Control and optimization of current profile under dominant electron heating in HL-2A

Establishment of the current profile like in the hybrid scenario is studied under the condition of dominant electron heating in HL-2A. In the discharge of qa∼3.4 a sawtooth-free period was produced following the pellet injection. The discharge is analysed with TRANSP. It is shown that a q-profile of weak negative shear is produced immediately after the pellet injection, and it then evolves to a broad flat profile with q0 > 0. The measured MHD mode structures evidence consistencies of the evaluated q-profile with the locations of q = 1 and q = 2 surfaces in the sawtoothing period and q = 2 surface in the sawtooth-free period. Both the diamagnetic measurement and TRANSP analysis indicate that the energy confinement is enhanced substantially after pellet injection, which would be resulted from the q-profile optimization. The discharges with injecting LH and EC waves are simulated with TRANSP. Carefully adjusting the position of non-inductive current driven by EC, an optimized q-profile was obtained with qa = 3.78 and low shear region extending to x∼0.45 in the low-density discharges (line averaged density = 1.0x1019m-3). When 0.5 MW LH power in CD mode and 0.9 MW EC power mainly for plasma heating are used to control the current profile, a q-profile of low shear region extending to x = 0.6 and qa = 3.21 is established through controlling the EC absorption position in the low-density plasma (1.0x1019 m-3). Corresponding to the optimized current profile an electron-ITB is developed on the Te profile of Te(0) = 3.5 keV. With the similar control scheme the q-profile of low shear region extending to x = 0.45 and qa = 3.36 can be produced in a higher density plasma of 2.32x1019 m-3. As the constraint imposed by wave propagation condition in the HL-2A plasma limits nparallel -upshift, the LH wave absorption is bounded by the strong Landau-damping limit and the boundary of wave propagation domain. This mechanism of the LH wave absorption causes interplay of the distribution of the LH driven current with the modification of the plasma configuration, which constitutes non-linearity in the LH wave deposition. The LH wave deposition position changes spontaneously because of the non-linearity. Therefore, the feedback control of the plasma current profile through controlling the LH driven current is a challenge in the high performance operation. (author)