Nonlinear effects of lower hybrid wave absorption on location of power deposition in the HL-2A reversed-shear plasma

In the simulations of the reversed-shear plasma of the HL-2A tokamak [Q. Gao , Nucl. Fusion 40, 1897 (2000)], the absorption of the lower hybrid (LH) wave is weak and makes many passes through the plasma until the initial launched wave spectrum is sufficiently broadened to be absorbed. As the constraint imposed by the wave propagation condition limits the maximum allowed n(parallel to) upshift, the LH wave absorption is bounded in the region defined by the strong Landau-damping limit and the boundary of the wave propagation domain. This mechanism of the LH wave absorption causes interplay of the distribution of the LH wave driven current with the modification of the plasma configuration, which constitutes nonlinearity in the LH wave deposition. Due to the nonlinear coupling between the LH power deposition and the profiles of both pressure and current, the LH wave deposition position changes spontaneously, generating two distinct quasistationary reversed magnetic shear (RS) configurations in a single discharge without changing the discharge condition. Therefore, the feedback control of the plasma current profile through controlling the externally driven current by the LH wave is a challenge in the RS plasma regimes for steady-state high performance tokamak operations. (c) 2005 American Institute of Physics.