Long-Pulse Heating and Plasma Production by Neutral Beam Injection in Large Helical Device

Abstract We have achieved long-pulse NBI heating in Large Helical Device (LHD). A quasi-steadystate plasma was sustained for 21 sec with an injection power of 0.6 MW, where the central plasma temperature was around 1 keV with a line-averaged electron density of 0.3 x 10 19 m -3 . At higher densities, relaxation oscillation phenomena were observed for 20 sec at a period of 1 -3 sec. We have also achieved plasma production by NBI alone. The density build-up time was several hundreds msec, and the produced plasma showed the same characteristics as an ECH-initiated plasma. These results indicate unique characteristics of LHD where only external superconducting coils generate the plasma confinement magnetic field. Introduction Negative-ion-based neutral beam injection (NBI) heating started in September, 1998, in Large Helical Device (LHD), which is the world largest superconducting helical system On the other hand, plasma initiation by ECH is a unique characteristic in the helical system producing currentless plasmas, and neutral beams are usually injected into the ECHproduced target plasmas as an additional heating. However, since the ECH utilizes a given EC resonance magnetic field, the magnetic field strength is restricted in experiments to the resonance field. We achieved NBI-initiated plasma production for the first time, which gives no experimental constraint on the magnetic field strength. In this paper, we focus on the results of both long-pulse NBI plasma heating and plasma production by NBI itself