Residual Gases in CEBAF Warm Beam Line Sources and Evacuation Rongli Geng October 12, 2015 CEBAF Performance Improvement Team Meeting
What is the vacuum level in warm beam line between cryomodules? Questions What is the vacuum level in warm beam line between cryomodules? a. 10-11 Torr b. 10-9 Torr c. 10-6 Torr d. 10-3 Torr
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
Residual Gas Species – an Example Thanks Anthony Dipette and John Heckman for assistance in accessing the data.
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
35 l/s specification 22 l/s @ 10-9 torr best pumping speed Typically @ 10-6 torr 22 l/s @ 10-9 torr
Case I: Gases Due to Field Emission via ESD This surface intercepts lots FE electrons F. Marhauser et al., SRF2013, TUP095
Thanks Jim Henry for creating and exporting the 3D model of the vacuum space between cavities, needed for pumping speed computation
Case I: Effective Pumping Speed ~ 2 l/s ~ 0.3 l/s
Case I continued: Launch Gas at Varied Locations
twice likely cryosorbed by cold cavity than evacuated by ion pump 100 mm away from step transition (close to slotted beam tube)
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
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
FE Electron Bombardment Sites (Simulation) F. Marhauser et al., SRF2013, TUP095