Assessments of the performance of neutral beam injection on the spherical tokamak EXL-50

Abstract

The numerical simulation of the neutral beam injection (NBI) heating in the spherical tokamak EXL-50 has been performed in order to offer a guideline for the practicable engineering design. A series of parameter scans on injected beam energy, plasma density, electron temperature, plasma current and injection direction have been performed with the NBI heating analysis code NUBEAM. The simulation results show that the injected beam energy should be matched with the plasma density to avoid the excessive shine-through loss. For the scenario with central plasma density of 3.0 × 1019 m−3, the 20 keV beam injection has a power absorption rate about 77%, while for the 50 keV beam injection, only 57% injected power has been absorbed by plasma. When the central plasma density increases to 5.0 × 1019 m−3, a power absorption rate more than 70% can be obtained even for the 50 keV beam injection. As the increase in the electron temperature, the injected power is more likely to heat ions while the total absorbed power by plasma roughly remains stable. The increased plasma current leads to the stronger poloidal magnetic field. A qualitative elucidation on the fast ion trajectory has been given to reveal that the orbit loss has been remarkably suppressed by the enhanced poloidal magnetic field. In addition, a comparison of power absorption rate in the co-current and counter-current injection cases has been carried out, which suggests that counter-current injection leads to more orbit loss and hence less power absorption rate.