RF Upgrades and Experience at JLab: CEBAF Enhancements and Future Directions

1. RF Upgrades & Experience At JLab Rick Nelson

2. Outline Background: CEBAF / Jefferson Lab History, upgrade requirements & decisions Progress & problems along the way Present status Future directions & concerns

3. CEBAF at Jefferson Lab • Design • 4 GeV, 200 uA • 3 Experimental Halls • Present (pre-upgrade) • 6 GeV, 200 uA • 3 Experimental Halls • Upgrade • 11 GeV, 200 uA • 3 Experimental Halls • 12 GeV, 200 uA • 4th Hall D only

4. Upgrade magnets and power supplies CHL-2 Upgrade Existing Halls From 6 to 12 GeV

5. RF + Upgrades • Original • 42.5 cryomodules/338 SC cavities • 340 klystrons: 5 to 6.5 kW CW, 1497 MHz • Incremental upgrades to C50 ongoing • C25, C50 cryomodules • Upgrade • 10 cryomodules, 80 SC cavities • 80 klystrons: 13 kW CW, 1497 MHz • New designs for klystrons, power supplies, circulators, controls

6. Key RF Requirements 10 new zones of RF power for new accelerating structures: • 1497 MHz • Operating Gradients Required • >17.5 MV/m • RF Power per cavity • 13 kW saturated • Regulation requirements • (table) • Cavity QL • ≥ 2x107 Cavity de-tuning curve

7. How Many RF Sources? 1 per cavity (existing configuration) Minimum impact of failures 1 per zone or 1 per linac Larger impact on faults High power splitters High power amplitude and phase control required with high precision. Additional controls and high power modulators found to be more $$ than individual RF sources. Single LINAC upgrade shown

8. CEBAF Klystrons Compared

9. HV DC Power Supply • Design adapted from electrostatic precipitator application (higher volts/lower amps & in oil) 1000+ units in the field at award • Highly tolerant to load faults • Lower stored energy than T-R, fast turn off on fault, series resistor limits output current (no crowbar) • Each system powers 8 klystrons (as before) • Resonant mode switcher design (15-20 KHz) • 4 separate supplies. Each feeds 2 klystrons • Minimizes klystrons taken offline due to power supply failure • Controlled as a “unit” • Each adjustable to -15kV • 15 A total

10. Additional Views • HV Deck (4 per system, on rollers) Rear View

11. Typical RF Installation All zones installed and commissioned with beam

12. Tunnel Connections Waveguide installation

13. RF Commissioning Selected Data

14. Good Times / Bad Times • Everything according to plan? • Delivery delays on several key components • Klystron & general WG close to schedule with no problems • HOM filters, isolators, solenoid power • Revisions and rework for problem areas • One contract cancellation • Multiple installation delays with starts & stops • Largely from budget constraints • Resulting in reassignment of workforce • Inefficient to change tasks to often

15. Isolator Requirements • 13 kW CW, full reflection • 0.2 dB insertion loss • 21 dB isolation (any phase & power) • Water cooled • PM only - no TCU • Operates adjacent to others • Awarded to Ferrite (also supplied 350+ units for original CEBAF) • Full power testing at JLab • Vendor test capabilities missing • Several rounds of testing with sliding short • Using FA klystron at L-3 • At JLab using 2 x 6.5 kW and 13 kW klystron

16. Events • Initial tests looked OK and first lots were installed • Tests into WG shorts not as good • Results not repeatable/consistent with similar test connditons • And, performance varied based on phasing (Distance to short) • Two rounds of measurements & adjustments to understand the fix • Tried to characterize performance and make adjustments before next production batch • Next production units still variable

17. Testing

18. Sensitive to Match & Phase • Isolation affected by • Ferrite temperature • Magnetic field strength • Could be adjusted to maintain good performance • Less field needed at higher temperature • Other solutions include TCU, active field control (VSWR) • “Automatically” handled in small units • Reflected phase • Match (all ports) • 2 of 3 need to be good for high isolation • Load OK, short bad, klystron needs to be good (but not easily measured)

19. Early Test Results

20. Early Test Results

21. Isolation vs. Short Position

22. Temperature • Initial measurements done steady-state • Find position for lowest isolation (worst case, run plots) • Changes then observed at turn-on • Concerns for off-resonance conditions at turn-on • Must avoid tripping on high reflected power at turn-on • Must work under varying conditions due to differing distances to cavities

23. Isolation vs. Heating/Time • RF heating of ferrite resulted in significant changes over (short) time

24. Adjusting Magnetic Fields

25. Isolator

26. Tuning

27. Resolution • Vendor reworked dome – new domes, improved cooling, full rebuild all units • … results were still variable • LL tuning abandoned in favor of full power setup at JLab • 100% re-tested at power • Some adjustment to requirements allowed – lower isolation at lower power • Reflected power well below threshold for klystron damage or performance degradation • Final solution meets operational needs including credible fault conditions • All 84 units modified, tested, reinstalled • Considerable extra work -- all units were handle multiple times installed/removed/reinstalled.

28. Isolation vs. Short Position

29. HOM Filters • Uncertainty of need, though requirements known • Originally not needed, later added back in • Belief was only 2 of 8 cavities would require HOM filter • Normal procurement process • 2 vendor offers • Final units essentially identical to what we had from multiple purchases • Performance met, cost lower though alternate proposal was more robust • Small tweaks to reduce fundamental absorption • Manufacturing relied on external shops (as before) • Vendor a small concern, limited resources & staff • Fabrication subcontracted (metal fab, Iridite, dip brazing) • Dummy spool pieces installed in other positions • Had expected this to come in last

30. Klystron HV PS • Performance has been good overall • Switcher design and controls work well - good reliability • DSP-based controls with hardware safety interlocks • Code changes needed to address timing issues • 1 unit tested OK, but all 4 might trip external breaker • Extended step-start to deal with high inrush & breaker trips • Possible race condition for contactor control vs. status reporting (several contactors changed but seem to be OK) • Control transformers (480:120) shorted out • Loose connections/loosening connections • Contactors, IGBT • Suggestion: check connections… • No similar problems with old supplies, but a lot less connections • DC power guys regularly check transistor connections • New doesn’t mean perfect -- especially after x-country trips • A couple noticeable events

31. Flash, Boom – Tripped 3 Breakers

32. Cause & Effect

33. IGBT Connection Another loose connection IGBT overheating & short circuit Checked torque on all connections, all systems No early signs noted – just tripped

34. Problem Procurements • Isolator (discussed) • HOM filter (discussed) • Klystron solenoid PS • Offer looked good on paper • Major design effort resulted in delays and cancellation • FA looked like a lab prototype – and didn’t work long • Test results failed to meet their results (tried 3 ways) • Ultimately cancelled and purchased from Sorensen

35. We Didn’t Buy This One • Liberal use of RTV to insure components didn’t shift during shipping… • Kluge board • Not First Article class • Order cancelled

36. Installation Challenges Funding shortages resulted in work reassignments & delays Techs reassigned to dismantle other systems(multi-month delays) Start/stop/start not efficient and required relearning (In spite of this RF finished on time and below budget)

37. A Wet Year • Brazing issues and water leaks • Multiple new components, nuisance problems • Both believed to be of similar origin – but different suppliers • Pressure tested (but not long enough) • Trapped flux dissolved out resulted in small leaks on a few pieces • Circulator load assemblies • New loads built, and testing refined • Solenoid leaks on plumbing • Longer pressure testing with hot water

38. Ongoing Circulator (old style) • Reliable for a lot of years, but load failures becoming more frequent • LC DI water • 15 years+ erosion and leaching • Self-rebuilding w/o retuning • Same load back to its circulator • New circulator loads won’t experience this failure mode

39. Water Flows Downhill Water level – horizontal run Bleed hole was for air… All LCW was turned off during extended down (~1 year) Circulator load seals lost their seal Water in select waveguides (not our selection) HOM filters soaked

40. Maintenance Issues • New systems to be learned and maintained • New systems to be re-checked • Old systems getting older • Spares needed for new (and old alike) • Major PM efforts planned for summer • Pushed off many times already

41. 20 Year Old Transformers

42. Summary • All new LINAC RF has been installed and commissioned, though not without issues along the way • Operating requirements met • Staff still learning operational maintenance differences from old systems • Maintenance activities scheduled for summer down – old and new systems • Lobby to purchase spares with new equipment

43. 12 GeV Timeline 2009 12GeV Upgrade construction starts in May with ground breaking ceremony at the Hall-D site. 2011 First C100 installed in the 2L23 slot in CEBAF, July. 2012 C100 module successfully operated at design specifications: 108MeV of energy gain with 465μ A of beam loading on May-18. 2013 North and South Linac 2K LHe operations established, Dec-09 for the first time with two CHL's plants connected to a "split CEBAF". 2013 12GeV CEBAF Beam Commissioning begins Dec-13. 2014 Beam successfully transported to the 2R dumplette with 2.2GeV/pass energy gain on Feb-05. Establishing RF capability to support 12GeV 5.5pass operation with greater than 50% availability. 2014 Injector achieves 12GeV design energy of 123MeV on Mar-10 2014 3-pass beam established to Hall-A Mar-20. Multi-pass capability established in the 12GeV era. 2014 3-pass beam with E>6GeV established to Hall-A on Apr-01 and beam-target interactions recorded. First time beam transported to an end-station with energy that exceeds maximum energy set during the 6GeV CEBAF era. 2014 10.5GeV 5.5 pass beam established to Hall-D Tagger dump on May-07. 2014 First RF separated beams in 12GeV era on Oct-??. Establishes multi-beam capability in the 12GeV era.

44. Thank You!