1. Gradient Degradation Experience at Fermilab (NML/CM-1) Elvin Harms TTC Meeting, Beijing 5-8 December 2011

2. Harms - TTC Meeting WG-1 5-8 December, 2011 Outline Introduction CM-1 experience CM-2 to date Cross-check - ACC39 Identified sources A real problem? Summary 2

3. Harms - TTC Meeting WG-1 5-8 December, 2011 Introduction Fermilab has limited but increasing statistics to gauge cavity performance degradation as SRF cavities go through process from bare cavity to completed cryomodule.  CM-1  CM-2  ACC39 3 CM-1 CM-2 ACC39 ‘bare’ ‘dressed’

4. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-1 Experience Cavities were delivered to Fermilab separately under vacuum from DESY Long time from delivery to cool down and first operation (~3 years) On average, cavities went through four vacuum cycles at Fermilab before cool down and first operation  Venting and pump down was done carefully but not as controlled as recent cavities

5. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-1 Experience 5 Fermilab mean Gradient = 23.7 MV/m Average gradient degradation is 19%

6. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-1 Experience 6 620  s flattop 120  s flattop Cavity 1/Z89 Change in Peak gradient 620 vs. 120  s flattop 0 - 30 MV/m Q L = 0 - 5E6 13% 8% 2% 3% 24% 18% 24%

7. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-1 Experience: Degradation ‘Soft’ quenches seen on some cavities  Especially 1/Z89, 3/AC73, 7/Z91  Drop in Q L as gradient increases  Cryogenics sees accompanying heat load, but no quench  Not accompanied by significant field emission  HOM heating? no thermal straps to HOM feedthroughs One Tuner motor non-functional after 20 minutes of cold operation  Motor leads One piezo non-functional One cavity shows change in Q L  Thermal time constant of ~20 minutes  Spontaneous changes Coupler vacuum activity  Related to adjusting Q L /motorized couplers  Makes 5 Hz/higher gradient operation unstable

8. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2 to Date - Vertical Results 8 = CM2 ILC S0 spec

9. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2 to Date 9 Administrative Limit of 35 MV/m enacted ILC S1 spec = CM2

10. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2 Horizontal Results Courtesy of Andy Hocker One CM2 cavity (A16) had a miserable horizontal test  Extremely high heat loads, hard to even do a meaningful test  Upon disassembly of input coupler, glitter-like flakes of copper found stuck to antenna tip and elsewhere FNAL and SLAC working with vendor to understand plating process and improve QA  HPR and subsequent vertical test of cavity (dressed) showed very good performance, so used for CM2 One “good” cavity was removed from HTS, and then immediately reinstalled and re-tested  Result: FE observed where there had previously been none Changed cavity handling procedures to eliminate contamination risk State of previously “successful” cavities (four) called into question  HPR and subsequent horizontal test showed good performance again Re-HPR’d the four CM2-ready cavities, but no horizontal re-test

11. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2 Summary Courtesy of Andy Hocker For a dressed cavity, Q 0 must be measured from heat dissipated to cryo 1 W or less, DIFFICULT measurement Q vs E in general agrees with VT results to within a factor of 2 But usually a little lower Plan to re-visit the methodology, investigate alternate approaches

12. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2: Those left behind 8/14 cavities tested at HTS were deemed CM-2-worthy, what about the rest?  2 performed just as well as they did in VT, but were not 35 MV/m One went to KEK for the S1-Global project, one was a CM-2 backup  1 quenched early (33 MV/m) due to heating from FE Has since been HPR’d, likely OK now  1 had extremely high FE that was NOT fixed by HPR  1 quenched early (29 MV/m) for reasons unknown (no FE) Very few quench diagnostics possible with a dressed cavity  1 had an input coupler breakdown that contaminated the cavity No FE before breakdown, extremely high FE after breakdown Breakdown happened at ~37 MV/m, led to administrative HT limit of 35 MV/m Courtesy of Andy Hocker

13. Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2: Reasons for Optimism Most phases of cavity production under Fermilab supervision Most cavity tests were successful  Demonstrates that high performance can be maintained through the dressing process  The failed tests have helped point out areas of improvement for the CM production chain Coupler QA Handling, etc. Decreased lag time compared to CM-1 More experience

14. Harms - TTC Meeting WG-1 5-8 December, 2011 Cross-Check: ACC39 14

15. Harms - TTC Meeting WG-1 5-8 December, 2011 Cross-Check: ACC39 15

16. Harms - TTC Meeting WG-1 5-8 December, 2011  Cavity performance results through chain are counter-intuitive: improved performance?  ACC39 performance limit controlled by HOM heatingof ‘old’ style/2-leg F-piece on 2 of the 4 cavity HOM’s  Attention - especially transport of cavity string under vacuum ACC39 16

17. Harms - TTC Meeting WG-1 5-8 December, 2011 Data from Fermilab shows some cavity degradation CM-1- a realistic situation? In addition to cavities, one must pay attention to other contributors to System degradation  Tuning systems (fast and slow)  HOM’s  Couplers  Handling  Sources of Field Emission What about uncertainties in RF calibration? Is Degradation a Real Problem? 17

18. Harms - TTC Meeting WG-1 5-8 December, 2011 Fermilab now has experience with assembling three Cryomodules - varying levels of involvement Performance degradation is on the radar System degradation is bigger cause for concern that simply cavity performance CM-1 has been a valuable test bed CM-2 comes on line in first half of 2012 - its performance will tell us much Thank you for your attention! Summary 18