1. TTC 2019 – WG1 summary 05-08 Feb. 2019 TRIUMF, Vancouver
Julien Branlard, Rongli Geng

2. WG1 Performance limitation in operational facilities
WG mission:
Performance limitation in operational facilities
including both elliptical and non-elliptical variants
longer term operation of test facilities
cavity performance experience
Safe limits
Multipacting limitations and mitigations
Field emission
Voltage and phase stability
microphonics,
Lorentz force detuning
Vector sum (pulsed and cw)
4K vs 2K operation,
trapped flux (local magnetic sources)
cooldown dynamics
TTC WG1 summary

3. WG1.1 Cavity performance experience-safe limit
Title
Speaker
Affiliation
Long Term Operation of a Large Ensemble of SRF Cavities in CEBAF
Arne Freyberger
JLAB
Long term experience with SRF cavities at ELBE 
Peter Michel
ELBE
Summary of the maximum SCRF voltage in XFEL 
Nick Walker
DESY
Operation and performance limits of the LHC cryomodule
Katarzyna Turaj
CERN
Long term experience at INFN Legnaro 
Giovanni Bisoffi
INFN
Overview of 12-year operation of the SCL at SNS 
Sang-Ho Kim / John Mammosser
ORNL
TTC WG1 summary

4. WG1.1 Cavity performance experience-safe limit
Long term experience with SRF
CEBAF running with original cryomodules (1990’s) and added new cryomodules since 2014
ELBE, LHC 17 years of operation
ALPI running since 25 years and needs to go for another 20 years
Eacc: general trend is down
when you’re operating at the top, the only way is down
Across the labs, mostly due to field emission
TTC WG1 summary

5. CEBAF
TTC WG1 summary

6. ELBE
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7. WG1.1 Cavity performance experience-safe limit
BUT
SRF is a “survivor” (A. Freyberger)
Mitigation approaches
LHC: building spares
CEBAF: refurbishment, maintenance, helium proc., avoid field emission by lowering gradient
SNS: in situ plasma processing, spare cryomodules
ELBE: partial gradient recovery through HPP
ALPI : generation transition
TTC WG1 summary

8. CEBAF
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9. SNS
TTC WG1 summary

10. WG1.1 Cavity performance experience-safe limit
XFEL new but high statistics
Particular case of VS (one cavity limiting 31)
No degradation from HTS to tunnel (within error bars)
On-going effort to assess and maintain max gradients
TTC WG1 summary

11. XFEL
TTC WG1 summary

12. WG1.2 Field emission and multipacting: experience and mitigation
Title
Speaker
Affiliation
Field Emission and Mitigation in the CEBAF Linacs 
Bob Legg / Rongli Geng
JLAB
Exceptional events during operation of the European XFEL
Mathieu Omet
DESY
Operational Experience of Cavity/CMs in ISAC-II QWR linac
Zhongyuan Yao
TRIUMF
TTC WG1 summary

13. Field emission and multipacting
CEBAF
Increased field emission over time
He processing successful but not always
Current approach is mitigation via cryomodule re-work and particulate control
Plasma processing being considered
EuXFEL
Couple of examples of successful processing of field emitters
Some cavities routinely operated in quench regime (cryo acceptable “soft” quench)
Observed plasma discharges (e- quench)
ISAC-II
Maintenance and refurbishment program essential to increase available gradient
Common issues: Q disease, magnetic pollution from solenoids, multipacting happens < 100 kV for QWR, field emission (residual gas after thermal cycle),
Yearly warm up cool down (cost saving reasons but increase nb of issues)
Multipacting processing in SEL  speed up process
TTC WG1 summary

14. CEBAF
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15. XFEL
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16. ISAC-II
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17. WG1.3 Voltage and phase stability
Title
Speaker
Affiliation
LLRF system performance limiting factors during CW operation of high Ql cavities cryomodule
Wojciech Cichalewski
TUL
LLRF compensation and mitigation of two cavity instability
Ramona Leewe
TRIUMF
CEBAF C100 Fault Classification Based on Time Domain RF Signals
Tom Powers
JLAB
Experience with LCLS-II cavity and cryomodule tests at Fermilab
Elvin Harms
FNAL
TTC WG1 summary

18. Field stability LCSL-II CMTB TRIUMF
Cryomodule tests at FNAL (14/20), procedure
Microphonics issues now under control
Mass flow as preferred technique for Q0 measurement
Coupler heating not an issue
CMTB
R&D program towards a CW EuXFEL
Main challenge is CW in VS
Static drops (manageable in SEL, but can be a real challenge in VS)
Coupler heating leads to QL drop  more power for same gradient
Cross coupling between neighboring cavities
IOT as power source: non linear behavior
TRIUMF
E- linac (2 cavities operated in VS)
Ponderomotive oscillations
Analytical derivation to identify conditions leading to unstable behavior
Unclear if piezo can offer a possible mitigation : to be tested
TTC WG1 summary

19. CW in VS
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20. CW tests at CMTB
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21. Field stability CEBAF Fault analysis using fault logger
Time consuming task to identify and categorize faults
Use data to perform machine learning and help identify common fault signatures
Help operators take the right action ( smaller impact on gradient)
Fault classification
TTC WG1 summary

22. WG1.4 2K versus 4K operations, He induced microphonics
Title
Speaker
Affiliation
The 8T solenoid and the local magnetic shielding in the FRIB cryomodules
Kenji Saito
MSU
Comparision of 4K/2K operation performance CEBAF injector cryomodules
Grigory Eremeev
JLAB
Experience with the CeC PoP 4K 1/4 Wave Photocathode Gun and 2K 5-Cell Cavity at RHIC
Kevin Smith
BNL
Experience with cryogenics-induced microphonics on LCLS-II cryomodules
Tom Peterson
SLAC
Commissioning of 704 MHz SRF Booster Cavity for LEReC
Wencan Xu
TTC WG1 summary

23. WG1.4 – Microphonics, 2K versus 4K
FRIB
Local magnetic shielding developed and applied
Demonstrated comparable Q0 with mu-metal for magnetic shield
JLAB
4K vs 2K comparative test of two cryomodule designs
Original cryomodule design permits large gradients at 4K close to 2K
4K test with beam : not significant difference in terms of beam quality
BNL
Multipacting issues  hard to condition away
Developed a procedure for overcoming multipacting
FNAL
Valve leak at the origin of He gas bubbles
Once problem understood, detuning +/- 40 Hz  +/- 20 Hz
BNL (LEReC)
Conversion of SRF gun cryomodule to and SRF booster cryomodule
Issue with microphonics
Conditioning of field emitter during operation
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24. From TTC chairman’s report
TTC WG1 summary

25. Foster synergies and collaborations
Microphonics oscillations mitigation in vector sum control
TRIUMF
DESY
Efforts on-going for root cause studies – more needed
Instrumentation needed for FE studies and perhaps operation control.
More effective techniques for rapid performance recovery from FE needed.
JLAB
SLAC …
TTC WG1 summary

26. Lessons learnt
Fortunate to have speakers who are “not conventional TTC participants”, valuable point of view for TTC
Not enough time for discussion during session
Schedule less talks
More time for discussion
We proposed a structure for WG1 talks:
Problem statement, known solutions, future tests, proposed collaboration
Several talks followed
More needed - Focus on issues, (not a PR talk)
TTC WG1 summary

27. Thank you to all WG1 speakers and thank you for your attention!
TTC WG1 summary