2-D Fluid Model for Discharge Analysis of the RF-Driven Prototype Ion Source for ITER NBI (SPIDER)

The experimental fusion reactor ITER will be heated by injection of a fast neutral beam generated by acceleration and neutralization of negative ions. The negative ion source used for this purpose (SPIDER), constructed at the Consorzio RFX (Italy), consists of eight driver volumes where radio frequency (RF) power is inductively coupled to the plasma electrons and an expansion chamber containing a magnetic filter (MF). This article presents self-consistent 2-D fluid description of the source, including neutral gas flow, plasma chemistry, and plasma transport through the magnetic filter. The numerical method is based on finite volume approximation, and nine-point discretization is used to account on anisotropy due to magnetic field. Semi-implicit numerical solver allows for large time steps producing steady-state solution in a reasonable time (few hours for the typical mesh). Simulation results for the simple configuration show code numerical stability and efficiency. Influence of neutral gas pressure, RF power, bias potential, and magnetic field on the plasma properties is investigated. It has been found that the reduced electron mobility perpendicular to the magnetic field reduces electrostatic plasma screening, allowing for easier extraction of negative charges from the plasma by means of an applied voltage.