Overview of LHD experiments

During the first two years of the LHD experiment the following results have been achieved: (i) higher T-e (T-e(0) 4.4 keV at [n(e)] = 5.3 x 10(18) m(- 3) and P-abs = 1.8 MW); (ii) higher confinement (tau (E) = 0.3 s, T-e(0) 1. 1 keV at [n(e)] = 6.5 x 10(19) m(-3) and P-abs = 2.0 MW) (iii) higher store d energy, W-p(dia) = 880 kJ at B = 2.75 T. High performance plasmas have be en realized in the inward shifted magnetic axis configuration (R = 3.6 m) w here helical symmetry is recovered and the particle orbit properties are im proved by a trade-off of MHD stability properties due to the appearance of a magnetic hill. Energy confinement was systematically higher than that pre dicted by the International Stellarator Scaling 95 by up to a factor of 1.6 and was comparable with the ELMy H mode confinement capability in tokamaks . This confinement improvement is attributed to configuration control (inwa rd shift of the magnetic axis) and to the formation of a high edge temperat ure. The average beta value achieved reached 2.4% at B = 1.3 T, the highest beta value ever obtained in a helical device, and so far no degradation of confinement by MHD phenomena has been observed. The inward shifted configu ration has also led to successful ICRF minority ion heating. ICRF powers up to 1.3 MW were reliably injected into the plasma without significant impur ity contamination, and a plasma with a stored energy of 200 kJ was sustaine d for 5 s by ICRF alone. As another important result, long pulse discharges of more than 1 min were successfully achieved separately with an NBI heati ng of 0.5 MW and with an ICRF heating of 0.85 MW.