Tungsten as target material in fusion devices

Abstract

Several experiments were conducted in ASDEX Upgrade to prove the suitability of tungsten as a divertor target material under the conditions of a high density and low temperature divertor. The observed fluxes from a tungsten tile into the plasma are low, in keeping with the extremely low sputtering yields. In addition, the very favourable effect of `prompt redeposition' (redeposition during the first gyration) could be confirmed by the experiments. Cooling of the edge region by neon injection seems permissible, i.e. neon impurity sputtering did not increase the eroded fluxes of tungsten. The transport and accumulation behaviour were investigated by means of the laser blow-off technique. No accumulation effects could be observed in ohmic discharges. In discharges with NBI heating but without ICRH, strong accumulation can occur. High heat flux tests were performed on graphite tiles coated with plasma sprayed tungsten, which withstood a thermal load of 15 MW/m2 lasting 2 s as well as 1000 cycles of 10 MW/m2 for 2 s without disabling damage. Owing to the encouraging results, an experiment using a tungsten divertor is planned in ASDEX Upgrade

Molybdenum sources and transport in the Alcator C-Mod tokamak

“…The important elements of the erosion/ redeposition model used in this study are based on a study by Naujoks et al [9]. With boron being the main low-Z impurity following boronization of the Alcator CMod vessel, the molybdenum influx is calculated from…”

“…the fraction of the ions redeposited within the first gyration. The calculation of r is based on the description given elsewhere [9,15]. The result, without including the sheath potential, is (see Eq.…”

“…An average charge Z Mo = +1.5 is used for the promptly redeposited Mo ions. This is based on the fact that at high electron densities, such as is the case for Alcator C-Mod, particles can be ionized multiple times before completing their first orbit [9]. Although these additional ionizations will reduce the probability of prompt redeposition, some of the ions will make it back to the target.…”

“…Source rate calculations have also been routinely made, especially in tokamaks with low-Z plasma interaction surfaces [5,6]. However, calculations involving high-Z plasma interaction surfaces have become more frequent the last few years [7][8][9].…”

Spectroscopic data for atomic tungsten transitions of interest in fusion plasma research

“…Federici et al 2001, Pospieszczyk 2006. Some years ago, several experiments were carried out in the ASDEX Upgrade device to prove the suitability of W as a divertor target material under the conditions of a high-density and low-temperature divertor (Naujoks et al 1996). The International Thermonuclear Experimental Reactor (ITER) will be the next step experimental fusion device which will use tungsten, together with beryllium and carbon-fibre reinforced composites, as plasma-facing materials.…”