Recent Results from LHD Experiment
with Emphasis on Relation to Theory from Experimentalist's View

H.Yamada for LHD Experimental Group

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

Experiments in the Large Helical Device (LHD) have been progressed steadily for these 5 years and many encouraging physical achievements have been obtained in conjunction with engineering demonstration of superconducting technology, increase of heating power and development of sophisticated diagnostics. Simultaneously, a variety of new findings which are not necessarily in scope of the existing theories have emerged. This paper sorts out experimental observations which can be described by available theories or cannot be done. The most distinguished remark is that stable plasmas can be maintained in the Mercier unstable region. Due to this unexpected but favorable characteristic, plasmas with high performance have been demonstrated in the configuration with the inward shifted magnetic axis. This observation poses an issue about the limitation of a linear stability theory on an interchange mode. Confinement improvement by the inward shift of the magnetic axis is partially explained by reduction of helical ripple transport, however, a clear improvement has been confirmed also in the collisional regime where the neoclassical transport does not play an essential role. Computation of linear stability of ITG/TEM has not provided a coherent picture with the experimental observation of anomalous transport yet. In the collisionless regime, transition of the radial electric field has been observed. Increase of helical ripples by shifting the magnetic axis outward results in increase of collisionality where the transition to the positive electric field takes place. This observation agrees with the generation of the electron root predicted by the neoclassical ambipolar diffusion intrinsic to asymmetric configuration. The accompanying formation of the internal transport barrier still remains a challenging issue for theory. Experimental exploration of confinement of energetic particles has been much facilitated by the 3-D Monte-Carlo simulation. Comparison is now shifting from qualitative argument to quantitative confirmation. Generation and healing of magnetic islands have been investigated experimentally. A study on magnetic islands is complementary to tokamaks with the opposite sign of the magnetic shear, which contributes to deepening understanding of the neoclassical tearing mode. Flexibility of magnetic configuration in LHD has been expanding an experimental envelope. Equilibria with magnetic islands and an internal separatrix, and transport in these eccentric configurations attract theoretical as well as experimental interests.