C. L. Foerster and C.Lanni

Brookhaven National Laboratory

Upton, N.Y. 11973, USA

A good UHV or XHV vacuum design is a critical consideration for the successful operation of high energy accelerators and synchrotron light sources. The choice of materials for construction is a major factor in achieving a good vacuum during operation. By utilizing; good design, cleaning processes, assembly procedures, and vacuum technique, the desired vacuum can be achieved. Problems arise when the circulating beam particles cause a pressure increase due to bombardment of chamber walls by particles and/or photons produced by the beam. The bombardment causes a pressure increase due to the desorption of surface bound molecules and electrons. The increase in pressure adversely affects the operation of the accelerator or light source. In order to store circulating beam in the storage ring for extended periods of time, UHV as a minimum, or XHV is needed during operation. The loss of beam particles and the reduction of beam particle density are directly proportional to the product of residual gas pressure and path length of the beam particles. Third and fourth generation synchrotron light sources with lower emittance, smaller beam chamber cross sections, and smaller gap insertion devices require vacuum approaching XHV.

Photon Stimulated Desorption (PSD) from electron accelerator and storage ring beam chamber inner surfaces impose a serious limit on beam quality as well as beam lifetime. Aluminum, copper, and stainless steel are the common materials of choice, for the construction of the ring chambers in which particles circulate. PSD from these materials have been measured at the National Synchrotron Light Source (NSLS) on X-ray ring and VUV ring white light experimental beamline set-ups. The X-ray source critical energy is approximately 5KeV and is approximately 500eV for the VUV source. PSD yields from these materials were measured after various preparations, which include; no vacuum bake, vacuum baking, glow discharge conditioning, chemical cleaning, surface geometry, thin oxide surface and thick oxide surface. In addition to NSLS chambers, PSD measurements were performed on chamber materials from other laboratories such as; ALS, APS, DaFiNe, KEK B-factory, SLAC B-factory, and SSC. Results for room temperature measurements are reported. Chamber walls were typically exposed to at least 10E23 photons per meter, directly from the source, striking the sample at an incident angle of 10, 12, 25, or 100 mrad depending on the experimental beam line set-up.

The major PSD yields for hydrogen, carbon monoxide, carbon dioxide, methane, and water vapor are reported as a function of accumulated photon flux and sample preparation. Some typical PSD yields for chamber construction materials, measured on the NSLS beamlines, are presented. The PSD measurements are compared with those of other laboratories published for similar materials. Recent results for KEK B-meson coppers chambers and thick oxide surfaces are also given.

*Work performed under the auspices of the Department of Energy, under contract DE-AC02-98CH10886