Modeling a high power fusion plasma reactor-type ion source: Applicability of particle methods

Abstract

A simple two-dimensional (2D) Cartesian, three-velocities (3 V), particle-in-cell Monte Carlo collisions (PIC MCC) model of a tandem type inductively coupled plasma (ICP) discharge is presented. The conditions are similar to those of negative ion sources for fusion applications, i.e., a high absorbed power (on the order of 100 kW) and a high density plasma (typically 1018 m−3) in a large volume vessel with a magnetic field barrier. We show that the plasma transport properties may be calculated with sufficient accuracy by performing simulations at low plasma density (low absorbed power) and rescaling the results to large plasma density by assuming linear variations of plasma density with power (in case of non-linear plasma chemistry, appropriate scale of the cross-sections or reaction rates is also necessary). This is equivalent to performing the simulations with the real absorbed power but using a larger value of the vacuum permittivity in Poisson’s equation, to force quasineutrality. Rescaled results from explicit PIC MCC simulations compare well with those from direct implicit PIC MCC simulations (performed with real power and plasma density). Implicit simulations have less stringent constraints on time step and grid spacing but numerical heating may be more difficult to control and verification of energy conservation during the calculation is necessary. Rescaled explicit simulations may be employed for non-turbulent plasmas provided that the sheath length is small with respect to the plasma dimensions. The simulation results are used to discuss the physics of plasma transport across the magnetic filter and the role of the drift currents (E×B drift and diamagnetic drift).