Comparison of synchrotron radiation induced gas desorption from Al, stainless steel and Cu chambers

Abstract

The synchrotron radiation induced neutral gas desorption coefficients for Al, stainless steel and Cu have been measured by exposing test vacuum chambers made from those materials to 2.95 keV critical energy synchrotron radiation in a dedicated beam line on the DCI positron storage ring at the LURE Laboratory, Orsay, France. In addition to the initial desorption coefficients, the decrease of the gas desorption with photon dose is also described. Al was found to have the largest initial desorption coefficients, with Cu a factor of about 10 lower and the lowest desorption coefficients were from stainless steel, where a 950 °C vacuum degassing treatment had little effect. Integration of the desorption coefficients with respect to photon dose gave the total quantity of gas desorbed. For a photon dose of 3×1021 photons/m, almost 1 Torr 1/m of H2, CH4, CO and CO2 were desorbed from Al, 10−1 Torr 1/m from Cu and 5×10−2 Torr 1/m from stainless steel. In parallel, measurements of the photoelectron currents, believed to be the primary cause of the neutral gas desorption, were carried out. The highest photoelectron currents were measured in the Al chamber, followed by Cu with the lowest in the stainless steel chambers. Using the results from the Al test chamber, the predicted pressure increases in the LEP vacuum system, when exposed to synchrotron radiation, agreed well with those actually measured.