Establishment of Configuration with Optimized Current Profile in Pellet Fuelled Discharge on HL-2A

A sawtooth-free period is produced following the pellet injection in the pellet fuelled discharge of q_a=3.4 (where q_a is the safety factor at the plasma boundary) in the HL-2A tokamak. Establishment of the current profile such as in the hybrid scenario is studied under the condition of pellet injection in HL-2A. 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>1 (where q₀ is the safety factor at the centre). The measured MHD mode structures evidence consistencies of the evaluated q-profile with locations of the q=1 surface in the sawtoothing period and of the q=2 surface
in the sawtooth-free period. TRANSP analysis indicates that the energy confinement is enhanced substantially during and after pellet injection, which is resulted from the q-profile optimization and the plasma density peaking.