Partial Warm-Ups at CLS Chris Regier, Ph.D., P.Eng. Mechanical/Cryogenic Engineer

Cryogenics at CLS - Outline The storage ring SRF system Gas deposition and cryopumping The CLS Partial warm-up Overview Basic data RGA results Dirty versus clean cavities Observations

The Storage Ring SRF System 500 MHz CESR-B elliptical single- cell SRF cavity Eacc = 2.05 MV to 2.4 MV Ibeam = 250 mA, decay to 150 mA 310 kW Thales TH2161B klystron RF User – no repair capability High reliability requirement

Gas Deposition and Cryopumping Pressure drops due to common gases condensing < 100 K

Gas Deposition and Cryopumping Vacuum pressure increases as cryopumping continues At CLS, issues begin to occur above 1x10-9

The CLS Partial Warm-Up Overview: Purpose: To regularly remove some contaminant gases in reasonable time without inducing large thermal stresses Full Warm-Up (290 K) Partial Warm-Up (50 K) Pulse Conditioning (Cold) Gets most condensed contaminants Gets H2, Air Gets almost nothing out – just moves it around Takes days ~14 hours ~ 4 hours Causes thermal stresses Low thermal stresses No thermal stresses Can’t do often Every 2 weeks Weekly

The CLS Partial Warm-Up Basic Data: Sublimation of gases off of the cavity inner wall Peaks indicate contaminants subliming at those temperatures

The CLS Partial Warm-Up RGA Data: RGA indicates H2, Ar, N2/CO, but no O2 H2O requires warmer temperatures

The CLS Partial Warm-Up “Dirty” vs. “Clean” Cavities – Cavity Clean Up Hydrogen peak is similar, but argon/nitrogen peak is much larger and starts earlier for dirty cavity.

The CLS Partial Warm-Up Cleanest cavity occurs after full warm-up Ar/N2 bump building backward

The CLS Partial Warm-Up Observations Partial warm-ups better than pulse conditioning but not as effective as full warm-up A good compromise Cavity vacuum creeps up over time as cryopumping continues First pressure peak is H2, next is H2, Ar, N2/CO O2 does not appear in CLS partial warm-up RGAs Second peak builds over time, and is bigger for “dirty” or new cavities Repeated warm-ups (full and partial) appear to improve cavity performance and reduce the level of condensed gases other than hydrogen. This takes a long time! Months!

The Canadian Light Source CLS is a 3rd Generation Synchrotron Canada’s only synchrotron Located in Saskatoon, Canada Population 250,000 Western Canadian prairie ~1200 km to Vancouver, ~2200 km to Toronto 13 active beamlines + 4 under construction 2.9 GeV, 170.88 m circumference E loss ~ 1.1 MeV per turn

The Storage Ring SRF System 500 MHz CESR-B elliptical single-cell SRF cavity CLS has 2 units (one operating, one offline spare) Cavity 1: Q0 = 8x108 at 8 MV/m Qext = 192,000 Eacc = 2.2 – 2.4 MV Cavity 2: (currently operating) Q0 = 1.1x109 at 6.6 MV/m Qext = 222,000 Qext reduced to 154,000 using iris in waveguide upstream of RF window Allows lower voltage (2.05 MV) for same power input

Gas Deposition and Cryopumping SR vacuum not perfect Contaminant gases migrate throughout SR Based on molecular flow principles (P < 10-4) Statistical flow – molecule moves in “random” direction after collision Molecules have better chance of colliding with chamber wall than with each other Still a “net” flow from higher to lower pressure Higher number of molecules in higher pressure region – statistically will be more moving toward lower pressure region than the other way Eventually migrate into a pump, or into the SRF cavity

Gas Deposition and Cryopumping Gases in SRF cavity at 4K Form monolayers on contact with cold surface Add “skin” to inner cavity surface RF will run through this NC skin instead of SC cavity Can trapped gas molecules form structures that act as emitters? CLS experience suggests yes May build on existing contaminant that is close to being an emitter

Gas Deposition and Cryopumping

The CLS Partial Warm-Up Overview: As temperature increases, desorbed gases sublime Temp = saturation temp at cavity vacuum pressure Note: CO has same molecular mass as N2 and similar saturation curve to Ar

The CLS Partial Warm-Up Overview: Full warm-up