Cold beam tube photodesorption and related experiments for the Superconducting Super Collider Laboratory 20 TeV proton collider

Abstract

The next generation of proton colliders that has been contemplated—the Superconducting Super Collider (SSC) in the U.S. and the Large Hadron Collider (LHC) at CERN—will be the first to encounter significant intensities of synchrotron radiation within the cold bore tube of superconducting magnets. Aside from removal of the synchrotron radiation heat load, the main problem encountered is how to deal with the magnitudes of photodesorbed gases. Choosing the beam tube to coincide with the cryosorbing magnet bore tube has the advantages of simplicity and, in principle, of providing very high pumping speed. Tightly bound H, C, and O in the near-surface layer (∼100 Å) are converted by photodesorption to a steadily increasing surface density of physisorbed molecules. However, the effective pumping by the bore tube is greatly reduced by the photodesorption of relatively weakly bound physisorbed molecules. In addition, the saturation vapor density of H2 at the ∼4.2 K temperature of the SSC cryostats exceeds, by a factor of fifty, the upper bound allowed by the nuclear scattering deposition of energy in the magnet cryostats. Consequently, accumulation of a monolayer of physisorbed H2 must be avoided even locally. An alternative approach is to install a coaxial perforated tube or liner inside the magnet bore tube which allows the photodesorbed gases to be pumped out of the view of the synchrotron radiation photons. The purpose of the work described in this paper is to develop a methodology that will allow prediction of the SSC beam tube vacuum for simple, 4.2 K beam tubes and for distributed pump or liner configurations—and to provide the technical data required for choosing among the alternative possibilities. The first photodesorption experiments have been completed on the VEPP2M storage ring at the Budker Institute of Nuclear Physics (BINP). Additional photodesorption experiments are underway at BINP and are being planned for a beamline at the UV ring of the Brookhaven National Laboratory National Synchrotron Light Source (BNL NSLS). Related experiments at BNL measuring molecular sticking coefficients and at the State University of New York-Albany (SUNY-Albany) measuring the depth profile of hydrogen on beam tube surfaces are also beginning to yield data. New ideas for directly measuring molecular density inside a cryosorbing beam tube are under development—neutralization of H− and H+ beams at BINP and positron annihilation at BNL. A status report of these activities is given in this paper.