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

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

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) 20-32. http://dx.doi.org/10.1016/j.nima.2017.02.009

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

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

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.

Backup slides

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 50-60 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

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