Photon stimulated desorption of an unbaked stainless steel chamber by 3.75 keV critical energy photons

Abstract

A stainless steel vacuum chamber, which had been going through a usual laboratory ultrahigh vacuum process, was exposed to a white synchrotron radiation photon beam, 3.75 keV critical energy, 70 W power at DCI. The vessel was however not baked “in situ” in order to test the necessity of this intermediate step. This is an issue with far reaching consequences for new synchrotron radiation sources. Beam scrubbing from a photon beam dose of 2.3×1023 photons m−1 was found to decrease the molecular yields from an initial value of 10−2 molecules per photon to below 10−5 for the main species CO and CO2. The cleaning effect of the secondaries was clearly demonstrated, thus justifying the definition of the photon beam dose. It also explains the strong reduction of the static pressure. Wall pumping developed by the irradiation of the 3.6 m long tube was found to develop a pumping speed of, respectively, 750 and 1900 l s−1 for CO and CO2 and nil for H2, CH4, and N2. The resulting pumping capacity amounts to an equivalent of 10−4 monolayer for both CO and CO2 to be compared to a total of 2 monolayers removed by beam scrubbing. These results are finally applied to predict the gas beam lifetime for a new synchrotron radiation source SOLEIL. It appears that a reasonable lifetime can be reached in a short time in the absence of in situ bakeout.