Gas desorption and electron emission from 1 MeV potassium ion bombardment of stainless steel

Abstract

Gas desorption and electron emission coefficients were measured for 1 MeV potassium ions incident on stainless steel at grazing angles (between 80° and 88° from normal incidence) using a new gas-electron source diagnostic (GESD). Issues addressed in design and commissioning of the GESD include effects from backscattering of ions at the surface, space-charge limited emission current, and reproducibility of desorption measurements. We find that electron emission coefficients γ_{e} scale as 1/cos⁡(θ) up to angles of 86°, where γ_{e}=90. Nearer grazing incidence, γ_{e} is reduced below the 1/cos⁡(θ) scaling by nuclear scattering of ions through large angles, reaching γ_{e}=135 at 88°. Electrons were emitted with a measured temperature of ∼30 eV. Gas desorption coefficients γ_{0} were much larger, of order γ_{0}=10^{4}. They also varied with angle, but much more slowly than 1/cos⁡(θ). From this we conclude that the desorption was not entirely from adsorbed layers of gas on the surface. Two mitigation techniques were investigated: rough surfaces reduced electron emission by a factor of 10 and gas desorption by a factor of 2; a mild bake to ∼220° had no effect on electron emission, but decreased gas desorption by 15% near grazing incidence. We propose that gas desorption is due to electronic sputtering.