Enhancement of plasma ion temperature by impurity seeding in H-mode plasmas

Abstract

Effect of impurity seeding on plasma global confinement has been investigated in H-mode plasmas of the HL-2A tokamak. Metal and gas impurities can be externally seeded by laser blow-off (LBO) and supersonic molecular beam injection (SMBI) systems, respectively. Using the LBO system to seed aluminium impurities into H-mode plasmas, it is observed that the ELM frequency after the impurity seeding is reduced by about 50%. The plasma stored energy is enhanced. The corresponding energy loss caused by each ELM increases with the decrease of ELM frequency. Besides, the neon and argon gas impurities have been seeded into H-mode plasmas by SMBI. The ELM frequency decreases to 0.3–0.5 times lower than that before the SMBI. The prolonged inter-ELM periods allow the plasma to build a higher pedestal density. It is observed that the energy confinement of the H-mode plasma is improved by the edge-deposited impurities, which is mainly attributed to the enhancement of plasma ion temperature. Both the edge and core ion temperatures are increased by 20%–40% after the impurity seeding. The quasi-linear simulations predict that the ion heat flux induced by ion temperature gradient mode is deceased in the present of impurity. The result suggests that the seeded impurity could reduce the edge ion thermal transport, resulting in the formation a higher edge ion temperature, which is a boundary condition for further increasing the core temperature through the profile stiffness.