1. Field Emission and Mitigation in the CEBAF Linacs R Legg, R Geng Jlab SRF Ops Dept.
TTC,

2. Upgrade magnets and power supplies
new Hall 12 11
6 GeV CEBAF
Upgrade is designed
to build on existing facility:
vast majority of accelerator
and experimental equipment
have continued use.
Upgrade magnets and power supplies
CHL-2
help me
Two 0.6 GV linacs
1.1
Enhanced capabilities
in existing Halls
Maintain capability to deliver lower pass beam energies: 2.2, 4.4, 6.6….

3. Problem Description
Field emission (FE) is generated and transported in CEBAF
Reaches energies of 100’s of MeV
FE strikes cavities and causes beam trips
FE charges cavity windows which cause beam trips
Pipe size step downs and magnets convert the field emission stopped to gamma radiation which damages organic molecules (cables, epoxy, etc.) on girders between modules

4. Other Issues Spurious trips correlated to CM Activation
Cavity Trips
Gas load increase around cryomodules
Spurious valve closures
Frozen gas in cavities
Electronic quenches

5. One Source of Problem
C100 (12 Gev Upgrade) Cavities are seven cell “Low-loss” design
Worked well in horizontal test bed
Field emission threshold was not specified
End groups larger diameter than cavity irises
Cavity design transports FE from cavity to cavity
Operated above known FE onset gradient
Corollary: Don’t put machine energy in project name.
Field emission has increased over time

6. Increase of Field Emission with time
How do we mitigate the problem?

7. Mitigation: Local Shielding
For C100s, we attempted to locally shield equipment with lead collars around beampipe adjacent to modules
Pb Collar
OSLs
BL-1
BL-2
MAG-BL
OPTICHROMIC
EC-1
EC-2
Girder

8. Good and Bad News
Good – Data fits closely with earlier dataset. Lead collars did attenuate radiation by 3x from one side to the other.
Bad – 10 HVL of lead should attenuate 1000x.
Core of radiation cone which passes through the shielding in the beam pipe gave a dose at the girder 3x higher than measured on the face of the cryomodule.
Local shielding is not practical in this case.
Normalized Dose
Distance, in

9. Mitigation: Post Processing
Helium processing of cavities performed July 2015
Plot shows the distribution of changes in the gradient at which radiation becomes detectable by the DecaRad system *
*Courtesy of Mike Drury

10. Mitigation: Reduce source of Field Emission
Cavities were field emission free when qualified.
Rebuild C100 cryomodules using LCLS-II tools and procedures for clean assembly
When module is replaced, clean the adjacent warm girder too. * *
* Courtesy of C. Reece

11. Mitigation: Modeling of Problem
Use Fowler-Nordheim fit to cavities from commissioning data
Also used on C20 modules in CEBAF (arXiv: )
For C100 cryomodules, an additional multiplier is added for energy gain in adjacent cavities
Straight forward solver to maximize EACC vs Dose rate for a cryomodule.
For C20s we maximize EACC vs trip rates
Reality is a problem though;
Field emitters pop-up and process away in cavities
FE characteristics change over time; (gas load?)
Users want the maximum EACC possible.

12. Mitigation: Reduce Radiation by Gradient Redistribution
CEBAF Injector Area Test w/ w CM’s in series
0L03(C20) total output voltage maintained at constant
Lowering gradients in two FE source cavities
Raising gradient in two FE free cavities
0L04(R100) at constant gradient
Radiation reduced by
5 order of magnitude
While preserving total
accelerating voltage output
0L03-3 lowered by 3 MV/m
0L03-7 lowered by 3 MV/m
0L03-5 raised by 3 MV/m
oL03-6 raised by 3 MV/m
Requires knowledge on FE source cavity and FE onset
May not always work due to other constraints
In-situ FE instrumentation

13. Thank you for your attention.
Next Steps
Field emission is a problem, but
Can be modeled and optimized
Mitigation can be realized
He processing
Run below the FE onset threshold
Particle free, clean assembly of linac hardware
Routine testing to stay within limits
Jlab is in process of rebuilding the first C100 Cryomodule using LCLS-II clean room and techniques to mitigate field emission by raising onset thresholds.
Thank you for your attention.