1. Strategy for operation and recovery of performance at SNS -Background -History of performance -What is our operational strategy -Performance recovery and -improvement
SCL Systems Group, SNS
S-H. Kim, R. Afanador, D.L. Barnhart, B.D. Degraff, M. Doleans, S.W. Gold, M.P. Howell, J. Mammosser, C.J. McMahan, T.S. Neustadt, J.W. Saunders, K. Tippey, D.J. Vandygriff, and D.M. Vandygriff

2. SNS Superconducting Linac
Two types of cavities (βg=0.61 medium beta and βg =0.81 high beta) covers acceleration from 186 MeV to 1000 MeV (or beyond for upgrade)
33 medium beta cavities in 11 cryomodules and 48 high beta cavities in 12 cryomodules
7 new cryomodules will be installed through Proton Power Upgrade (PPU) project (1.3 GeV) FE
DTL
CCL
SRF, b=0.61
SRF, b=0.81
1000 MeV
upgrade
Accumulator Ring
Liquid Hg Target
259 m
157 m (SCL)
71 m
Empty slots
for upgrade
Helium transfer lines in the tunnel
Cryomodules
in the tunnel

3. Numerous lessons have been learned to achieve a stable and reliable operation of the SCL*
Availability last 7 years:
Whole SCL including RF, HVCM, Control, Vacuum, etc.: ~98 %
SRF cavities, cryomodules, and CHL: >99 %
Average trip or downtime: <1 trip/day or <5 min./day
*S-H. Kim, R. Afanador, D.L. Barnhart, M. Crofford, B.D. Degraff, M. Doleans, J. Galambos, S.W. Gold, M.P. Howell, J. Mammosser, C.J. McMahan, T.S. Neustadt, C. Peters, J.W. Saunders, W.H. Strong, D.J. Vandygriff, D.M. Vandygriff, “Overview of ten-year operation of the superconducting linear accelerator at the Spallation Neutron Source,” Nuclear Inst. and Methods in Physics Research A, 852 (2017)

4. Performance degradation during operation
Degradation related to vacuum activity
Trigger by electron activity, gate valve operation, ion pump pressure spike, beam halo, errant beam, etc.
Desorption and redistribution of gas and particulate contamination  could create conditions for vacuum breakdown or hot spots
Effect of event can be intensified by interaction with RF (one of the worst case is surface damage)
Not every event makes a cavity trip. But the probability for degradation increases with frequency and intensity of events
Most frequent symptoms are the creation of hot spots and partial quench in the end group
Actions have been taken to minimize these unwanted events
Fast beam abort at errant beam condition
Minimize valve operation
Turn off ion pump on CM23

5. Operational strategy Every cavity trip is investigated by SRF experts
In-depth involvement for machine operation by SRF experts
SRF experts are responsible for Eacc setup, machine conditioning, and troubleshooting
Operators contact SRF experts for any changes
When a cavity shows a symptom of performance degradation, the gradient is lowered slightly (typically lowered by 1 MV/m) to avoid further degradation and to minimize downtime
Early diagnostics and prompt involvement from system experts are the key to minimize additional degradation
Usually recover performance during maintenance period

6. Performance recovery and improvement
Recovery of cavity performance to previously attained operating gradients
RF conditioning and/or thermal cycling
Out of all the thermal cycles conducted, only two cavities not fully recovered
Improvement to new higher operating gradients by in-situ plasma processing* starting Jan. 2016
Five cryomodules have been successfully plasma processed:
One offline cryomodule as a part of R&D and four cryomodules in the tunnel
25% Eacc gained on average
Beam output energy has been increased from 939 MeV to 990 MeV
Cryomodule repairs
All repairs have been done through access ports
5 cryomodules were removed from the tunnel for offline repairs
~50 thermal cycles of cryomodules competed to date
*M. Doleans, et al, "In-situ plasma processing to increase the accelerating gradients of superconducting radio-frequency cavities." Nuclear Inst. and Methods in Physics Research A, 812 (2016) 50–59.

7. Backup slides

8. Examples of cryogenic load changes due to hot spot
An example of performance degradation resulting from a hot spot. In this example, the cavity accelerating gradient can be kept stable with much higher dynamic load. 10-% increase of the JT valve position in this example indicates that dynamic heat load increased by W. The JT valve position refers to an amount of the JT valve opening in percentage
An example of performance degradation resulting from a hot spot. In this example the cavity accelerating gradient needs to be lowered by 0.5 MV/m to avoid a quench

9. End group model with hot spot
Temperature (K)
2.1
4.6
5.8
7.0
8.2
9.4
10.6
11.8
13.0
14.2