Improvement of plasma performances using carbon pellet injection in Large Helical Device

摘要

A cylindrical carbon pellet with a size of 1.2-1.8mm and a velocity of 100-300m/s has been injected in Large Helical Device for the purpose of efficient fueling based on its deeper deposition instead of H_2 gas puffing and ice pellet injection. Electron density increment of Δn_e=10x10~(13)cm~(-3) is successfully obtained by single carbon pellet injection without plasma collapse. Typical density and temperature of the ablation plasma of the carbon pellet are examined by spectroscopic method, e.g., 6.5x10~(16)cm~(-3) and 2.5eV for CII, respectively. The confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in relatively lowdensity range of ne=2-4x10~(13)cm~(-3) and high ion temperature of T_i(0)-6keV is also observed with internal transport barrier at n_e=1.2x10~(13)cm~(-3). These improvements largely exceed the values appeared in usual hydrogen gas-puffed discharges