1. SRF Operating Experience at JLab
Andrew Hutton

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Backup Slides

12. 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)

13. 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