The optimizing numerical simulation of beam ions loss due to toroidal field ripple on EXL-50U spherical torus

Realization of high performance plasma of EXL-50U is very sensitive to NBI (neutral beam injection) heating, and it is expected that the fast ions of NBI are confined well and their energy is transferred to the background plasma by collision moderating. In this paper, the loss of fast ion ripple is simulated based on the equilibrium configuration, fast ion distribution and device waviness data given by the integrated simulation. It is found that the loss fraction of fast ion ripple is about 37%, and the local hot spot is about 0.6 MW/m², which is unacceptable for the experimental operation of the device. The optimization method includes moving the plasma position and adding FI (ferritic steel plug-in) to reduce the ripple degree, increasing the I_p (plasma current) and optimizing the NBI injection angle. The results show that the ripple distribution must be controlled and the I_p must be increased to more than 600 kA, so that the fast ion loss can be reduced to 3%–4% and the local heat spot can be reduced by an order of magnitude. In this paper, the evaluation methods of fast ion ripple loss in device design are summarized, including the fast ion distribution in phase space, the overlap degree of ripple loss area, and the particle tracking on the time scale of total factor slowing down. The engineering and physical ways to reduce ripple loss are also summarized to provide simulation support for integrated simulation iterative optimization and plant operation.