1. Residual Gases in CEBAF Warm Beam Line Sources and Evacuation
Rongli Geng
October 12, 2015
CEBAF Performance Improvement Team Meeting

2. What is the vacuum level in warm beam line between cryomodules?
Questions
What is the vacuum level in warm beam line between cryomodules?
a Torr
b Torr
c Torr
d Torr

3. What is the residual gas species in CEBAF beam line vacuum?
Questions
What is the residual gas species in CEBAF beam line vacuum?
a. H2
b. H2O
c. CO
d. CO2
e. He
f. O2

4. Residual Gas Species – an Example
Thanks Anthony Dipette and John Heckman for assistance in accessing the data.

5. What is the effective pumping speed of the beam line vacuum pumps?
Questions
What is the effective pumping speed of the beam line vacuum pumps?
a. 35 l/s
b. 22 l/s
c. 2 l/s
d. All of the above

7. 35 l/s specification 22 l/s @ 10-9 torr best pumping speed
torr
torr

8. Case I: Gases Due to Field Emission via ESD
This surface intercepts lots FE electrons
F. Marhauser et al., SRF2013, TUP095

10. Thanks Jim Henry for creating and exporting the 3D model of the vacuum space between cavities, needed for pumping speed computation

11. Case I: Effective Pumping Speed
~ 2 l/s
~ 0.3 l/s

12. Case I continued: Launch Gas at Varied Locations

13. twice likely cryosorbed by cold cavity than evacuated by ion pump
100 mm away from step transition
(close to slotted beam tube)

14. Summary
Initial data mining reveals that the warm beam line vacuum can be as high as 10-6 torr.
Is this typical in the whole machine?
Is not this too high?
Are the cavities constantly under gas loading?
First result on effective pumping speed
Beam line ion pumps provide feeble evacuation compared to the strong cryosoption by cold cavities
Beam line ion pumps shut off once the cavities are cold?
Not much pumping benefit
Possible risk of shedding particulate field emitters

15. Gas Sources Outgassing from ceramic RF window
Inevitable RF heating
Outgassing from warm beam line surface
First/last cavity more vulnerable
Desorption stimulated by surface bombardment from field emission electrons
Accidental air leaks at warm beamline components
Gas release from cold cavity surface by thermal quenching, multipacting

16. FE Electron Bombardment Sites (Simulation)
F. Marhauser et al., SRF2013, TUP095