1. Summary, workshop for “Operating SRF reliably in a dirty machine”
Younguk Sohn
Wilhelm-Conrad-Röntgen-Campus Berlin
September 14th-15th, 2017.
※Material available at

2. Focus was on the operating machine !
AIM of Workshop
To compile experience with operating conditions of high- voltage SRF
To develop recipes for the reliable operation
Aspects to explore include:
cleanliness and vacuum requirements up/downstream of the cavities,
long-term performance degradation and mitigation schemes,
how to deal with synchrotron radiation, particle transport etc.
Focus was on the operating machine !

3. Workshop Statistics
62 Participants from Asia, Europe and North America
31 talks and 7 in-depth discussion

4. “Conclusion”
To distinguish ‘clean’ and ‘dirty’ is not clear, depending on each group and person. (~1E-9 mbar ?)
But ‘Superconducting cavity’ must be clean as possible as you can during fabrication, handling and operation.
Lessons from happenings and trials in other groups and institutes would be valuable to keep our SRF clean

5. Summary- 1 Q0 degradation between vertical test and horizontal test
Q0 degradation from long term operation, but not all
Touching warm section near SRF module for repair would produce cavity contamination degradation
Multipacting or field emission happens once, a process to mitigate is inevitable in both of test modules and operation ones
Helium processing have been soft measure against performance degradation like FE, but not
High power processing would be also helpful to recover performance partially.

6. Summary- 2
Cavity partial warmup would recover performance partially for operation cavities (usually in light sources)
Slow purging & pumping of cavity & coupler to avoid contamination. Purging with particle filter
Mechanical vibration like from valve operation and hitting produce particles
A differential pumping system between warm and cold sections, to intercept particulates by reducing the gas flow into SC part
Reasonable slow cooling of cavity

7. Recommendation for ESS and FREIA
During module acceptance tests, proper recovery procedures are to be prepared against light events like multipacting, field emission, vacuum spike and other expectable. (Even slight Eacc degradation can be managed with Q eternal adjustment outside cavity !!)
And also to prepare related systems.
Rare gas analyzer is recommended during module tests
Subsidiary devices and tools for SRF cryomodule must be compatible with clean environment.
Checking Eacc degradation with nominal performance at module test. (criteria like accept, conditionally accept, reject with simple repair, reject, ?)

8. Presentation Summary

9. General Aspect for Handling Cavity & CM -1
FPC implementation without removing the CM out of the ring under laminar air flow and slight N2 gas overpressure inside the
It takes several hours in vented vacuum systems for all particles to settle on the surface: (comment)After final cooldown, the stabilizing time would be necessary to push RF power to coupler and cavity
Mechanical vibration like valve operation and hitting produces particles transportation.
Laminar gas flow during purge and pumping vacuum system (slow purging & pumping)

10. General Aspect for Handling Cavity & CM-2
Mechanical pump like scroll pump would produce particles
Cavity gradient degradation of cERL Main Linac cryomodule at KEK (VT to HT)
From start of beam operation at compact-ERL Injector Cryomodu le at KEK, severe increase of radiation from FE. High RF power puls ed conditioning with different pulse lengths provided performanc e recovery.
Vacuum particle counter is useful to figure out particle contamination during (slow) N2 purging & pumping
During N2 purging, a particle filter is very effective to prevent particles intake to cavity

11. General Aspect for Handling Cavity & CM-3
Vibration (hitting bellow) is source of particle DESY)
Ion gun blow before installing bellow is very helpful to keep clean
Slow pumping cavities recommended.
Observed gradient degradation between VT and HT, but no obvious during yearly operation at FLASH
Gate valve opening procedure at cryomodule, with slow pumping system
During powering of cavity, pumping procedure of outgassing from cavity and coupler

12. General Aspect for Handling Cavity & CM-4
(XFEL-Desy) Repair of leaked modules in a local cleanroom onto the cavity string, without re-entering higher cleanroom.
Permanent nitrogen overpressure from inside of string to guarantee minimized particle contamination.
(HAEVY-ION LINAC AT RIKEN) A compact differential pumping system to intercept particulates by reducing the gas flow into SC part from room temperature region

13. BESSY-VSR Variable pulse-length storage ring
Long and short photon pulses simultaneously for all beam lines through a pair of superconducting bunch compression cavities
Each individual user to freely switch between high average photon flux and picosecond pulses up to 500 MHz repetition rate for dynamic studies.
BESSY-VSR preserves the present average brilliance of BESSY II and adds the new capability of user accessible picosecond pulses at high repetition rate.

14. Possible Cures for Field Emission: High Power RF Conditioning
In some cases applying high RF power to the cavity can cause the destruction of field emitters and improve the cavity gradient
In-situ processing can sometimes cause new problems
By Lutz

15. Pump & Purge, Down of “Particle Free” Sections
Adequate pump & purge procedures by means on in-vacuum particle counter. No particles are transported if either:
Flow ≤ 3 l/min, or
Pressure < 1 mbar
Automatic pump & purge units is recommended
Constant flow of 3 l/min of nitrogen or argon, by means of mass flow controllers.
Units have been widely used for XFEL
By Lutz

16. Taiwan Light Source, 13 Years Operation
No observation of prominent Q0 degradation
Operation with big margin of Eacc and forward power
Also some uncertainty in Q0 measurement

17. Statistics from TLS and TPS
TLS (2.0 GeV storage ring)
One cavity with 80 kW CW RF power
TPS (3.0 GeV storage ring)
Two cavities with 230 kW CW each

18. Turbine Problem in TPS He Refrigerator
The SRF module at TLS has been kept at 4.5 K since its last warm-up in summer of 2009

19. General Aspect for Handling Power Coupler - 1
(TPS, one and half years operation)
Development of multipacting owing to gas condensation on cold outer conductor of a high-power input coupler, even applying weekly RF conditioning for the power coupler. Obviously, we should seek a more effective technique of processing the power coupler.
Increased gas loads during the course of SRF operation even after applying RF conditioning (coupler aging) weekly.
Unable to ignite the multipacting during RF conditioning (coupler aging) does not ensure that the SEC of a conditioned surface is already less than unity! Perhaps, sufficient number of seeding electrons are not available to ignite the multipacting.
A satisfactory or even highly successful result of a SRF module during its horizontal test does not mean problematic-free operation
Add over-sized external pumping units for hydrogen

20. General Aspect for Handling Power Coupler - 2
(BEPC-II, 10 years operation)
After troubles in beam operation, a DC bias voltage (1.5kV) was used on the coupler during beam operation
Local overheating in window with big outgassing resulted in arc
Bad vacuum → serious discharging → produced a large number of ions → which bombarded the copper surface and reacted with residual gasses and formed some kind of copper compounds of CuC2 or CuH → window overheating
3 times 150 ℃ baking, the overheating relieved much
After 2 years operation, the window was cracked
Serious discharging would be related to window cracking. Snow figure in outer conductor, fish scale in ceramic surface and dark smudge in inner conductor

21. General Aspect for Handling Power Coupler - 3
(BEPC-II, 10 years operation, Jianping Dai)
Cracked coupler was replaced without dismount from tunnel, with portable clean room and with pure N2 flow over atmospheric pressure
Horizontal test with window baking (150 ℃, 63 hours). Severe Q0 degradation with high radiation observed.
Baking outer surface of cavity with hot N2 (~90 ℃) to Helium vessel, which was useful for outgassing (indium sealing module) → Q0 recovered

22. General Aspect for Handling Power Coupler - 3
Window heating in ALICE, ASTeC, STFC DL
He Processing improved cavity performance definitely, but not all.

23. PLS-II Definite Q0 degradation, between VT to HT
Also, Q0 degradation during operation
Vacuum spikes at window, mitigated with module partial warmup (up to 45 K, stay 1 hour) every two weeks at beginning of beam operation
(Model simulation experiment) Vacuum spikes took place when reflected power approaches to its optimum (minimum) region during beam operation. (Y. Joo and Y. Sohn, et al, J. Instrumentation, March 22, 2017)
For given beam current and required RF power, vacuum spikes were avoided by Increased Vacc
Position of max E-field around window changed with beam current at given Vacc
Signal spikes of cavity pickup prove, suspected by condensed gases or particles around prove

24. Gases’ Behavior during Partial Warmup
~22K
N2: Mass 28
1 Ion on by mistake
(03:40)
H: Mass 2
TMP on
1.4e-8 mbar
(01:00)
C, Mass 12
All Ion pump on
TMP off
Cavity top:43K
Cavity bottom: 31K
(06:07), 2.9e-9 mbar
Ion pump off
(00:28)
O:Mass 16
N:Mass 14
~15K
~26K
Data from PLS-II
※ Temperature would not be exact due to using non-calibrated sensors

25. DIAMOND
Cavity fast vacuum trips due to discharge in high field near coupling tongue. Substantial removal by lowering Vacc than that of specification
Leakage between beam and insulation vacuums (indium seal), maybe due to frequent thermal cycling for warmups
By every week’s cavity conditioning with pulse RF power with detuned mode, cavities’ performance maintained.
In 2015, failure of ceramic-metal braze at window during normal operation after standard conditioning
Keeping the cavities permanently cold
Installation of normal conducting cavities for resilience

26. CEBAF, JLab
Dominant gradient limitations with 52 cryomodules are “Field Emission” resulted in periodic window arcs, high radiation & heating
Original CEBAF design (old) suffers from vulnerability to periodic arcing at cold ceramic waveguide window
Usable gradient reduced by 20-30% or more
Physics program can tolerate up to 10 such trips/hr
Integrated over ~240 cavities (~34 MV net usable) voltage lost/year (~3.8%/yr)
Dirt transport through non-sealing gate valve during warm section maintenance >> poor Qs and 6 MV/m quench limits
High radiation levels due to FE around new installed cavities has led to material damage near cryomodule, some cavities turned down as a result.
CEBAF C50 CM was very significantly degraded by unexpected leak- through of beamline valve during warm girder servicing.

27. Helium Processing, JLab

28. Effect, Helium Processing, JLab
Reduced Field Emission ⇒ Reduced Dynamic Heat Load
⇒ In some cases, reduced heat load leads to higher gradients

29. Effect, Helium Processing, JLab
Sometimes, some obstacles such as
Several cases of new field emitter creation
Large increase in field emission
Lowered quench gradient
Helium migrates into waveguide vacuum
Difficult or impossible to establish gradient because of arcing
ttt

30. Horizontal High Pressure Rinsing, KEK
Q0 degradation and FE from long term operation in KEKB and by
repair of vacuum leak
replacement of coupler gaskets to change Qext.
Applying horizontal HPR (HHPR) system, without cavity dismount from module. But dismount of end groups and coupler
Recovered cavities’ performance up to original performance, without cavity baking.
It was only with single-cell cavities.
It’s for reuse in Super-KEKB.

31. FE Degradation from VT to HT, LCLS-II
In vertical test 2/3rd of the cavities (296) show no FE up to 24 MV/m. (Requirement > 17 MV/m)
Those that do field emit above 17 MV/m, none should exceed 1 R/hr, and that do are re-rinsed prior to CM assembly
In CM testing FE onset degrades. Distribution in cavities is random. Some FE starts as low as 8 MV/m
Situation is complicated by:
Assembly of RF input coupler
Copper plated bellows between cavities
The fact that JLab and FNAL do the assembly in a different manner

32. Thank you