Simulations of the radial electric field induced by neutral beam injection in a tokamak

A model based on Poisson's equation has been proposed to study the radial electric field induced by neutral beam injection (NBI). The radial electric field is produced by charge separation of beam induced electrons and beam ions. The charge separation is produced by redistribution and loss of NBI ions, which is due to the magnetic drift and collision effects. The birth and steady-state distributions of NBI fast ions related to charge separation were numerically computed by the orbit code GYCAVA and the NBI code TGCO. According to our model, the neoclassical polarization density of bulk ions has a great shielding effect on the radial electric field due to charge separation. The simulation results show that NBI ions can produce a significant radial electric field (∼10 kV m⁻¹) in the core region. The counter-current NBI (injection anti-parallel to the plasma current) can produce a negative radial electric field, which is due to the magnetic drift and collision effects of NBI ions. While the co-current injection (injection parallel to the plasma current) produces a positive radial electric field in the core region, which is mainly related to the magnetic drift of NBI ions. The dependence of the radial electric field on the injection direction and the beam energy of NBI has been investigated. As the beam energy increases, the magnitudes of the electric field for counter-injections increase and the magnitude of the electric field for co-current perpendicular injection decrease. (paper)