Abstract
Experimental studies on the Large Helical Device during the last two years are reviewed. After the start of LHD experiment in 1998, the magnetic field has been gradually raised up to 2.89 T. The heating power has been increased, up to 4.2 MW for NBI, 1.3 MW for ICRF, and 0.9 MW for ECRH. Upgrading the key hardware systems has led to the extension of the plasma parameters to (i) higher Te [ Te(0) = 4.4 keV at <ne> = 5.3 x 1018 m-3 and Pabs = 1.8 MW ], (ii) higher confinement [ tE = 0.3 s, Te(0) = 1.1 keV at <ne> = 6.5 x 1019 m-3 and Pabs = 2.0 MW ] and (iii) higher stored energy Wp dia = 880 kJ. High performance plasmas have been realized in the inward shifted magnetic axis configuration (R=3.6m) where the helical symmetry is recovered and the particle orbit properties are improved by trade off of MHD stability properties due to the appearance of the magnetic hill. The energy confinement was systematically higher than that predicted by the International Stellerator Scaling 95 up to a factor of 1.6 and was comparable with ELMy H-mode confinement capability in tokamaks. This confinement improvement is attributed to the configuration control (the inward shift of magnetic axis) and to the formation of the high edge temperature. The achieved average beta value reached 2.4 % at B=1.3 T, the highest beta value ever obtained in helical devices, and so far no degradation of confinement by MHD phenomenon is observed. The inward shifted configuration has also led to successful ICRF minority ion heating. ICRF power up to 1.3 MW was reliably injected into the plasma without significant impurity contamination and a plasma with a stored energy of 200 kJ was sustained for 5 sec by ICRF alone. As another important result long pulse discharges of more than 1 minute were successfully achieved separately with NBI heating of 0.5 MW and with ICRF heating of 0.85 MM.
