Tin re-deposition and erosion measured by cavity-ring-down-spectroscopy under a high flux plasma beam

Abstract

Cavity-Ring-Down Spectroscopy (CRDS) was implemented to measure the re-deposition of liquid tin under a high flux plasma beam in the linear plasma device Pilot-PSI. A capillary porous system (CPS) consisting of a molybdenum cup and tungsten meshes (pores diameters of 0.2 mm and 0.44 mm) was filled with tin and exposed to argon plasma. The absorption of a UV laser-beam at 286.331 nm was used to determine a number of sputtered neutral tin atoms. The incoming flux of argon ions of ~50 eV was 1.6−2.7x10 23 m-2 s-1 , and the sample temperature measured by pyrometry varied from 850 °C to 1200 °C during exposures. The use of CRDS for measuring absolute number of particles under such plasma exposure was demonstrated for the first time. The number of sputtered tin particles in the cavity region assuming no losses would be expected to be 5.5x10 11 −1.2x10 12 while CRDS measurements showed only 5.7−9.9x10 8. About 98−99.8% of sputtered particles were therefore found to not reach the CRDS observation volume. Spectroscopic ratios of Sn I to Sn II ions, as well as equilibrium considerations, indicate that fast ionization as well as plasma entrainment of neutrals is responsible for the discrepancy. This would lead to high re-deposition rates, implying a lowered contamination rate of core plasma and lower required replenishment rates at high-flux conditions than would otherwise be expected.

Liquid metals as a divertor plasma-facing material explored using the Pilot-PSI and Magnum-PSI linear devices

“…It may be feasible to design components where Sn evaporation is the limiting factor, especially given there are very large uncertainties in the tolerable erosion flux. In this case, a high redeposition rate as measured in [82] and as would be expected in the highly dense partially detached divertor conditions in DEMO, would be able to increase the operational temperature range and power handling by as much as an additional 5 MW m −2 [26]. Erosion by stannane production may be of concern as an additional source of Sn and little is known about its behaviour under fusion-relevant conditions.…”

Power handling and vapor shielding of pre-filled lithium divertor targets in Magnum-PSI

“…This seems to be contradicting the literature. In [33] and [34], the redeposition of tin and lithium respectively has been investigated and R is found to be >0.999 in both cases. A possible explanation for this discrepancy could be that both these works experiments are conducted in Magnum-PSI where the redeposited species was at an impurity level density several orders of magnitude below the electron density.…”

Vapour shielding of liquid-metal CPS-based targets under ELM-like and disruption transient loading

“…It should be noted that Sn has a much lower vapour pressure and retention rate [8,14]. Therefore, tin-based CPSs potentially have a higher power handling [2,8,10,14]. A post-mortem examination of the Sn-CPS target exposed to the heat plasma fluxes of up to 18.1 MW m −2 showed no essential damage caused by the exposure of the CPS.…”

“…Capillary porous systems (CPSs) filled with liquid metal (Li, Sn) are considered now as an alternative approach for plasma-facing components of heavily loaded divertor in a fusion reactor [1,2]. This approach is comprehensively tested now using different plasma-surface interaction (PSI) devices [2,3,[5][6][7][8][9][10][11][12][13][14] as testbed facilities to analyse the material response to extremely high particle and heat fluxes attributed to the transient events like disruptions and repetitive giant ELMs. Among the favorable effects for the LM divertor approach could be a strong vapour shielding of exposed surfaces that decreases essentially both the resulting surface load and erosion.…”