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.