Re-deposition of lithium and boron coatings under high-flux plasma bombardment at normal and grazing magnetic incidence

Summary form only given. Thin coatings composed of low-Z elements (lithium, boron, beryllium, carbon) are applied to graphite or metal substrates in tokamaks. Because these coatings improve plasma performance, it is important to characterize how fast the film thickness changes. The erosion rate of these low-Z elements is strongly dependent on ion flux Γ
_i
and surface temperature T
_surf
. Re-deposition rates vary with electron temperature T
_e
and density n
_e
. Thus it is crucial to understand the evolution of these films during plasma bombardment at similar n
_e
(> 10
¹⁹
m
^-3
), T
_e
(<; 15 eV), Γ
_i
(> 10
²³
m
^-2
s
^-1
), T
_surf
(<; 800°C), and magnetic geometry found in the divertor. An analytic/numerical model was developed to simulate the mechanisms of particle loss/gain from thin films during plasma bombardment. Predictions of Li erosion rates
¹
were obtained through empirical extrapolation from existing measurements. The model predicts re-deposition fractions exceeding unity for Li-coated TZM Mo during bombardment at normal magnetic incidence for n
_e
= 10
²⁰
m
^-3
and T
_e
> 1.5 eV. The time evolution of Li and B coatings on graphite and TZM Mo were studied in Magnum-PSI
²
, a linear plasma device with Γ
_i
>10
²⁴
m
^-2
s
^-1
and T
_e
<; 4 eV. A series of 2-7 s plasma exposures at both normal and grazing magnetic incidence were run on bare samples of each substrate and after a deposition of 100-1000 nm of Li or B. Line radiation was monitored using a camera with an optical filter. Parameter scans were performed in T
_surf
(250-1000°C), T
_e
(0.5-4.0 eV) and ne (0.7-4.0*10
²⁰
m
^-3
). Measurements of the Li erosion rate on TZM Mo at normal incidence were significantly lower than predictions from the empirical formula
¹
. A mechanism involving chemical interactions between Li and D was hypothesized. Results from exposures of Li-coated TZM to neon plasmas will be discussed. Initial observations indicate Li redeposition fractions between 0.9 and 0.98. Further measurements of Li and B re-deposition fractions at normal and grazing incidence will be reported.