SRF Cryomodule 2 Cavity Test Results

Andy Hocker

TD/SRF Dev Dept.

All Experimenters Meeting, 10-OCT-2011

Context

• ILC “S1” goal: 31.5 MV/m in an 8-cavity cryomodule– DESY did it once with an XFEL prototype– Yet to be demonstrated in Asia/Americas regions

• Industry/labs are getting good at high-gradient (>35 MV/m) cavities (~50% yield)

• High-gradient cryomodules still challenging– Maintain high performance of cavities– Functionality of auxiliary components (tuners, couplers, etc.)

bare dressed CM1

x 8

Recent History

• DESY sent us a “cryomodule kit” in 2007 and helped us assemble it --- CM1– An educational tool… mainly mediocre cavities

• In parallel, New Muon Lab was re-habbed as a SRF cryomodule test facility– And eventually, a mini-ILC (e- beam, up to 6 cryomodules)– Infrastructure: cryo, RF power, shielding block caves, etc.

• CM1 was installed at NML and used to commission the facility– First cooldown in Dec 2010– Mostly successful, lessons learned…. future talk from Elvin

Harms

• CM2 now under assembly as first US-built ILC CM

Vertical Tests

ILC S0 spec

= CM2

CM2 Cavity VT Results

Q0 vs. Eacc Field Emission (X-rays)

Horizontal Tests

= CM2

ILC S1 spec

(some) CM2 Cavity HT Results

• For a dressed cavity, Q0 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

Those left behind

• 8/14 cavities tested were deemed CM2-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 CM2 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

Input Coupler Failure

Void in Cu plating

“vapor trail”

Tuners

• Slow blade tuner for static tuning of the cavity frequency

• Modifications to stepper motor linkage to address failure mode uncovered in horizontal testing

Dynamic compensation for Lorentz force detuning using fast piezoelectric tuners

Piezo OFF Piezo ON

Discussion and caveats

• One CM2 cavity 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

Conclusions

• Eight cavities have been certified in the HTS for CM2 assembly– 7 reach at least 35 MV/m, one reaches 33 MV/m

• ILC S1 goal theoretically attainable

– Cavities look good from a Q0 and FE standpoint as well

• 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.

• Will good cavities make a good cryomodule?– I’m cautiously optimistic

Thanks

CM2 under assembly at MP-9