2D 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 Consorzium RFX (Italy) consists of driver volumes where radio-frequency (RF) power is inductively coupled to the plasma electrons and an expansion chamber containing a magnetic filter (MF). This paper presents the physical and numerical principles of a comprehensive fluid model of this source. The model gives a qualitative but self-consistent two-dimensional description of the source, including the neutral gas flow, plasma chemistry, RF coupling in the source driver and plasma transport through the magnetic filter. The different particle species (electrons, the three types of the positive ions: H+ , H2 + ,H3+, negative ions H- and the neutral species: hydrogen atoms H and molecules H2 ) are described by separate continuity equations and the electron temperature is governed by the electron energy balance equation. The particle fluxes are found from momentum equations neglecting the inertia terms (drift-diffusion approximation). The model accounts for the bias potential applied at the plasma grid (PG) and the losses of particles and electron energy in the third dimension. The electrostatic coupling between electrons and ions is described by the Poisson equation. The numerical method is based on finite volume approximation and 9- point discretization is used to account on anisotropy due to magnetic field. Semi implicit numerical solver allows for large time steps (> 1000 x explicit time step) producing steady-state solution in a reasonable time (few hours for 100x100 mesh). The results for the simple configurations are shown to illustrate the code numerical stability and efficiency as well the influence of the neutral gas pressure, RF power, and magnetic field on the plasma properties. It is shown how the fluxes associated with the diamagnetic drift and the E×B-drift as well as the electron heating and the negative ion drift in the dc electric field are involved in the formation of the pattern of the plasma parameters. Effects due to the partial penetration of the MF in the driver are also investigated.