SRF Operating Experience at JLab Andrew Hutton

Topics to Consider 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

CEBAF Overall Operating Experience >7 cryomodule-centuries operating experience at cryogenic temperature 4 cryomodule-centuries under power >56 cavity-centuries (>30 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)

Probability of replacing cryomodules 11 cryomodules have already been removed from CEBAF for refurbishment Probability of replacement is 11 in 7 cryomodule-centuries 1 – 1.5 replacement per cryomodule-century CEBAF refurbishment plan called for 2 cryomodules per year with 42¼ operating cryomodules 5 replacements per cryomodule-century Required to catch up

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

Example of “Event” Courtesy of Jay Benesch Cavity 2L145 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

Summary 5 cryomodule-centuries of operating experience at CEBAF 1 – 1.5 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

Halo CEBAF has excellent control of Halo Hall B profile is Gaussian for ~5 orders of magnitude But an FEL has even more stringent requirements

DarkLight Experiment on FEL Beam Established low beam loss in transmission of CW high power sub-100 micron beam at 100 MeV, 60 pC, 75 MHz Independently controlled betatron match of primary beam and halo. Measured beam interception as low as 3.2 PPM in <2 mm x 40 mm long aperture while running 450 kW of beam power Target Chamber Tube block Stepper motor DarkLight Collaboration

Tour New SRF Facilities: Refurbished Test Lab Vertical Test Area (VTA), Cryomodule Test Facility (CMTF) Test Lab Addition (TLA) Cavity fabrication, cavity processing, R&D facilities, cryomodule assembly CEBAF Tunnel with new C100 modules installed New CHL2 FEL

Backup Slides

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)

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 ~25K