Improvement of Plasma Performance Using Carbon Pellet Injection in Large Helical Device

A cylindrical carbon pellet with a size of 1.2L × 1.2 mm to 1.8L × 1.8 mm and a velocity of 100 m/s to 300 m/s was injected into large helical device (LHD) for an efficient fueling based on its deeper deposition instead of hydrogen gas puffing and ice pellet injection. Electron density increment of Δne = 1014 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, e.g., 6.5 × 1016 cm−3 and 2.5 eV for CII, are examined respectively by spectroscopic method. A confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in a relatively low-density regime of ne = 2×1013 cm−3 to 4×1013 cm−3, and high ion temperature Ti(0) of about 6 keV is also observed with an internal transport barrier at ne = 1.2×1013 cm−3. In particular, the improvement in the ion temperature largely exceeds that observed in hydrogen gas-puffed discharges, which typically ranges below 3 keV.