PHOTON DESORPTION MEASUREMENTS OF COPPER AND COPPER PLATED BEAM TUBES FOR THE SSCL-20 TEV PROTON COLLIDER

Relativistic protons circulating in the SSCL 20 TeV collider emit synchrotron radiation (10(16) photons/m/s, E(crit) = 284 eV). This radiation will photodesorb gases from the walls of the beam tube. The 4.2 K superconducting magnet bore tubes will act as very long distributed pumps. Build up of cryosorbed gases could severely limit collider operation and may require a bore tube liner to hide cryosorbed molecules from the photon flux and to increase the H-2 pumping capacity. To predict collider vacuum performance, photon stimulated desorption experiments have been running at the National Synchrotron Light Source (NSLS) at BNL and on the VEPP2M storage ring at BINP, Novosibirsk, Russia. At the U10B beamline of the VUV ring at NSLS, beam tube samples were exposed to over 10(23) photons/m with E(crit) = 486 eV and incident angle = 12 mrad. Unbaked copper and copper plated stainless steel have been tested. In addition, results have been obtained with a 1100 G dipole magnetic field and with a 350-degrees-C vacuum bake. Results are presented and compared with other measurements at the NSLS and at other laboratories.