Neutral beam injection in stellarators/heliotrons is studied with Monte Carlo codes that treat the initial beam deposition and the fast-ion thermalization process. The birth deposition model carefully treats the geometry of the vacuum vessel and includes beam divergence, focusing, and aperture losses. The thermalization process is determined by integrating the guiding centre equations of the fast ions and simulating collisions with the plasma by Monte Carlo collision operators. This process may include charge exchange and neutral reabsorption. For the purposes of this benchmark, we review the different formulations of the guiding centre equations and the Monte Carlo collision operators. We studied perpendicular injection into Heliotron-E, which is located at the Plasma Physics Laboratory at Kyoto University. The magnetic fields of Heliotron-E are computed using the Biot-Savart law with realistic filament models. The sensitivity of the computed heating efficiency to the modelling of the particle loss boundary and to the numerical procedures is examined. The results of three different codes were compared. When the codes solve the same problem, the answers agree quite well. However, changing some of the modelling assumptions (such as the loss boundary location) can create significant differences in the results.
Modeling of neutral beam injection into a compact spherical torus by a Monte Carlo deposition and a Fokker–Planck code