Field emission mitigation studies in LCLS-II cavities via in situ plasma processing
Bianca Giaccone, Martina Martinello, Paolo Berrutti, Oleksandr Melnychuk, Dmitri A. Sergatskov, Anna Grassellino, Dan Gonnella, Marc Ross, Marc Doleans, John F. Zasadzinski
FField emission mitigation studies in LCLS-II cavities via in situ plasmaprocessing
Bianca Giaccone, a) Martina Martinello, Paolo Berrutti, Oleksandr Melnychuk, Dmitri A. Sergatskov, AnnaGrassellino, Dan Gonnella, Marc Ross, Marc Doleans, and John F. Zasadzinski Fermi National Accelerator Laboratory, Kirk Rd andPine St, Batavia, IL 60510 USA SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, CA 94025 USA Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37830 USA Physics Department, Illinois Institute of Technology, 10 W 35th St, Chicago,IL 60616 USA (Dated: 6 October 2020)
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 developingplasma processing for 1 . in situ in the cryomodule tomitigate 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. Itwas proved that plasma cleaning does not negatively affect the nitrogen doping surface treatment; on thecontrary, it preserves the high quality factor and quench field. Plasma processing was also applied to cavitieswith natural field emission or artificially contaminated. It was found that this technique successfully removescarbon-based contamination from the cavity iris and that it is able to remove field emission in a naturallyfield emitting cavity. Vacuum failure experiments were simulated on four cavities, and in some cases plasmaprocessing was able to achieve an increase in performance. I. INTRODUCTION
A collaboration among Fermi National AcceleratorLaboratory (FNAL), SLAC National Accelerator Lab-oratory and Oak Ridge National Laboratory (ORNL)is working to develop plasma processing for LCLS-II . cavities. LCLS-II (Linac Co-herent Light Source II) is the LCLS XFEL (X-ray FreeElectron Laser ) upgrade, and will utilize a supercon-ducting linear accelerator, along with other cutting-edgecomponents, to produce an X-ray laser beam 1E4 timesbrighter than LCLS .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 theaccelerating gradient at which a cavity can operate ; itconsists of electron emission from regions of the cavitysurface with intense applied electric field . The emit-ted electrons are accelerated by the electric field andimpact on the cavity walls depositing heat and creatingbremsstrahlung X-rays. These electrons can also inter-act with and disrupt the beam passing through the cav-ity. The X-rays produced by the field emission can causeradiation damage in the cryomodule’s components, de-creasing their lifetime. Once FE is activated, it limitsthe cavity’s accelerating field and it causes a degradation a) Electronic mail: [email protected]; Physics Department, IllinoisInstitute of Technology, 10 W 35th St, Chicago, IL 60616 USA in quality factor, due to the additional dissipation intro-duced by the emitted electrons. If field emission is severeit can cause the cavity’s thermal breakdown and it canalso activate the beamline, causing induced radioactivityin the cavity.Sources of field emission are contaminants (dust ormetal particles) or cavity surface defects that cause alocal enhancement of the FE current. In addition, thepresence of few monolayers of hydrocarbons, or other ad-sorbate gases, on the cavity surface can further decreasethe Nb work function , facilitating field emission. Theorigin of hydrocarbon contamination on the cavity innersurface is not completely understood; however, its pres-ence has been reported in literature in multiple cases and carbon has been observed both in the form of adven-titious and as a local contamination on the Nb surface.Doleans et al report in that evidence of volatile hy-drocarbon has been found through residual gas analysison thermally cycled SNS (Spallation Neutron Source )cryomodules; they explain that these signals must orig-inate from the released gases that were previously con-densed on the cavity walls at cryogenic temperature orfrom species produced during accelerator operation bythe interaction of electrons with the cavity surface con-taminants.Plasma processing can be used on superconducting ra-dio frequency (SRF) cavities in situ in the cryomodulesto remove the hydrocarbon contamination and restorethe niobium work function obtaining a decrease in fieldemission and a corresponding increase in the accelerat-ing gradient. This technique was first applied to SRFcavities at ORNL, where Doleans et al developed plasma a r X i v : . [ phy s i c s . acc - ph ] O c t cleaning for SNS high beta 805 MHz cavities . Plasmaprocessing has been applied to multiple SNS cryomod-ules, both offline and online, showing improvement in theaccelerating gradient . Starting from SNS experience,a new method of plasma ignition for LCLS-II 1 . cavities has been developed by Berruttiet al at FNAL . Studies on plasma ignition are be-ing conducted also at the Institute of Modern Physics,CAS (Chinese Academy of Sciences) on half wave res-onators (HWR). Wu et al used an experimental setupthat replicates the cavity assembled in the cryomodule tostudy plasma ignition with Ar/O gas mixture in HWRcavities and studied the effect of plasma processing ona HWR cavity contaminated with methane gas . Huanget al applied helium and plasma processing to low betaHWR.In this paper we present the results of plasma process-ing applied to multiple 1 . ) and radiation versus accelerating field (E acc ).A N-doped single cell was used for the first plasma pro-cessing test in order to study the possible effect of theplasma on the surface treatment. Afterwards, plasmacleaning was applied on two 9-cell cavities cavities withnatural field emission (meaning FE of unknown source,not caused by a contamination intentionally introducedin the cavity). Given the results of these tests, it wasdecided to investigate the efficacy of plasma processingon cavities artificially contaminated with carbon-relatedsources or through vacuum failure simulations. II. EXPERIMENTAL SYSTEM AND PLASMAPARAMETERS
Plasma cleaning uses a glow discharge ignited in-side the cavity volume to remove the hydrocarbons fromthe niobium surface, restoring the Nb work function andcausing a decrease in field emission . An inert gas (neon)is injected into the cavity to ignite and sustain the plasmaand a small percentage of O is added to the Ne. Theoxygen molecules are dissociated in the plasma and thereactive oxygen binds with the hydrocarbons on the sur-face, creating volatile by-products that are easily pumpedout of the cavity.The glow discharge is ignited inside RF volume, cell bycell, using the cavity’s resonant modes. Scope of plasmaprocessing is to be used in situ in the cryomodule, so thistechnique relies only on the hardware present in the cry-omodule cavity’s assembly. The SNS method for plasmaignition is the dual tone excitation and uses the funda-mental pass-band. For LCLS-II 1 . ,the cavity quality factor Q and the FPC Q ext are highlymismatched at room temperature . Therefore, the ap-proach taken here for LCLS-II cavities, is a new plasma ignition method, developed by Berrutti et al, using thehigher order modes (HOMs) and the HOM couplers .Modes belonging to two dipole pass-bands are used to ig-nite the glow discharge in the central cell and to plasmaprocess the entire cavity. The procedure is composed oftwo identical rounds; during each round all the cavitycells are plasma processed. Using the newly developedHOM ignition method, the glow discharge is ignited inthe central cell and then immediately transferred to cellnumber 9, passing through adjacent cells. Once arrivedin the desired cell, the RF driving frequency is tuned inorder to increase the resonant peak’s frequency shift andto maximize the plasma density . The cell is pro-cessed for 50 min, then the plasma density is decreasedand the glow discharge is transferred to the adjacent cellusing a combination of resonant dipole modes. The pro-cedure is repeated until all the cavity cells, from and the hydrocarbons on the cavitysurface.Plasma processing on LCLS-II cavities is performed atroom temperature, using a mixture of neon with approx-imately 1-1.5% oxygen, for a total gas pressure around75 mTorr; the experimental setup in use at FNAL isshown in Fig. 1. The set of parameters currently used hasbeen developed during the first plasma processing experi-ment on a 9-cell cavity, however it has not been optimizedyet. Studies to identify the set of parameters (pressure,duration, oxygen percentage, plasma density/frequencyshift) that maximizes the plasma efficiency are currentlyongoing. A. Removal study of C-based contamination from cavityiris
After the plasma ignition studies on 1 . ,the first plasma processing test was performed on a 9-cellcavity assembled with viewports on the beam tubes. Apermanent marker was used to introduce a carbon-basedcontamination on the iris of one end cell. Permanentmarker ink is composed of hydrocarbon chains, as shownby the EDS (Energy Dispersive X-ray spectroscopy) anal-ysis in Fig. 2, and it was previously used for plasmaprocessing studies at SNS . Two permanent markers FIG. 1. Experimental setup used for plasma processing: RF rack on the left side, gas and vacuum system on the right, andcavity in the center. A portable cleanroom, not shown in the picture, has been added to the setup and placed around thecavity. All the connections between the cavity and the gas/vacuum system take place inside the cleanroom.FIG. 2. Permanent marker ink analyzed with Scanning Elec-tron Microscopy (SEM) on the left and Energy DispersiveX-ray spectroscopy (EDS) on the right. A Nb sample is usedas substrate. (black and red ink) were used to draw 8 dots on the cavityiris. We applied plasma processing to the contaminatedcell for 19 h: Fig. 3 shows the initial and final state ofthe cavity, while Fig. 4 offers a close up of the initial,intermediate and final state.With this test, we confirmed that plasma processing isable to remove a C-based contamination from the cavityiris, the area most problematic for FE. The experimenthas also allowed us to develop a first plasma recipe interms of duration of the process, O percentage, pressure,and plasma density. This set of parameters was later usedto apply plasma processing on multiple LCLS-II cavities,both single cell and 9-cell cavities. III. PLASMA PROCESSING AND RF TESTS ON . CAVITIES
Plasma processing was applied to LCLS-II cavities,both single cell and 9-cells, and the effectiveness ofplasma cleaning was measured in terms of Q versus E acc and radiation versus E acc curves. All the cavities werecold tested before and after plasma processing in orderto compare their performance.The first test was carried out on a clean nitrogen doped (a) (b) FIG. 3. Glow discharge ignited in the end cell contaminatedwith permanent marker dots. (a) initial state, (b) final stateafter 19 hours of plasma processing. (b)(c)(a)
FIG. 4. Close up of the contamination showed in Fig. 3.Initial state of the contamination is shown in (a), final state in(c); (b) shows the progress after 5 hours of plasma processing. cavity, the following two on naturally field emitting 9-cell N-doped cavities. The subsequent studies were con-ducted on cavities artificially contaminated in order toinvestigate the efficacy of plasma processing under dif-ferent circumstances. Table I summarizes the tests con-ducted on LCLS-II 1 . . The cryogenic dewars areequipped with two radiation detectors positioned onthe top and on the bottom of the dewar where the cavityis placed for the cold test. In order to reliably comparethe results of the RF tests done before and after plasmaprocessing, we have attempted to test each cavity alwaysin the same VTS dewar and, when possible, in the sameposition inside the dewar. In some cases it was not possi-ble to test the cavity in the same dewar and it is indicatedin the text.All the plots in this paper use the following symbols:solid symbols for Q versus E acc curves, empty and halffilled symbols for the radiation versus E acc curves, withempty symbols used for the radiation detector located ontop of the cryogenic dewar, vertically half filled symbolsfor the bottom radiation detector. A. Baseline test on N-doped Cavity
N-doping is a surface treatment developed at FNALthat has allowed to increase the cavity Q by a factorof 3 . The recipe used to N-dope LCLS-II cavities iscalled ’2/6’, which consists of baking the cavity in vac-uum (p < ° C, once the temperature isstable nitrogen is injected in the furnace at a pressure of25 mTorr for 2 min. Afterwards the vacuum is restoredand the cavity undergoes 6 min of annealing. After thedoping, 5 µ m are removed from the inner cavity surfacethrough electropolishing (EP) in order to eliminate possi-ble nitrides. After 5 µ m of EP, only the interstitial nitro-gen remains in the niobium with a concentration in theorder of 100 ppm. N atoms are absorbed as interstitialimpurities in the niobium lattice and cause a reductionof the Mattis-Bardeen surface resistance R BCS with theaccelerating field .Since all 1 . , we have started our studies applying plasmacleaning to a N-doped single cell cavity in order to under-stand if this processing can affect the surface treatment.The single cell used for the test was built by welding to-gether two end-cells of a 9-cell cavity. The result is asingle cell cavity with HOMs couplers on the beamtubes(sew panel (a) in Fig. 5). This characteristic makes itsuitable for plasma processing, as it allows to ignite theglow discharge using the HOMs and, with only one cell toprocess, drastically reduces the duration of the cleaning.The results of the RF cold tests obtained on the singlecell before and after plasma processing are shown in panel BaselineAfter Plasma Q E acc (MV/m) Test on N-doped cavity R ad i a t i on ( m R / h ) (a)(b) FIG. 5. In (a) single cell cavity with HOM couplers. Figure(b) shows the results of the RF tests measured at 2 K before(green) and after (red) plasma processing. The baseline RFtest has been intentionally limited at 28 MV/m at 2 K to avoidquench. The quench field was reached at E acc = 33 . . vs E acc curve, emptysymbols for the radiation detected from the top detector, halffilled symbols for the bottom radiation detector. (b) Fig. 5. We intentionally stopped the baseline test be-fore quench in order to measure the cavity Q at 1 . / O plasma.Comparing the RF tests measured before and afterplasma processing, it is clear that plasma cleaning doesnot negatively affect the performance of the nitrogendoped single cell; on the contrary, it preserves the highquality factor and quench field characteristic of N-dopedcavities. TABLE I. Summary of the 1 . n minutes in 25 mTorr of nitrogen and m minutes of annealingin vacuum.Cavity Surface Treatment Contamination Test ScopeSingle cell ’2/6’ N-doped Plasma Processing Effect on N-doping9-cell ’3/60’ N-doped Natural FE Removal of Natural FE9-cell ’3/60’ N-doped Natural FE Removal of Natural FESingle cell ’2/6’ N-doped Aquadag ® Removal of C-contamination9-cell EP Vacuum Failure Simulated Inside Cleanroom FE MitigationSingle cell ’2/6’ N-doped Vacuum Failure Simulated Outside Cleanroom FE Mitigation9-cell ’2/6’ N-doped Vacuum Failure Simulated Outside Cleanroom FE Mitigation9-cell ’2/6’ N-doped Vacuum Failure Simulated Outside Cleanroom FE Mitigation
B. Naturally Field Emitting N-doped Cavities
We used two cavities with natural field emission to testthe efficacy of plasma processing on FE with unknownsource, not caused by an artificial contamination. Thetwo 9-cell cavities showed X-rays during the first verticaltests performed at FNAL. Both cavities were assembledwith a second valve to allow the flow of gas for plasmaprocessing and RF tested again after valve assembly. Af-terwards the two cavities were plasma processed and coldtested again.The top plot in figure 6 shows the results of thecold tests of the first 9-cell; the curves registered be-fore plasma show that the cavity quenched at E acc =18 . acc = 18 MV/m showingno X-rays. In this case plasma processing completely re-moved field emission. The fact that no change in quenchfield has been registered suggests that is a hard quench,not due to FE.The bottom plot in figure 6 shows the performance ofthe second cavity: the radiation onset before plasma pro-cessing was registered at 7 MV/m; the test was stoppedbefore the quench field due to intense radiation levels(1.1E4 mR/h at 16.5 MV/m) and final FE onset was mea-sured at 7.8 MV/m. The cold test conducted after plasmaprocessing showed the X-ray onset decreased to 7 MV/mand the quality factor degraded. Also in this case, the RFtest was interrupted due to intense FE. The Q degrada-tion could be due to higher ambient magnetic field duringcooldown, which results in increased trapped magneticflux in the cavity. The two RF tests measured on thiscavity were carried out in different cryogenic dewar, thiscould also explain the difference in trapped flux.The fact that plasma processing was effective on onecavity with natural field emission but not on the sec-ond indicates that the FE may originate from differentsources. The evidence suggests that in the first cavity theFE was caused by a hydrocarbon contamination and thatplasma processing was effective in removing it thanks tothe reactive oxygen, present in the glow discharge, thatbinds with the H x C y and creates volatile byproducts. Inthe second cavity instead the field emission may not be caused by a hydrocarbons but it could be due to surfacedefects or metal flakes on the cavity surface. If that is thecase, it is expected that the plasma cleaning would havelittle effect in mitigating FE since no volatile byproductsare generated in the reaction.
1. Residual Gas Analyzer spectrum
A Residual Gas Analyzer is used to monitor the com-position of the gas pumped out of the cavities for theduration of the entire plasma cleaning. Figure 7 showsthe RGA data acquired during the first round of plasmaprocessing applied to the first naturally field emittingcavity and it represents a typical example of the RGAspectrum registered during plasma processing. Duringthe first round of plasma cleaning, the spectrum oftenshows peaks in C, CO, CO in correspondence with themoment when the plasma is ignited in, or transferred to,a new cell. The increase in the C-related signals showsthat the oxygen is reacting with the H x C y on the cavitysurface.The RGA can record masses from 1 to 300 u, howeverin this plot are shown only the elements of interest. Infigure 7 the C-related peaks are clearly visible in corre-spondence with the plasma being initially ignited in cell C. Carbon-based contamination
We contaminated the single cell cavity, previously usedfor the N-doping test, using a carbon-based paint in or-der to study the effectiveness of plasma processing on anartificial carbon contamination. Aquadag ® , a conduc-tive paint made of graphite and ultra pure water, was Natural FEAfter Plasma Q E acc (MV/m) Natural field emission R ad i a t i on ( m R / h ) Natural FEAfter Plasma Q E acc (MV/m) R ad i a t i on ( m R / h ) Natural field emission (a)(b)
FIG. 6. Figure (a) shows the results of the first naturallyfield emitting cavity: in blue Q and radiation level vs E acc performed before plasma processing. The X-ray onset is at16 MV/m before plasma processing; after plasma processingthe FE was completely removed: the RF test measured afterthe treatment shows no X-rays. Figure (b) shows the RF testsof the second cavity. The test was stopped before quenchdue to intense X-rays. The red curves show the performanceafter plasma processing; no increase in performance has beenregistered after the cleaning. used to contaminate the cavity. A small drop of highlydiluted paint was deposited on the cavity iris. Figure 8shows images of pure and diluted Aquadag acquired withthe Scanning Electron Microscope (SEM). The Aquadagused to contaminate the single cell iris was diluted by afactor 2E4, where the dilution was calculated as the ratiobetween the H O and the Aquadag mass.For this study it was not possible to cold test the cav-ity in the same cryogenic dewar before and after plasma P a r t i a l P r e ss u r e ( T o rr) time C O H O Ne CO O CO FIG. 7. Example of RGA spectrum acquired during the firstday of plasma processing on a 9-cell cavity. Cell processing.Figure 9 shows the results of the RF tests. The con-taminated single cell was plasma processed for 17 h be-tween the two cold tests. In blue is the curve mea-sured on the contaminated cavity; comparing it withthe baseline test (here in green, same curve shown inred in Fig. 5), it can be seen that the cavity showsa degradation in the quality factor and quench field: Q = 1.7E10 at E acc = 16.2 MV/m, quench field is reg-istered at 18.8 MV/m. The radiation detector positionedat the bottom of the cryogenic dewar was not workingcorrectly during this cold test, however the top radi-ation detector was connected and no X-rays were reg-istered. After 17 h of plasma processing, the cavityexhibits an increase in quality factor (Q = 2E10 atE acc = 16.4 MV/m). Plasma processing increased thequench field by almost 15 MV/m, restoring the initialquench field at E acc = 33.5 MV/m. D. Vacuum Failure Experiments
A possible cause of cavity contamination is a vacuumleak or a complete vacuum loss. Multiple experimentswere conducted on cavities exposed to air in order tounderstand if plasma processing can be effective in mit-igating field emission in these scenarios. The tests werecarried out under different conditions, on both 9-cell andsingle cell cavities. We refer to these tests as vacuumfailure experiments (or simulations). (a) (b)(c)
FIG. 8. Scanning Electron Microscope images of pure (a) anddiluted Aquadag ® on Nb substrate. The dilution factor hasbeen calculated as the ratio between H O mass and Aquadagmass. Figure (b) and (c) show respectively Aquadag dilutedby a factor of 100 and 2E4. Q0 E a c c ( M V / m ) B a s e l i n eA f t e r C o n t a m i n a t i o nA f t e r C o n t a m i n a t i o n + P l a s m a
Radiation (mR/h)
C a r b o n - b a s e d c o n t a m i n a t i o n
FIG. 9. Quality factor versus accelerating field curves mea-sured on the contaminated cavity and after plasma cleaning.The contaminated cavity (blue triangle) shows a degradationin quench field and Q . After plasma processing it shows anincrease in quality factor (Q = 2E10 at 16.4 MV/m) and acomplete recovery in E acc (quench field is 33.5 MV/m). Ingreen is shown the baseline performance of the cavity beforecontamination.
1. Vacuum Failure Experiment Inside the Cleanroom
We conducted the first test inside a cleanroom envi-ronment, in order to introduce a controlled amount ofparticulate. High pressure rinsing (HPR) was used toclean the cavity and, after drying, it was slowly evacu-ated to high vacuum. To simulate the vacuum failure,the mini right angle valve (RAV) was opened while thecavity was in a class 100 cleanroom. The cavity quicklyreached atmospheric pressure and, after sitting at thispressure 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 h 40 min per cell). After each plasmaprocessing, the cavity was RF tested at 2 K. The RFtests on the contaminated cavity and after the secondplasma processing were conducted in the same cryogenicdewar, while the cold test after the first plasma cleaningwas carried out in a different dewar.Panel (a) of figure 10 summarizes the performance ofthe cavity during the cold tests. In blue the curves reg-istered before plasma processing (on the contaminatedcavity): the 9-cell reached a first quench at 7.5 MV/m,it was then possible to increase the power and measurethe Q versus E acc curve up to 23 MV/m (with interme-diate quenches at 20.5 and 22 MV/m), when the cavityreached the final quench. The X-ray onset was regis-tered at 18.5 MV/m for the bottom radiation detector,at 20 MV/m for the top detector. The cavity was testedalso at 1 . =2.5E10 at 16.4 MV/m) and bothradiation detectors show no X-ray activity, confirmingthat the field emission was eliminated.
2. Vacuum Failure Experiments Outside the Cleanroom
Following the experiment in cleanroom, additional vac-uum failures were simulated outside the cleanroom. Theprocedure was repeated with small variations on threecavities: one single cell and two 9-cells. After the contam-ination, the single cell cavity was plasma processed for20 h, and the 9-cell cavities for approximately 1 h 40 minper cell. After the venting, the cavities were evacuated
BaselineAfter ContaminationAfter Contamination + 1 st PlasmaAfter Contamination + 1 st & 2 nd Plasma Q E acc (MV/m) Radiation measuredduring last power riseon contaminated cavity R ad i a t i on ( m R / h ) Vacuum failure simulation in cleanroom Q E acc (MV/m) BaselineAfter ContaminationAfter Contamination + Plasma R ad i a t i on ( m R / h ) Vacuum failure simulation outiside cleanroom
BaselineAfter ContaminationAfter Contamination + Plasma Q E acc (MV/m) R ad i a t i on ( m R / h ) Vacuum failure simulation outside cleanroom
BaselineAfter ContaminationAfter Contamination + Plasma Q E acc (MV/m) R ad i a t i on ( m R / h ) Vacuum failure simulation outside cleanroom (a)(c) (b)(d)
FIG. 10. Vacuum failure experiments simulated on four cavities. Figure (a) shows the curves of the 9-cell cavity vented incleanroom; the results of the cold test perfored after the cavity was contaminated is plotted in blue; the final radiation curveswere acquired during this test at 1 . to the low E-6 - high E-7 Torr range; RF tests were per-formed before and after plasma processing. The plots inFig. 10 show the Q and radiation versus E acc curves.The single cell cavity was quickly vented through themini RAV from high vacuum to atmospheric pressure.Panel (b) in figure 10 contains the RF tests of thesingle cell cavity. The green curve shows the baselinebefore the vacuum failure experiment (red curve inFig. 9, Q =1.9E10 at 5 MV/m). The contaminatedcavity (blue curves) exhibits a degradation in qualityfactor (Q =9.4E9 at 5 MV/m) and radiation onset at 6.2 MV/m. The test was limited before quench at10 MV/m by the available RF power. Due to the severityof FE registered during the first RF test it was decidedto process the cavity for a time longer than usual (20 h).The cold test conducted after plasma processing showsa moderate increase in quality factor (Q =1.2E10 at5.1 MV/m) and decrease in radiation (8.4 mR/h at10 MV/m versus the 28 mR/h at 9.8 MV/m registeredbefore plasma) with FE onset at 7 MV/m.Panel (c) in figure 10 shows the results of the 9-cellcavity quickly vented through the mini RAV. In greenis plotted the baseline performance of the cavity beforeventing (Q =3.1E10 at 8.8 MV/m); in blue are the curvesmeasured on the cavity after the contamination: qualityfactor degradation (Q =2.6E10 at 8.7 MV/m) and FEonset at 8.7 MV/m, reaching final quench at 17 MV/mwith 1E4 mR/h; the FE onset after quench was regis-tered at 10.3 MV/m. The after plasma curves are shownin red: FE onset at 8.8 MV/m, with less severe slope inQ versus E acc curve until 12.6 MV/m; at this point anincrease in radiation was registered along with a Q drop.The Q versus E acc curve was measured again showingoverlap with the test measured on the contaminated cav-ity in terms of quench field (17 MV/m), quality factordegradation and slope due to FE.We simulated the final vacuum failure experiment byslowly opening the mini RAV on a 9-cell cavity, vent-ing the cavity over a 18 min time interval. The resultsof the RF tests are shown in panel (d) Fig. 10. Thegreen curve shows the baseline performance; in blue areplotted the curves of the contaminated cavity (beforeplasma): the cavity shows intense FE as evident fromthe slope in quality factor. A first quench was reachedbelow 3 MV/m, we then increased the accelerating field:at 4 MV/m FE started and between 4.5-5 MV/m therewas a switch in quality factor (from 7.3E9 to 1.75E10).The sudden jump in Q suggests that a field emitter wasprocessed by the RF, eliminating the radiation and en-ergy dissipation that it was causing. After this event thequality factor followed a new curve that started bendingabove 4 MV/m while the radiation increased. AnotherQ switch occurred at 6.4 MV/m, indicating that a newfield emitter was likely processed. The test was stoppedat this point due to intense FE ( > curve overlaps with the finalbefore plasma (blue) curve. Also in this case a Q switchwas observed at 7.5 MV/m, indicating RF processing ofthe field emitter. The test was stopped at 10.4 MV/m.The partial performance recovery observed in this cavityis to be attributed to RF processing occurred during the2 K vertical tests, not to plasma processing. IV. CONCLUSIONS
Using the newly developed technique of plasma igni-tion with HOMs, it was demonstrated that plasma pro-cessing successfully interacts with the cavity iris and re-moves carbon-based contamination.It was also proved that plasma processing does not af-fect negatively the performance of nitrogen doped cavi-ties, on the contrary it preserves their high quality factorand quench field. Plasma cleaning was applied to multi-ple LCLS-II cavities with natural field emission or arti-ficially contaminated. The comparison between the RF tests conducted before and after plasma cleaning showedan increase in performance in the carbon contaminatedsingle cell, in one out of two naturally field emitting cav-ities and in one 9-cell cavity exposed to vacuum failuresimulation inside the cleanroom. A second cavity withnatural field emission was processed but still showed X-rays activity after the plasma, suggesting that the sourceof FE may not be carbon-related in this case, but due tometal flakes or surface defects. The three cavities usedfor vacuum failure simulation outside the cleanroom haveshown little or no improvement attributable to plasmaprocessing.Plasma processing has shown positive results in cavi-ties with field emission onset registered at high field level(above 16 MV/m), while it has not been able to reduceFE in cases when the onset started at low fields. This canbe related with the source of FE: plasma processing, ap-plied with the current recipe and parameters, is effectiveon 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 ofthe field emitters in the cavities exposed to vacuum fail-ure simulations, it was decided to collect and analyze theparticles introduced in the cavities and not removed withplasma cleaning. Preliminary results from the analysis ofparticles collected from the single cell cavity suggest thatmetal flakes were introduced into the cavity during thevacuum failure experiment performed outside the clean-room. The SEM/EDS analysis of the particles collectedfrom the vented cavities is currently ongoing and will bethe subject of future publication.We intend to apply plasma processing to more LCLS-II 9-cell cavities and cold test them (before and after) inorder to acquire a greater statistics, focusing in particularon cavities that exhibit field emission of unknown source(natural FE) during the RF tests.
ACKNOWLEDGMENTS
This work was supported by the U.S. Department ofEnergy (DOE), Office of Science, Basic Energy Sciences(BES). Fermilab is operated by Fermi Research Alliance,LLC under Contract No. DE-AC02-07CH11359 with theU.S. Department of Energy.We thank LCLS-II-HE for providing the N-doped1 . J. Stohr, “Linac coherent light source ii (LCLS-II) conceptualdesign report,” Tech. Rep. (SLAC National Accelerator Lab.,Menlo Park, CA (United States), 2011). J. Galayda, “The Linac Coherent Light Source-II Project,”in
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