Collisional and thermal effects on liquid lithium sputtering

Abstract

The lithium sputtering yield from lithium and tin-lithium surfaces in the liquid state under bombardment by low-energy, singly charged particles as a function of target temperature is measured by using the Ion-surface Interaction Experiment facility. Total erosion exceeds that expected from conventional collisional sputtering after accounting for lithium evaporation for temperatures between 200 and $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Lithium surfaces treated with high-fluence D atoms are bombarded by ${\mathrm{H}}^{+}$, ${\mathrm{D}}^{+}$, ${\mathrm{He}}^{+}$, and ${\mathrm{Li}}^{+}$ at energies between 200 and $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and 45\ifmmode^\circ\else\textdegree\fi{} incidence. Erosion measurements account for temperature-dependent evaporation. For example, $700\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ ${\mathrm{He}}^{+}$ particles bombarding the D-treated liquid Li surface at room temperature result in a sputter yield of 0.12 Li/ion and at temperatures $\ensuremath{\sim}2.0{T}_{m}$ (where ${T}_{m}$ is the melting temperature of the sample), a yield near and above unity. The enhancement of lithium sputtering is observed to be a strong function of temperature and moderately on particle energy. Bombardment of a low-vapor-pressure lithium alloy (0.8 Sn-Li), used for comparison, also results in nonlinear rise of lithium erosion as a function of temperature. Measurements on both pure liquid Li and the alloy indicate a weak dependence with surface temperature of the secondary ion-induced secondary ion emission. Treatment of liquid Li surfaces with D, yields reduced sputtering under ${\mathrm{He}}^{+}$ impact by a factor of 5--6 when measured at room temperature due to preferential sputtering effects.