Overview of Results from the Large Helical Device

Full text: Physical understanding of net-current free helical plasmas has progressed remarkably in the Large Helical Device (LHD) since the last Fusion Energy Conference in Geneva, 2008. Highlighted results from the LHD experiments in these two years are overviewed. In parallel with parameter improvement, important physics processes of transport and MHD for fusion energy development have been identified and assessed by the analysis based on profile diagnostics with fine spatial resolution and numerical computation to cope with a real 3D geometry. LHD is based on the heliotron employing a pair of superconducting helical coils. The primary heating source is NBI with a heating power of 23 MW, and ECH with 3.5 MW plays an important role in local heating and power modulation in transport studies. The maximum central density has exceeded 1 x 1021 m-3 due to the formation of the Internal Diffusion Barrier (IDB) at the magnetic field of 2.5 T. A Resonant Magnetic Perturbation (RMP) with m/n = 1/1, which has resonance in the plasma periphery, has demonstrated the radial expansion of a super-dense-core surrounded by an IDB through the density reduction in the mantle outside the IDB. The plasma with a central ion temperature reaching 5.6 keV exhibits the formation of an Internal Transport Barrier (ITB). The ion thermal diffusivity decreases to the level predicted by neoclassical transport. This ITB is accompanied by spontaneous toroidal rotation and an Impurity Hole which generates an impurity-free core. This phenomenon is due to a large outward convection of carbon impurities in spite of the negative radial electric field. The magnitude of the Impurity Hole is enhanced in the magnetic configuration with larger helical ripple and for higher Z impurities. Another mechanism to suppress impurity contamination is impurity screening at the plasma edge with a stochastic magnetic field. The operational envelope of high beta has been extended to 5.1%. The demand for 3D modeling is becoming inevitable for accurate and detailed studies in tokamaks. A helical system shares common 3D related physics issues with tokamak such as documentation of 3D equilibria, transport in a stochastic magnetic field, plasma response to RMP and divertor physics. Results from LHD would accelerate tokamak research in addition to advancing the prospect for a helical reactor. (author)