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Gradient Degradation Experience at Fermilab (NML/CM-1) Elvin Harms TTC Meeting, Beijing 5-8 December 2011
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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
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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’
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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
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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%
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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%
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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
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Harms - TTC Meeting WG-1 5-8 December, 2011 CM-2 to Date - Vertical Results 8 = CM2 ILC S0 spec
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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
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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
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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
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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
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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
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Harms - TTC Meeting WG-1 5-8 December, 2011 Cross-Check: ACC39 14
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Harms - TTC Meeting WG-1 5-8 December, 2011 Cross-Check: ACC39 15
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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
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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
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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
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