SRF Operating Experience at JLab Andrew Hutton Thomas

SRF Operating Experience at JLab Andrew Hutton Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 1

Overview § Description of the SRF installation at CEBAF § Operating gradient compared to peak gradient § Protecting the cryomodule against catastrophic failure § Protecting the cryomodule performance against long-term degradation § Optimum operating conditions (temperature, vibration, magnetic fields, etc. ) § Probability of replacing cryomodules § Summary Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Jefferson Lab Accelerator Site CEBAF SRF recirculating linac Test Lab at the Institute for Superconducting Radio-Frequency Science and Technology FEL CHL Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

CEBAF Operation Optical fiber-based, RF-pulsed drive lasers 499 MHz ΔΦ= 120 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

SRF Installation in CEBAF Cryomodules in the accelerator tunnel Superconducting radiofrequency (SRF) cavities undergo vertical testing. An aerial view of the recirculating linear accelerator and 3 experimental halls. A B C Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

SRF Installation in CEBAF 42¼ Cryomodules 2¼ in the Injector 20 in each Linac Each cryomodule has eight 5 -cell cavities, installed as 4 pairs Total: 338 5 -cell cavities Each Cavity has individual klystron and RF module Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

RF Commander Screen Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

CEBAF Cryomodule Performance § CEBAF cryomodules were designed for CW operation § Maximum beam power 1 MW (typically > 0. 5 MW) § Original CEBAF cryomodules: § Design gradient = 5 MV/m § Achieved gradient* = 5 - 8 MV/m § Improved cryomodules § Design gradient = 12. 5 Me. V/m § Achieved gradient* = 12. 5 - 15 MV/m § CEBAF 12 Ge. V Upgrade cryomodules § Design gradient = 19. 2 MV/m * Average over 8 cavities Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy in the cryomodule Andrew Hutton SRF Operations 9/27/06

CEBAF Overall Operating Experience § >5 cryomodule-centuries operating experience at cryogenic temperature § 3 cryomodule-centuries under power § >40 cavity-centuries (>25 cavity-centuries under power) § Initial performance of all cavities was improved by Helium processing § Total failures § 3 cryomodules (damaged during warm-up) § 2 cryomodules with severely degraded performance § All five were early production cryomodules § 6 cavities (mostly stuck tuners) § Cavity degradation § 65 MV/year/CEBAF (due to new electron emission sites) Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Cryomodules Removed from Service Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

System Availability FY 05 Q 4 – FY 06 Q 3 Downtime due to SRF 0. 5% Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Operating gradient compared to peak gradient Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 12

Peak Gradient § For a few CEBAF cavities, peak gradient is quench limited § The maximum operating gradient is set 1 Me. V/m below quench gradient limit § If all cavities were quench-limited this would be the end of the story § ILC should be in this condition § Unfortunately, most CEBAF cavities are limited by fieldemission § No hard cut-off § Difficult to define peak gradient § Effects can have long-term impact Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Effects of Field Emission (1) § Arc trips § Arcs are due to field-emitted electrons charging up the ceramic windows § The particular RF station and the beam is tripped off to remove all sources of energy from arc § Arc rate is function of gradient § Fits to Fowler-Nordheim plot § New CEBAF cryomodule design eliminates arc trips by putting dogleg in internal waveguide § Gradient is “nuisance” limited to about 10 trips per hour for the accelerator, but we have run up to 15 trips per hour § Recovery from each trip takes <30 seconds Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Effects of Field Emission § Field emission also causes radiation near the beam line § Initially established a limit of 1 rad § Beam loss monitors were set to trip above 1 rad § Then we got greedy (Users pushed for more energy) § Radiation limits were relaxed § Viton seals on beamline vacuum valves in cryomodules suffered radiation damage § Most do not seal properly now § Major problem when removing cryomodules from tunnel § Must prevent adjacent cryomodules being vented to air Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Protecting the cryomodule against catastrophic failure Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 16

Linac Energy History 1997 - 2007 6 Ge. V Test Hurricane Isabel Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Low Energy Operation Andrew Hutton SRF Operations 9/27/06

CEBAF Catastrophic Failures (1) § In September 18, 2003 Hurricane Isabel caused 4 -day power outage § Two cryomodules damaged by uncontrolled warm-up § Loss of insulating vacuum caused thermal gradients which flexed internal Indium beamline vacuum joints, causing Helium leaks into the cavities § Affected early production cryomodules § Our (partial) solution is to provide emergency generator power for vacuum pumps and controls § Insufficient emergency power to keep Central Helium Liquefier operational (start-up takes even more power) § Future machines should have emergency power capacity to maintain cryomodules at 4 K indefinitely Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

CEBAF Catastrophic Failures (2) § In January 2005, a module failed during removal of adjacent cryomodule for refurbishment § Cryomodule was taken to ambient temperature because beamline vacuum valve leaked allowing back-filling with air § Indium beamline vacuum joints leaked after cool down § Cryomodule ambient temperature cycle procedure was modified to reduce possibility of thermal transients § Cryomodules were probably already weakened by flexing during Hurricane Isabel § Cryomodules with slow Helium leaks into the beamline suffer performance degradation over ~ 1 month § Can be recovered by thermal cycle to ~25 K Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

CEBAF Maximum Energy 1995 -2010 Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

CEBAF Catastrophic Failures (3) § Lessons learnt § Minimize use of Indium seals § Already applied to new CEBAF modules § Minimize warming up the cryomodules § Do it slowly over temperature range of large expansion § Avoid radiation damage of viton seals in vacuum valves § Do not operate in heavy field-emission regime § Replacing viton seals with all-metal valves is not necessarily better - likely to create particulates § We now use procedure for warming cryomodule to ~20 K and backfilling with gaseous Helium to prevent air infiltrating the viton seals Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Protecting the cryomodule performance against long-term degradation Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 22

Cavity Degradation Courtesy of Jay Benesch § From January 1995 through February 2003, there were 64 instances of spontaneous onset or change in cavity field emission § This corresponds to 8 per year in CEBAF § 2. 4 events per cavity-century § Since 2003 (post Isabel), rate has climbed to 13 per year § 4 events per cavity-century § Drop in usable gradient averaging 1. 4 ± 0. 8 MV/m § 0. 7 MV per pass, 3. 5 MV top energy (5 pass) § Leads to loss of ~ 45 MV/year top energy § We add 50% safety margin for predictions – 65 MV/yr Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Example of “Event” Courtesy of Jay Benesch Cavity 2 L 145 Red circles are before 04: 40 on 9/21/2004; Blue squares are after At 8. 1 MV/m, interval changed from ~80, 000 seconds to ~500 seconds Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Response to Degradation § Indications are that “events” correspond to a particle that moves to a place with high fields in the cavity § May be from outside, maybe from lower-field position § We have no solution for avoiding this degradation § A regular program of Helium processing can diminish or eliminate emitters after they have appeared § There is some risk of degrading the cavities further § Long-term solution is regular, scheduled refurbishing of cryomodules § In situ liquid rinsing (CO 2, Ar) has been proposed by John Mammosser (but not yet attempted) Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Optimum operating conditions Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 26

Temperature § CEBAF operates at 2. 1 K § 2. 1 K Helium is produced at the Central Helium Liquefier (CHL) and distributed to cryomodules § Different from XFEL and proposed ILC configuration § Operating temperature should be derived from cost optimization when Q 0 dependence on temperature is known § Adequate Helium storage must be provided if the cavities are brought up to 4 K to during long downtimes § Liquid Helium increases in volume by 15% between 2 K and 4 K Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Cryomodule Overview Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Optimum operating conditions § Vibration § There was one instance of a large electrical transformer in the tunnel affecting adjacent cryomodules § Installed to provide power for additional pumps § Found by correlation analysis § Performance improved by relocating transformer § Vibration even more important at higher gradient Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Optimum operating conditions § Magnetic fields § CEBAF tunnel has magnetic fields of 2 -5 gauss § Impacts maximum gradient § Be careful using high currents in the tunnel, especially during construction § Welding, etc. § Magnetizes the iron rebar used to reinforce the concrete § Cannot be demagnetized later § For XFEL and ILC, need to be careful routing high-current cables in the tunnel to minimize current loops Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Probability of replacing cryomodules Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 31

Probability of replacing cryomodules § 4 cryomodules have already been removed from CEBAF for refurbishment § Another cryomodule is seriously underperforming § Probability of replacement is 5 in 5 cryomodule-centuries § 1 replacement per cryomodule-century § CEBAF refurbishment plan calls for 2 cryomodules per year with 42¼ operating cryomodules § 5 replacements per cryomodule-century § Required to catch up Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

CEBAF Cryomodule Refurbishment Plan Energies for trip rates <15/hour Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Summary § 5 cryomodule-centuries of operating experience at CEBAF § 1 cryomodule failure per cryomodule-century § Failures exposed design flaws § Did not appear for ~7 years (3 cryomodule-centuries) § New designs will avoid these problems § But will have different problems § Gradient degradation of ~1% per year from new field emission sites § Strongly recommend regular refurbishment program § 1 -2 refurbishments per cryomodule-century Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

The End Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Extra Slides Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations /SC-PAC 2001 -6. 19. 01 9/27/06 36

Linac RF Operation § Operation of the linacs requires sophisticated high-level software § Automated Cavity Tuning § Make it resonate at exactly the right frequency § Linac Energy Management § Set up exactly the right energy profile § KREST § Set up exactly the right phase for each cavity § MOMOD § Maintain the right phase for each linac Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Automated Cavity Tuning § Sweep Mode (Exact tuning) § The cavity frequency is modulated over a range of ± 200 Hz in steps of 5 Hz. § The response is typical of a resonant system to harmonic excitation § The measured de-tuning angle ψ as a function of modulating frequency is compared to the predicted curve to determine the resonance frequency as well as the phase offset ψ0 § Autotrack (locks cavity on frequency) § Uses the phase offset ψ0 determined above to keep the cavity tuned to the operating frequency Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Linac Energy Management (LEM) § For a given energy at the end of North or South Linac and available cavities: § Optimizes the cavity gradient distribution using individual cavity characteristics § Calculates energy profile along the linacs § Calculates Linac quad values consistent with calculated energy profile § Downloads and sets RF, Quads (including hysteresis), and skew quads § Required input § Maximum gradient permitted for each cavity § Current required in linacs § List of available cavities § Integrated into the maintenance log § Most useful number – energy overhead available (usually 6 – 10%) Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06

Global Procedures § Krest § Modulate the phase of each cavity § Observe the change in energy required by fast feedback system to maintain experiment energy constant § Change cavity phase so energy does not change with modulation § Repeat for each cavity § Each cryomodule is done by hand at start-up § Momod (parasitic) § Modulate the phase of the RF phase reference for each Linac using two different frequencies ~800 Hz during accelerator operation § Move the cavity phase until energy does not change with modulation § Linac is now on crest Thomas Jefferson National Accelerator Facility Operated by Jefferson Science Associates, LLC for the U. S. Depart. Of Energy Andrew Hutton SRF Operations 9/27/06