Tokamak plasma response to droplet spraying from melted plasma-facing components

Abstract

High-Z materials such as tungsten are currently the potentially best candidates for plasma-facing components (PFCs) in future fusion devices. However, the threat of melting under uncontrolled conditions and the associated material redistribution and loss can place strict limits on the lifetime of PFCs and plasma operation conditions. In particular, material losses in the form of fine sprayed droplets can provide a very intensive source of impurities in the plasma core. In this paper, the plasma response to radiation losses from impurity particles produced by droplet evaporation is modelled for the conditions found in the tokamak TEXTOR. The interplay between tungsten spraying and plasma behaviour, resulting in the reduction of power transferred to the limiter and diminution of droplet production, is taken into account. Calculations predict, in agreement with experimental observations, that this evolution results in a new steady state with significantly reduced central temperature and peaked impurity radiation profile. The efficiency of melt conversion into droplets, estimated by comparing experimental and computed plasma temperatures, is in reasonable agreement with the predictions from models for droplet generation.