Impact of high energy neutral beam injection in LHD plasma

Recent successful results of LHD experiments have relied on the high energy NBI heating that has been achieved by successful development of advanced ''negative ion technology''. The LHD experimental results are overviewed and discussed in conjunction with several features of the NBI system. High energy beam (180 keV hydrogen) heats electrons in plasma rather than ions, which is effective for production of high density and high pressure plasma. Tangentially injected beams are confined well in helical magnetic field, which enables plasma initiation by NBI and sustains high beta plasmas. The beam-induced toroidal current is utilized for changing the external magnetic configuration to study the MHD characteristics of the LHD plasma. The recent successful R and D results of the negative-ion-based NBI are also described. By introducing a new grid system and improving the arc efficiency of the ion source, the targeted output power of 5 MW per beamline has been achieved. (author)