Long-Pulse Neutral-Beam-Heated Discharges in Large Helical Device

Long-pulse neutral beam heating has been achieved in the Large Helical Device (LHD), with a negative-ion-based neutral beam injector (NBI). The pulse duration ofthe neutral-beam-heated discharge is extended easily without special feedback stabilization for plasma control. As a result, a hightemperature (1.5-1.8 keV) plasma with a density of (1.5 - 2.0 x 10re m-3) was sustained for 80 sec with a reduced injection power of 100 keV - 0.5 MW. Long-pulse quasi-steady-state plasmas, where almost no change of the plasma quantities is observed, are realized by feedback density control with fueling gaspuffing. Wall pumping effect enables the density control by the gas-puffing and is maintained in the present long-pulse discharge duration. On the other hand, slow relaxation oscillation phenomena with a period of l-2 sec were observed, lasting for 20 sec. They are characterized by altemate profile expansion and contraction of the electron temperature, observed just like "breathing" on a TV monitor. Core radiation loss by iron would play an important role in the "breathing" discharges. The temperature of the LHD vacuum vessel was increased by 10'C at maximum in the 80-sec injection. The injector operation including negative ion sources was stable, and the temperature rises ofthe cooling water for the heat load components were less than lO"C and all saturated. The injector will be up-graded step by step for an extension of the injection duration to 30 min.