Computational and theoretical studies of metallic dust transport in Tokamaks

Abstract

The plasma facing surfaces of the ITER are going to contain beryllium for the first wall and tungsten for the divertors. To test their benefits for the future operations, the JET is now run with the ITER-like walls. The ASDEX Upgrade has placed full tungsten surfaces inside it, so a large amount of tungsten dust grains are produced. The WEST divertor was recently set up in the Tore-Supra. There are some tokamaks which may not use the ITER-like materials but still metal, e.g. the FTU. Also the diagnostic tool can provide metallic dust grains in a chamber. With the high heat output of the future metallic tokamaks, much more metallic dust grains should be produced, the situation of which never occurs. We focus on studying two phenomena related to metallic dust grains in a plasma: the avalanche erosion done by high velocity impacts; and the misty plasma physics for charged droplet and bubble. For the former, we use the dust transport code, DTOKS to simulate iron dust grains re-entering the plasma, corresponding to the FTU, from the bottom. We find that only certain ranges of core plasma flow speed, launch direction and initial dust size result in acheiving a high velocity dust grain. In misty plasma, for a large droplet, we modify the electrostatic stability limit by the use of the MOML theory and the liquid pressure by the use of the conservation of the ion momentum flux. The bubble in the plasma may originate from the boiling molten layer on plasma facing surfaces or the transformation from the superheated droplets. We calculate the bubble electrostatic stability limit by the Lord Rayleigh's approach. It is surprisingly that the basic instability initiates at l = 3 rather than l = 2.