Optimisation of the RGA location in the evaluation of NEG coating pumping properties

Abstract

Non-evaporable getter (NEG) coatings are widely used in vacuum chambers of particle accelerators. Vacuum solutions group at STFC Daresbury Laboratory is continuously developing and optimising the NEG coatings are for various applications and vacuum chamber geometries. The pumping properties of the NEG coatings are characterised by sticking probability and pumping capacity. Initial sticking probability of the NEG coated tube can be measured by gas injection at one end of the tube and partial pressure measurements at either sides, P1 and P2 respectively. This requires the NEG coating facility to be modelled with the Test Particle Monte-Carlo (TPMC) method to obtain the pressure ratio R = P1/P2 as a function of sticking probability, R(α), for each type and geometry of the with each of the NEG coated tubes. The purpose of TPMC modelling reported in this paper was to find R(α) for the NEG coating evaluation facility with a sample in a form of the 0.5-m long NEG coated tubes with inner diameters of 5, 7, 9 and 38 mm. Furthermore, the effect of RGA2 location was also studied. It has been demonstrated that the RGA2 location could significantly (from a few percent up to a factor 3.1) affect the pressure ratio R(α). The effect of pressure measurement errors on calculated sticking probability was studied for ΔR/R = 10, 20 and 50%. The sticking probability accuracy varies: Δα/α = 0.04–0.5 for ΔR/R = 10%, Δα/α = 0.07–0.8 for ΔR/R = 20% and Δα/α = 0.23–2.0 for ΔR/R = 50%.