Erosion of lithium coatings on TZM molybdenum and graphite during high-flux plasma bombardment

• A formula for temperature-dependent lithium sputtering and evaporation is proposed.
• This formula was tested using the Magnum-PSI linear plasma device.
• Lithium-coated TZM molybdenum and graphite samples were exposed to plasmas.
• Measured Li erosion rates are significantly lower than the formula predicts.
• Evidence of lithium diffusion into graphite substrates was also observed.

The rate at which Li films will erode under plasma bombardment in the NSTX-U divertor is currently unknown. It is important to characterize this erosion rate so that the coatings can be replenished before they are completely depleted. An empirical formula for the Li erosion rate as a function of deuterium ion flux, incident ion energy, and Li temperature was developed based on existing theoretical and experimental work. These predictions were tested on the Magnum-PSI linear plasma device capable of ion fluxes >1024 m−2 s−1, ion energies of 20 eV and Li temperatures >800 °C. Li-coated graphite and TZM molybdenum samples were exposed to a series of plasma pulses during which neutral Li radiation was measured with a fast camera. The total Li erosion rate was inferred from measurements of Li-I emission. The measured erosion rates are significantly lower than the predictions of the empirical formula. Strong evidence of fast Li diffusion into graphite substrates was also observed.