Plasma startup by neutral beam injection in the Large Helical Device

Abstract

Plasma was successfully built up by neutral beams alone in LHD. After injecting 1.6 MW neutral beams through a low density neutral gas target of 10^-3Pa for about 0.3 s, fully ionized plasma of 10¹⁸ m^-3 was generated. Keeping the beams on, additional gas puff could increase the plasma density sufficiently for absorption of the beam, and a dense plasma of 5 × 10¹⁹ m^-3 was sustained afterwards. This technique is specific to a helical system that has a vacuum confining magnetic field structure, and no other assisting heating tools are required such as ECH, which has been used for producing target plasma with NBI.