Simulation of High-Field-Side and Low-Field-Side Helicon Wave Current Drive

This paper investigates the characteristics of helicon wave current drive (HCD) on the high-field-side and low-field-side of a spherical tokamak through numerical simulations. The simulations are based on the equilibrium configuration of the EXL-50U device, analyzing the current drive efficiency under different frequency, temperature, and density conditions. The results show that a helicon wave at 350 MHz can drive a significant current under the given temperature and density distribution. Under low-density conditions, the low-field-side HCD outperforms the high-field-side. However, as the electron density increases, the advantage of high-field-side HCD gradually emerges. Increasing the electron temperature is also beneficial for enhancing the total driven current. Additionally, although the driven current from the high-field-side injection is smaller, the radial current distribution is broader and shifts towards the plasma edge, which provides new insights for localized current drive control. This study offers theoretical support for the experimental design and optimization of helicon wave current drive in spherical tokamaks.