Comparative study of photodesorption from TiZrV coated and uncoated stainless steel vacuum chambers

Abstract

A three-gauge method of measurements (Nucl. Instrum. Methods A 359 (1995) 110) is described and used to study the photodesorption yields and sticking probabilities of a stainless steel test chamber coated with approximately 3 μm of TiZrV getter material. Using the measured sticking probability for hydrogen of 0.007 for the TiZrV coated chamber and the known sticking probabilities of CH 4, CO and CO 2 of 0, 0.5, 0.5, respectively (Vacuum 60 (2001) 57), the photodesorption yields are deduced. After activation, the H 2, CH 4, CO, and CO 2 desorption yields are found to be 1.5×10 −5, 2×10 −7, <1×10 −5 and <2×10 −6 molecules/photon, respectively. No saturation effect is observed even after a large photon dose. These results are compared to those from stainless steel test chamber measurements performed for the same geometry under identical conditions. After saturation with CO and during continuous CO injection, the effect of the irradiation is to reduce the pressure, contrary to the normal dynamic behaviour of a conventional vacuum system. This observation is explained in terms of photon induced pumping of the CO by the TiZrV coating.