Studies of a lower-hybrid wave driven plasma equilibrium with a hybrid-MHD model on the TST-2 spherical Tokamak

Removal of the central solenoid is considered essential to realize a spherical tokamak fusion reactor, but non-inductive plasma start-up is a challenge. Start-up using lower-hybrid (LH) waves has been studied on the TST-2 spherical tokamak at the University of Tokyo. The equilibrium poloidal field is believed to be generated mostly by the wave driven fast electrons, which are highly non-thermal and have large orbit excursions from the flux surfaces due to low plasma current. Such an equilibrium can be qualitatively different from the Grad-Shafranov equilibrium routinely used for internal magnetic field reconstruction in a tokamak. In this work, the effect of fast electrons on the MHD equilibrium was investigated by considering the equilibrium solution of the hybrid-MHD model [Y. Todo and A. Bierwage, Plasma Fusion Res. 9, 3403068]. The fast electron distribution function was estimated using a LH current drive simulation based on ray-tracing and an orbit-averaged Fokker-Planck solver. The equilibrium solution of the hybrid-MHD model was successfully fitted to the magnetic and kinetic measurements. The resulting poloidal flux function was more skewed towards the outboard side when fast electrons were introduced, which was more consistent with the density profile measured by the Thomson scattering diagnostic.