1. Field Emitter Workshop Summary Rong-Li Geng 2016 CEBAF StayTreat June 28-30, 2016

2. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 In seeking understandings of the root cause of the field emission onset degradation in SRF cavities placed in CEBAF tunnel, we organized a one day workshop on February 29, 2016. The focus was on beam line field emitter particulates in the CEBAF. We reviewed the current practices in cavity string assembly, cryomodule assembly and installation, warm girder UHV components installation, operation, and maintenance. The review session was followed by panel discussion and brainstorm sessions. The workshop was attended by 26 people, including 25 Jefferson Lab colleagues from several departments and one SLAC colleague from LCLS-II project (full list of participants is attached in the Appendix A). Some participants provided written notes and feedbacks. Although it is still too early draw any conclusion, many colleagues felt the workshop was a good start toward the understanding and the ultimate control of the “CEBAF gradient degradation” problem. Some immediate actions can be taken to improve procedures that are proven beneficial in reducing introduction of particulates. A Tech note (JLAB-TN-16-008) captures the main topics discussed at the workshop, enumerates list of key questions to be answered, and lays out some action items. Slides presented at the workshop can be found on JLab Indico web page and the web link is: https://www.jlab.org/indico/event/139/ Summary Slide 2

3. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Key Questions What are the sources of particulates? How are the particulates transported? How can they be prevented in the first place? How can they be stopped from moving from non-harm region into cavity space? How can they be removed from cavities? Slide 3

4. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 List of Topics Guiding Agenda Development Observations and experiences with beam line particulates and correlation with field emission. Sources of particulates in UHV beam lines. Roles of condensed residual gases. Mechanisms of particulate transportation in UHV beam lines. UHV beam line component design for lowest particulate generation. UHV beam line component cleaning, handling, assembly and operation procedure for lowest particulate generation. Mitigation against field emitter particulate in operational SRF linacs. Techniques for removal of accumulated field emission particulates. UHV beam line particulate detection technique. Fast detectors for in operando monitoring of field emission turn on. Slide 4

5. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 As a first step toward the ultimate goal of ending the CEBAF “gradient degradation” problem, it is imperative to characterize the CEBAF beam line components in the “as-is” condition. Therefore, the presentations made at the workshop were focused on three areas: (1) cavity string assembly; (2) cryomodule assembly and placement in the tunnel; (3) warm girder components. In the discussion sessions following the formal presentations, we encouraged the participants to ask questions and share insights along the line of “finding the current conditions and current practices”. Slide 5

6. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 6

7. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 FE Impacts CEBAF Energy Reach Average voltage loss 34 MV/pass-year (this value does not include any loss from the new C100 cryomodules as there is no data yet for cavities in these new modules). Cost to just keep the CEBAF energy reach constant via refurbishing of original-style cryomodules: annual 1.3 M$ (direct); Lifetime (15 year): 20 M$ (direct). Annual voltage gain must exceed the estimated voltage loss: > 64 MV/year. Slide 7

8. FE Impacts RF Trips, therefore CEBAF Reliability R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 8

9. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Heat produced by FE Radiation produced by FE FE Impacts Activation, therefore CEBAF Reliability Slide 9

10. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Sources of Field Emitters 90% FE onset degradation originated from field emitters introduced into cavities before they were placed in tunnel Slide 10

11. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 11

12. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Field Emitters Introduced After Module Placement FE onset degradation not limited to cavities at ends of module >>> field emitter transportation required Slide 12

13. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016  Many leaks detected in the C100 strings. o 80 Serpentine seals, 7 leaks o 56 BL radial wedge seals, 0 leaks o 20 BL cavity to valve seals, 1 leak o 204 HOM/ FP seals, 1 repaired with using a higher torque  7 (out of 10) Cryomodules had leaks. o Serpentine seal leak sizes ranged from 3e-9 TL/s to 4e-7 TL/s o The one beam line leak at the VAT valve was 4e-6 TL/s Cavity String Assembly Slide 13

14. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Cryomodule assembly, placement in tunnel Warm-to-cold beam line installation done in a portable clean room with real time particle counting for feedback. The work space is rather limited. 45 l/s DI ion pump at supply-end-can side takes over cavity string vacuum. Ion pump was turned off prior to module transportation to tunnel. Ion pump re-started when module is in place in tunnel Slide 14

15. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Beamline VAT gate valve uses Viton O-ring seals, which may fail due to radiation hardening. Some beam line VAT gate valves are known to leak. Two Warm girder viewer bellows leaked during the Spring2015 run. Replacement of all 12 viewer bellows in the summer of 2015 were done. VacSeal spray is used to repair leaks in viewer bellows. Worst case ion pump pressure 9×10-5 Torr (due to beam line gate valve leak). Warm Girder Components. Slide 15

16. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Direct Evidence of Particulate Transportation DI (noble) Ion pump (Ti/Ta) Conventional Ion pump (Ti) Pilot study with module FEL2 of identical design, previously operated with beam Slide 16

17. Beamline Gate Valve R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 17

18. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Beamline Threshold Vacuum Tripping RF/GV Closure Slide 18

19. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Workshop Action Items #ActionsOwner 1 Determine the source of field emission degradation during the string assembly and tunnel installation SRF 2 Determine the next generation beamline vacuum systems. Try to get a new system in time for the C50-12 installation this summer. SRF/VAC/ OPS 3 Halt all ion-pump hi-potting on CEBAF and LERF systems. Develop a path forward, which might include replacing the pumps that need hi-potting. VAC/ENG /OPS 4Develop a particulate transport model for CEBAF. SRF/CASA /OPS 5Develop an improved temporary clean room environment for tunnel work.VAC/SRF 6 Determine the particulate generation characteristics of ion pumps, gate valves*. SRF/VAC 7Develop a beam line venting procedure for low particulate movement.SRF/VAC Slide 19

20. Open Issues/Questions 90% of the observed FE onset degradation in new 7-cell cavities occurred before cryomodule placement CEBAF tunnel. How do we fix these “delivered field emitters” so as to restore the “lost gradient”? –More helium processing? –Cryo-cycling of modules? –Pull from tunnel and re-process? –develop new techniques for particulate removal without full disassembly of cryomodule? R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 20

21. Open Issues/Questions 10% of the observed FE onset degradation in new 7-cell cavities occurred after cryomodule placement CEBAF tunnel. How do we stop adding new field emitters? –What cavity/accelerator protection is provided by the frequent GV closing in response to pressure burst? –Did we learn anything about the trip in the decadal operation of CEBAF: which protects and which doesn’t? –Separate vacuum trip threshold for RF disabling from GV closing? –Re-work all warm girders particularly in C100 zones? –Replace all ion pumps with NEG pumps? R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 21

22. Open Issues/Questions Do we have a feedback loop to capture the issues during cryomodule fabrication and bring them to the attention of the end user? Is there a lack of transparency in sharing issues such that we end up with “inspecting problem out” as opposed to “build quality in”? Does JLAB need an ultra-high-vacuum group? Which would serve as SME and system support for SRF, cryo, CIS, Halls? –Does SRF need an ultra-high vacuum physicist? R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 22

23. BACKUP SLIDES R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 23

24. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Program Committee Arne Freyberger, Accelerator Operation, AD Rongli Geng, SRF R&D, AD Geoff Krafft, Accelerator Division Office, AD Tim Michalski, Mechanical Engineering, ED Tony Reilly, SRF Operation, AD Slide 24

25. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Participants Slide 25

26. CEBAF SRF Cavities Original CEBAF cavity CEBAF upgrade cavity R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 7-cell, Low-Loss Shape 80+8 cavities in 10+1 modules Design –Ea=19.2 MV/m –Q 0 =7.2×10 9 @ 19.2 MV/m Achieved – =22.2 MV/m – =8.1×10 9 2x (600 + 500) MV Add ~5 kW 2K cooling power Add ~ 5 MW liquefier operation power 5-cell, Cornell-Type 338 cavities in 42-1/4 modules Design –Ea=5 MV/m –Q 0 =2.4×10 9 @ 5 MV/m Achieved – =7.5 MV/m, =5×10 9 @ 5MV/m –Helium processing Achieved – =12.5 MV/m, =5×10 9 @ 5MV/m –Refurbishing 2x 600 MV 5 kW 2K cooling power 5 MW liquefier operation power Together for 12 GeV nuclear physics run Slide 26

27. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 R. Bachimanchi, J. Benesch, M. Drury, A. Freyberger, R. Geng, J. Mammosser, “2014 Update: CEBAF Energy Reach and Gradient Maintenance Needs”, JLAB-TN-14-024 CEBAF Gradient Degradation Loss rate: 34 MeV/pass-year. based on operations data since 1995. Mitigation: helium processing module refurbishment Root cause ??? Slide 27

28. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Slide 28

29. R.L. Geng, 2016 CEBAF StayTreat, June 28-30, 2016 Ion chamber used in VTA and CMTF: CANBERRA AM-IP100. Detector Sensitivities 0.4 μ R/Pulse. Detector Range: 100 μ R/h - 100 R/h. Energy Range 50 keV to 3 MeV. (data retrieved specification sheet published on vendor website) Geiger-Muller (GM) tubes used with OctiRad DAQ: Thermo Scientific HP- 270. Gamma Sensitivity: ~1200 cpm/mR/h (137Cs). Detector range: from back ground to 100 mR/h for non dead time corrected instruments or up to 3 R/h for those equipped with dead time correction capability. Energy Range: 20 keV to 1 MeV. (data retrieved specification sheet published on vendor website) GM tubes used with DecaRad DAQ: LND, INC 714 Gamma Detector. Gamma Sensitivity: 90 cpm/mR/h (60Co). Detector Range: 2 mR/h - 500 R/h (60Co). Energy Range: 40 keV to 500 keV. (Information provided by Omar Garza of Engineering Division, JLAB. Range data retrieved from information provided by Bill Lehnert of LND, Inc.) Slide 29