Cleaning and vacuum conditioning of RF cavities, proton storage rings and synchrotron radiation sources

Abstract

The optimization of a chemical cleaning method for Al alloy vacuum chambers using as criteria: surface analysis, thermal outgassing, electron, X‐ray and synchrotron radiation induced neutral gas desorption, is described.The results of the vaccuum conditioning of the 128 radio‐frequency (RF) accelerating cavities for the CERN LEP e+e− storage ring are presented. The gases desorbed with RF were H2, CH4, CO and CO2 and their decrease with running time recorded.Argon glow discharge cleaning of stainless‐steel UHV chambers revealed that up to 77 monolayers of gas—mainly CO—could be desorbed from the surface. Measurements of the so‐called roughness factor—the real surface area seen by the adsorbed gas—gave numbers as high as 15 for stainless‐steel, thus only about 5 monolayers of gas are desorbed.In a dedicated beam line at the DCI storage ring at the LURE Laboratory, Orsay, France, synchrotron radiation induced neutral gas desorption from baked Al alloy and stainless‐steel chambers was measured under controlled conditions.The gases desorbed were H2, CH4, CO and CO2 and with time the H2 and CO desorption descreased as D−1/2 where D is the radiation beam dose in mA hours. A model of gas diffusion from near surface layers and from the bulk to the surface which determined the desorption characteristics was proposed. This model well described the behavior of the desorption for H2.Measurements of the photoelectron currents produced at 11 mrad glancing angle of incidence and also at normal incidence, when compared with calculation, showed differences which could be explained if the photons with energies below about 1.5 keV were reflected at 11 mrad with a reflectivity approaching 1.In a special test chamber equipped with sets of photoelectron probes, it was established that, at glancing angles of incidence of 11, 20 and 40 mrad, up to 20% of the synchrotron radiation is scattered around the vacuum chamber from the point of impact.