Recent experimental results in the Large Helical Device have indicated that a large pressure gradient can be formed beyond the stability criterion for the Mercier (high-n) mode. While the stability against an interchange mode is violated in the inward-shifted configuration due to an enhancement of the magnetic hill, the neoclassical transport and confinement of high-energy particle are, in contrast, improved by this inward shift. Mitigation of the unfavourable effects of MHD instability has led to a significant extension of the operational regime. Achievements of the stored energy of 1 MJ and the volume-averaged beta of 3% are representative results from this finding. A confinement enhancement factor above the international stellarator scaling ISS95 is also maintained around 1.5 towards a volume-averaged beta, β , of

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Pellet injection has been used as a primary fueling scheme in Large Helical Device (LHD). Pellet injection has extended an operational region of NBI plasmas to higher densities with maintaining preferable dependence of energy confinement on density, and achieved several important data, such as plasma stored energy (0.88 MJ), energy confinement time (0.3 s), β (2.4 % at 1.3 T) and density (1.1×10 20 m-3). These parameters cannot be attained by gas puffing. Ablation and subsequent behavior of plasma has been investigated. Measured pellet penetration depth that is estimated by duration of the Hα emission is shallower than predicted penetration depth from the simple neutral gas shielding (NGS) model. The penetration depth can be explained by NGS model with fast ion effect on the ablation. Just after ablation, redistribution of ablated pellet mass was observed in short time (~ 400 µs). The redistribution causes shallow deposition and low fueling efficiency.

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During the first two years of the LHD experiment the following results have been achieved: (i) higher Te (Te(0) = 4.4 keV at ne = 5.3 × 10 18 m −3 and P abs = 1.8 MW); (ii) higher confinement (τE = 0.3 s, Te(0) = 1.1 keV at ne = 6.5 × 10 19 m −3 and P abs = 2.0 MW); (iii) higher stored energy, W dia p = 880 kJ at B = 2.75 T. High performance plasmas have been realized in the inward shifted magnetic axis configuration (R = 3.6 m) where helical symmetry is recovered and the particle orbit properties are improved by a trade-off of MHD stability properties due to the appearance of a magnetic hill. Energy confinement was systematically higher than that predicted 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 (inward shift of the magnetic axis) and to the formation of a high edge temperature. 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 configuration has also led to successful ICRF minority ion heating. ICRF powers up to 1.3 MW were reliably injected into the plasma without significant impurity contamination, and a plasma with a stored energy of 200 kJ was sustained 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 heating of 0.5 MW and with an ICRF heating of 0.85 MW.