TF Ripple Induced Stochastic Diffusion of Energetic Particles in Advanced Tokamak Configurations on HL-2A

A NBI heated discharge is modeled with TRANSP under the HL-2A condition. LH wave is injected into the target plasma heated by NBI with a nearly symmetric power spectrum to control the current profile. Due to the off-axis driven current and electron heating, an optimized q-profile, of which the magnetic shear is weak in the central region and negative in the mid-plasma region (x~0.5-0.65), is formed. The NBI heating discharge with the optimized q-profile enters into operation scenario of enhanced ion confinement. The ripple loss of the energetic particles is studied by using the Golaston-Whiet-Boozer threshold for stochastic ripple diffusion. Compared to the discharge of conventional tokamak configuration, the ripple loss fraction of the captured NBI power is much higher (more than 25%) in the discharge with the optimized q-profile. The factor of the ripple loss fraction of particles against that of energy, Ksp, is much higher than unity showing that the most of particles are lost during the slowing down of the injected ions. Ksp is dependent on the energy of injected particles, the injecting angle of neutral beam line, and electron temperature of the bulk plasma. With NBI of lower energy, the ripple loss fraction of the captured NBI power is larger than that for the case of higher NBI energy because the lost particles are richer in ones of relatively higher energy in the case of lower NBI energy.