1. Evolution of Field Emission Characteristics Selected C100 Cryomodules Michael Drury

2. Outline Measurement Hardware FE Onset – Definitions Overall Statistics C100-3 History C100-4 History Conclusions

3. Measurement Hardware Vertical Test Area (VTA) –Single Ion Chamber installed inside dewar lid Cryomodule Test Facility –Octirad – 8 channels of GM tubes In use in CMTF until C100-5 tests –Decarad – 10 channels of GM tubes In use during Commissioning tests beginning with C100-1 In use in CMTF starting with C100-6 Comparisons made between two systems during C100-7 tests

4. GM Tube Locations Top Center WG2 WG3 WG6 WG4/5 WG7 WG8 WG1 BL Sup BL Ret BL Sup BL Ret WG tubes and Top Center = Radial BL Supply and Return = BL Top Center WG3/4/5 WG4/5/6WG7/8 WG1/2 DecaRad OctiRad

5. OctiRad / DecaRad Comparison Cavity C100-7-2 on 06/01/12 Similar behavior for other cavities Compared beamline tube positions and top center tube position Relatively good agreement up to about 100 mR/hr Octirad tubes saturate at much lower dose rates Onset Gradients derived from octirad data tends to be higher by ~1 MV/m

6. DecaRad Calibration Two tubes co-located with ion chamber on Test Cave Wall

7. FE Onset Definition CMTF Decarad Channels have noise floor at just above or below 1 mR/hr Onset defined as the gradient at which slope intercepts 1mR/hr line

8. FE Onset Definition VTA VTA Noise floor ≤ 1E-2 mR/hr VTA Onset = 1 st Gradient ≥ 1E-2 mR/hr For comparisons in this presentation – using the CMTF definition

9. VTA Onsets Using the CMTF definition results in higher average onset gradient for VTA data x2 cavities that do not have FE onsets by definition

10. VTA and CMTF Onsets 1 mR/hr definition Average Onset drops by 6 MV/m from VTA to CMTF Number of Cavities with no Field Emission drops by more than half

11. CMTF and Linac Onsets Average Onset Drops by 1.4 MV/m from CMTF to linac Take into consideration octirad / decarad differences in onset measurement Actual gap might be as small as 1 MV/m Largest changes appear to take place between VTA and CMTF All linac measurements made with decarad Octirad used for first 5 modules in CMTF

12. After Commissioning Three cavities showed continued improvement in Onset gradient from 2011-2012 to 2015 Four cavities showed continued loss in onset gradient from 2011-2012 to 2015 Four modules in SL were measured repeatedly from commissioning until HeP in 2015

13. Histories C100-3 –Chosen because as of January 2015 - still contains several cavities with little or no FE C100-4 –String Assembly history includes cavity swap –Module subjected to two rounds of Helium Processing. In CMTF during Acceptance In 2015 in linac Possibly most measured module

14. C100-3 Evolution Two Cavities in this module have little or no measurable FE after two years after install Gains from He Processing tend to be small in comparison with early performance

16. Evolution of FE C100-3-1

17. Helium Processing C100-3-6

18. C100-3 Prior to He Processing The other Cavities in C100-3 prior to He Processing In several cases – low to non existent FE In others – generally lower than typical for C100 cavities Why?

19. C100-3 String and Final Assembly During String Assembly- No leaks reported During Final Assembly (Secondary Waveguide Attachment) No leaks detected

20. C100-4 Evolution During String Assembly – original cavity at position 3 was swapped out First Acceptance Test showed low FE onset gradients – between 6 MV/m and 11 MV/m Also several cavities with very low quench gradients Helium Processing used to recover gradient After processing –Onset gradients between 9 MV/m and 16 MV/m

21. C100-4-5 Evolution

22. Field emission Appears to have increased over time in the linac He Processing in 2015 succeeded in recovering some FE free gradient FE at return end of BL continues to dominate other measures

23. C100-5 String and Final Assembly Cavity 3 swap out during String assembly Serpentine seal leaks reported during both string assemblies During Final Assembly (Secondary Waveguide Attachment) BL leak reported during installation of Cavity 5 Secondary WG Lowest Onsets during initial Acceptance measurements were recorded for Cavities 1, 2 and 5

24. Conclusions Largest changes in FE Onset occur between VTA tests and initial CMTF Acceptance Tests C100-3 has maintained lower levels of FE throughout its history –Assembly process appears to have been leak free with respect to cavity vacuums C100-4 has suffered some of the worst increases in FE from VTA to CMTF –Several instances of cavity vacuum leaks during both string assembly and secondary waveguide installation. Increased field emission may be result of assembly issues A few cavities have apparently improved on their own while in operation in the linacs Some cavities have continued to degrade during operation in the linac Helium processing will reduce FE but will not recover cavities to the original VTA levels

25. He Processing Pump Cart Redundant Pfeiffer PKR 251 Full Range Vacuum Gauge Research Purity Helium 99.9999% To turbo pump To Cryomodule Valve actuators Agilent 951-5106 Metering Valve Mott POU-05-SV1 Filter (hidden)

26. Helium Processing Pump Cart Pfeiffer PKR 251 Read out VAT Valve for Turbo pump Valve for helium Bottle Baratron 100 torr To monitor helium volume

27. Helium Processing Pump Cart Pneumatic valve allows helium to flow through metering valve

28. Pump Cart with turbo pump Residual Gas Analyzer

29. Variable Leak Valve Variable Leak Valve Specifications Minimum leak rate 1 x 10-9 Torr-litres/sec. in normal operation; 1 x 10-10 Torr-litres/sec. with condensable vapours eliminated from leak gas