Modelling of Dust Transport in Reverse Field Pinch Configuration

Abstract: The DDFTU code has been modified and updated to study the transport of dust in the reverse field pinch (RFP) EXTRAP-T2R. The DDFTU code solves the coupled equations of charging, heating and motion of a dust grain immersed in a plasma whose profiles are given. Updates of the code have been introduced to include (i) non-steady state heat balance and phase transitions models, (ii) realistic boundary conditions for dust-wall collisions, and (iii) a radius and temperature-dependent thermal radiation model, for which a dedicated code has been developed. The models for secondary electron emission (SEE), thermionic emission and magnetic dipole force have been refined as well, the sensitivity to SEE and thermal radiation modelling being highlighted. Similarly to limiter tokamak configuration, the ion drag is confirmed to be the main force driving the dust dynamics. However for RFP configurations, the strong contribution of the electric force is demonstrated. The latter plays an important role in the RFP edge plasma due to (i) the large value of the radial electric field and (ii) its sign which is opposite to typical tokamak edge electric fields. A comparison with available experimental data - dust collection in the mid-plane - has been carried out. For this purpose, the dust toroidal transport asymmetry has been evaluated numerically from the computation of the time dependent velocity distribution of an ensemble of many particles launched from the vessel wall. An empirical scaling law for the asymmetry has been found in the high dust inertia regime.