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

Abstract

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 > 10 24 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.

Modeling the reduction of gross lithium erosion observed under high-flux deuterium bombardment

“…The procedure for obtaining measurements of the D ion flux and Li temperature, as well as the analysis methods used to infer Li erosion rates from a Phantom camera with a Li-I (670.8 nm) filter, have been previously described [8]. The temperature-dependent Li yields under 20 eV and 40 eV Ne ion bombardment are shown in Fig.…”

“…Details of the experimental apparatus and procedure for exposing thin-film (<1 lm) Li samples 25 mm in diameter to D ion fluxes C D+ 6 2 Â 10 24 m À2 s À1 at electron densities n e 6 8 Â 10 20 m À3 and temperatures T e 6 3 eV are described in [8]. The experimental procedure is similar for the neon plasma exposures described herein.…”

“…Li films on TZM molybdenum substrates were studied in the Magnum-PSI linear plasma device [7] during ion fluxes C i 6 2 Â 10 24 m À2 s À1 , electron temperatures T e < 5 eV, electron densities n e > 10 20 m À3 , and Li surface temperatures T Li < 800°C [8]. For comparison, the NSTX-U and ITER divertors are expected to experience C D+ > 10 23 m À2 s À1 and 10 24 m À2 s À1 , respectively [9,10].…”

“…It was proposed in [8] that the observed discrepancy in erosion rates was due to deuterium retention in the Li layer during Magnum-PSI experiments. In this paper, a model is developed to predict how fast a Li layer will saturate with D and what level suppression of Li erosion is expected as a function of the D/Li concentration.…”

Recent progress on spherical torus research

“…Future plans for NSTX-U PFCs include the phased replacement of graphite tiles with Mo tiles, and studies where lithium-coated Mo samples are exposed to discharges that simulate NSTX-U divertor plasmas are in progress. 299 Any effects specifically attributable to the LLD on NSTX plasma performance were less clear. For example, the fueling required to achieve and maintain stable discharges was comparable to what was needed when graphite PFCs were coated with solid lithium.…”