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\ifmmode^\circ\else\textdegree\fi{} and 88\ifmmode^\circ\else\textdegree\fi{} 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 ${\ensuremath{\gamma}}_{e}$ scale as $1/\mathrm{cos}(\ensuremath{\theta})$ up to angles of 86\ifmmode^\circ\else\textdegree\fi{}, where ${\ensuremath{\gamma}}_{e}=90$. Nearer grazing incidence, ${\ensuremath{\gamma}}_{e}$ is reduced below the $1/\mathrm{cos}(\ensuremath{\theta})$ scaling by nuclear scattering of ions through large angles, reaching ${\ensuremath{\gamma}}_{e}=135$ at 88\ifmmode^\circ\else\textdegree\fi{}. Electrons were emitted with a measured temperature of $\ensuremath{\sim}30\text{ }\text{ }\mathrm{e}\mathrm{V}$. Gas desorption coefficients ${\ensuremath{\gamma}}_{0}$ were much larger, of order ${\ensuremath{\gamma}}_{0}={10}^{4}$. They also varied with angle, but much more slowly than $1/\mathrm{cos}(\ensuremath{\theta})$. 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 $\ensuremath{\sim}220\ifmmode^\circ\else\textdegree\fi{}$ 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.