[FFA_CEBAF_Collab] vacuum with SR

[Jay Benesch]

Colleagues, 

The first few paragraphs of the following may be of interest.  
https://acceleratingnews.web.cern.ch/news/issue-39/future-circular-collider-fcc/fcc-ee-vacuum-system-towards-prototyping

I found most striking: The presence of SR, directly affects the vacuum in the beam pipes/in the accelerator via the photon-stimulated desorption mechanism (PSD). In the presence of SR, the internal wall of the vacuum chamber starts giving off molecules. At the start up of a new machine, the PSD coefficient η can be as high as 0.1 molecules/ph, depending on the material of the chamber and its surface treatments.  Past lepton accelerators have shown that when η=10-6 molecules/ph the corresponding vacuum lifetime is of the order of tens of hours and it becomes longer than that of other lifetime-limiting processes (e.g. Touschek lifetime). Historically such low value of η is reached after few hundred ampere-hours of integrated beam dose (the integral of the beam current multiplied by time).  

I estimate 40,000 hours max of beam since the 12 GeV era began.  At best 100 uA in each arc.   Product: 4 amp-hours.  It follows that CEBAF is unlikely ever to reach low photo-desorption. 

Jay