Modeling of neutral beam injection into a compact spherical torus by a Monte Carlo deposition and a Fokker–Planck code

The numerical codes NFREYA and FIFPC have been applied to optimize the neutral beam injection (NBI) parameters for a high-β, highly elongated, compact spherical tokamak plasma.The Monte Carlo code NFREYA uses the 2D-flux surface and the 3D beam line geometry, such as the focal lengths, duct length, and beamlet divergences, beam tangency radius and the position of the ion source to calculate the beam deposition profile H(ρ) in an arbitrarily shaped axisymmetric plasma. The deposition profile H(ρ), depending on the radial coordinate ρ, for an arbitrarily shaped axisymmetric plasma is computed by a Monte Carlo method. H(ρ) is proportional to the source terms in the Fokker–Planck equation to be solved at selected flux surfaces.The deposition profile is used in the FIFPC code to compute the fast ion distribution function and thus the neutron yield. FIFPC accounts for the slowing down of the fast deposited particles due to the drag of the ions and electrons, the energy diffusion and pitch angle scattering.Results concerning the beam geometry and the neutron generation by the injection of (D,T) ions into (D,T) background plasmas of different compositions are presented.