Simulations of Diffusive Lithium Evaporation onto the Lithium Tokamak eXperiment (LTX) Vessel Walls

The evaporation of lithium (Li) onto the shells of the LTX vacuum vessel has been shown to dramatically increase the magnitude and duration of plasma current pulses. This improvement in performance is believed to be due in large part to a decrease in the sputtering rate of impurities from the vacuum vessel wall, leading to a smaller concentration of impurities in LTX plasma discharges. Reduction in the strength of impurity sources with thicker Li coatings motivates maximizing the toroidal and poloidal uniformity of evaporative Li coverage. This optimization can be achieved by Li evaporation into a helium (He) background and exploiting the inverse scaling of He pressure with the mean free path of Li atoms. Three-dimensional DEGAS 2 neutral transport simulations of this process have been performed to determine an optimal sequence of He background pressures. The results of this optimization will be presented.