Ion desorption stability in superconducting high energy physics proton colliders
- E. O. Lawrence Berkeley National Laboratory, University of California--Berkeley, Berkeley, California 94720 (United States)
In this article we extend our previous analysis of a cold beam tube vacuum in a superconducting proton collider to include ion desorption in addition to thermal desorption and synchrotron radiation induced photodesorption. The ion desorption terms introduce the possibility of vacuum instability. This is similar to the classical room temperature case but is now modified by the inclusion of ion desorption coefficients for cryosorbed (physisorbed) molecules which can greatly exceed the coefficients for tightly bound molecules. The sojourn time concept for physisorbed H{sub 2} is generalized to include photodesorption and ion desorption as well as the usually considered thermal desorption. The ion desorption rate is density dependent and divergent so at the onset of instability the sojourn time goes to zero. Experimental data are used to evaluate the H{sub 2} sojourn time for the conditions of the Large Hadron Collider (LHC) and the situation is found to be stable. The sojourn time is dominated by photodesorption for surface density {ital s}(H{sub 2}) less than a monolayer and by thermal desorption for {ital s}(H{sub 2}) greater than a monolayer. For a few percent of a monolayer, characteristic of a beam screen, the photodesorption rate exceeds the ion desorption rate by more than two orders of magnitude. The photodesorption rate corresponds to a sojourn time of approximately 100 s. The article then turns to the evaluation of stability margins and the inclusion of gases heavier than H{sub 2} (CO, CO{sub 2}, and CH{sub 4}), where ion desorption introduces coupling between molecular species. Stability conditions are worked out for a simple cold beam tube, a cold beam tube pumped from the ends, and a cold beam tube with a coaxial perforated beam screen. In each case a simple inequality for stability of a single component is replaced by a determinant that must be greater than zero for a gas mixture. (Abstract Truncated)
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 285897
- Journal Information:
- Journal of Vacuum Science and Technology, A, Vol. 14, Issue 4; Other Information: PBD: Jul 1996
- Country of Publication:
- United States
- Language:
- English
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