Response of Plasmas to Tungsten Pellet Injection in Neutral Beam Heated Discharges in Large Helical Device

Response of plasmas in Large Helical Device (LHD) to the tungsten pellet injection depends on both the total port-through power of the neutral beam injection (NBI) for heating, P-NBI, and the line-averaged electron density, n(e). The plasma can be sustained in the range of high P-NBI and low n(e) while it collapses in the range of low PNBI and high n(e). When the number of tungsten atoms enclosed in a pellet, N-W, is small, plasma can survive in low P-NBI and high n(e) range. Parameter space for plasma sustainment after the pellet injection is limited to lower n e range in more outward-shifted magnetic configuration. n(e) < 4.0 x 10(13) cm(-3) and N-W < 1.2 x 10(18) for R-ax = 3.60 m, n(e) < 3.0 x 10(13) Cm-3 and N-W < 8.7 x 10(16) for R-ax = 3.75 m, and n(e) < 2.0 x 10(13) cm(-3) and N-W < 3.1 x 10(16) for R-ax = 3.90 m are appropriate parameter ranges for the plasma sustainment. This suggests that robustness against tungsten injection depends on the magnetic configurations. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research