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

2. 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

3. 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

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

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

6. 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

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

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

9. 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.

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

11. 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!

13. 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, m circumference
E loss ~ 1.1 MeV per turn

14. 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

15. 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

16. 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

17. Gas Deposition and Cryopumping

18. 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

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