Particle-In-Cell Modeling of Negative Ion Sources for Fusion Applications

A two- and three-dimensional (2D and 3D) 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), a high-density plasma (typically 5 × 1017 m-3), and a low neutral gas pressure (0.3 Pa) in a large volume vessel with a magnetic field barrier. We show that the plasma transport properties may be calculated with sufficient accuracy by implementing a larger than the real value of the vacuum permittivity in Poisson's equation. This approach is appropriate for nonturbulent plasmas, provided that the sheath length is small with respect to the quasi-neutral plasma dimensions. Furthermore, the calculation of the radio-frequency (RF) power coupling with the plasma (which is provided by an external antenna) is simplified by assuming that the electrons in the discharge interact with a uniform RF power profile and that their energy distribution function is Maxwellian in that region. Such approximation is relevant when the electron collision mean-free path is larger than the discharge dimensions and electrons are nonmagnetized. The simulation results are used to describe the plasma transport across the magnetic filter including the role of the Hall current (E × B and diamagnetic drifts), the dynamics of neutrals (notably the question of neutral depletion and physical chemistry), and lastly, the physical mechanisms involved with the extraction of negatively charged particles from the ion source, namely negative ions and electrons.