV/m) and radiation onset at 6.2 MV/m. The
test was limited before quench at 10 MV/m by the available RF power. Due to
the severity of FE registered during the first RF test it was decided to
process the cavity for a time longer than usual ($20\text{\,}\mathrm{h}$). The
cold test conducted after plasma processing shows a moderate increase in
quality factor ($\mathrm{Q_{0}}$=1.2E10 at 5.1 MV/m) and decrease in radiation
(8.4 mR/h at 10 MV/m versus the 28 mR/h at 9.8 MV/m registered before plasma)
with FE onset at 7 MV/m.

Panel (c) in figure 10 shows the results of the 9-cell cavity quickly vented
through the mini RAV. In green is plotted the baseline performance of the
cavity before venting ($\mathrm{Q_{0}}$=3.1E10 at 8.8 MV/m); in blue are the
curves measured on the cavity after the contamination: quality factor
degradation ($\mathrm{Q_{0}}$=2.6E10 at 8.7 MV/m) and FE onset at 8.7 MV/m,
reaching final quench at 17 MV/m with 1E4 mR/h; the FE onset after quench was
registered at 10.3 MV/m. The after plasma curves are shown in red: FE onset at
8.8 MV/m, with less severe slope in $\mathrm{Q_{0}}$ versus $\mathrm{E_{acc}}$
curve until 12.6 MV/m; at this point an increase in radiation was registered
along with a $\mathrm{Q_{0}}$ drop. The $\mathrm{Q_{0}}$ versus
$\mathrm{E_{acc}}$ curve was measured again showing overlap with the test
measured on the contaminated cavity in terms of quench field (17 MV/m),
quality factor degradation and slope due to FE.

We simulated the final vacuum failure experiment by slowly opening the mini
RAV on a 9-cell cavity, venting the cavity over a $18\text{\,}\mathrm{min}$
time interval. The results of the RF tests are shown in panel (d) Fig. 10. The
green curve shows the baseline performance; in blue are plotted the curves of
the contaminated cavity (before plasma): the cavity shows intense FE as
evident from the slope in quality factor. A first quench was reached below 3
MV/m, we then increased the accelerating field: at 4 MV/m FE started and
between 4.5-5 MV/m there was a switch in quality factor (from 7.3E9 to
1.75E10). The sudden jump in $\mathrm{Q_{0}}$ suggests that a field emitter
was processed by the RF, eliminating the radiation and energy dissipation that
it was causing. After this event the quality factor followed a new curve that
started bending above 4 MV/m while the radiation increased. Another
$\mathrm{Q_{0}}$ switch occurred at 6.4 MV/m, indicating that a new field
emitter was likely processed. The test was stopped at this point due to
intense FE ($>$1 R/h) and to avoid risking damaging the cavity surface through
RF processing of field emitters; the final FE onset was registered at 5.2
MV/m. In red is reported the RF test measured after plasma processing: the
$\mathrm{Q_{0}}$ curve overlaps with the final before plasma (blue) curve.
Also in this case a $\mathrm{Q_{0}}$ switch was observed at 7.5 MV/m,
indicating RF processing of the field emitter. The test was stopped at 10.4
MV/m. The partial performance recovery observed in this cavity is to be
attributed to RF processing occurred during the $2\text{\,}\mathrm{K}$
vertical tests, not to plasma processing.

##  IV  Conclusions

Using the newly developed technique of plasma ignition with HOMs, it was
demonstrated that plasma processing successfully interacts with the cavity
iris and removes carbon-based contamination.
It was also proved that plasma processing does not affect negatively the
performance of nitrogen doped cavities, on the contrary it preserves their
high quality factor and quench field. Plasma cleaning was applied to multiple
LCLS-II cavities with natural field emission or artificially contaminated. The
comparison between the RF tests conducted before and after plasma cleaning
showed an increase in performance in the carbon contaminated single cell, in
one out of two naturally field emitting cavities and in one 9-cell cavity
exposed to vacuum failure simulation inside the cleanroom. A second cavity
with natural field emission was processed but still showed X-rays activity
after the plasma, suggesting that the source of FE may not be carbon-related
in this case, but due to metal flakes or surface defects. The three cavities
used for vacuum failure simulation outside the cleanroom have shown little or
no improvement attributable to plasma processing.

Plasma processing has shown positive results in cavities with field emission
onset registered at high field level (above 16 MV/m), while it has not been
able to reduce FE in cases when the onset started at low fields. This can be
related with the source of FE: plasma processing, applied with the current
recipe and parameters, is effective on hydrocarbon contamination and not on
metal flakes, which are the most plausible cause of FE at low fields.

In order to better understand what is the nature of the field emitters in the
cavities exposed to vacuum failure simulations, it was decided to collect and
analyze the particles introduced in the cavities and not removed with plasma
cleaning. Preliminary results from the analysis of particles collected from
the single cell cavity suggest that metal flakes were introduced into the
cavity during the vacuum failure experiment performed outside the cleanroom.
The SEM/EDS analysis of the particles collected from the vented cavities is
currently ongoing and will be the subject of future publication.

We intend to apply plasma processing to more LCLS-II 9-cell cavities and cold
test them (before and after) in order to acquire a greater statistics,
focusing in particular on cavities that exhibit field emission of unknown
source (natural FE) during the RF tests.

###### Acknowledgements.

This work was supported by the U.S. Department of Energy (DOE), Office of
Science, Basic Energy Sciences (BES). Fermilab is operated by Fermi Research
Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of
Energy. We thank LCLS-II-HE for providing the N-doped
$1.3\text{\,}\mathrm{GHz}$ 9-cell cavities used in this study for plasma
processing and cold tests.

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