Recent development in the operational regime of large helical device toward a steady-state helical fusion reactor

Recent progress of the plasma performance and the related physics understanding is overviewed in Large Helical Device. The volume-averaged beta value is increased with an increase in the NBI heating power, and reached 5.0% of the reactor-relevant value. In the high-β plasmas, the plasma aspect ratio should be controlled so that the Shafranov shift would be reduced mainly to suppress the transport degradation and the deterioration of NBI heating efficiency. The operational regime of the high-density plasma with internal diffusion barrier (IDB) has been extended, and the IDB, which was originally found by use of the local island divertor, has been realized in the helical divertor configuration. The central density was recorded as high as 1x1021m-3, and the central pressure reached 130kPa. Based on these high-density plasmas with the IDB, a new ignition scenario has been proposed. This should be a specific scenario to the helical fusion reactor, in which the helical ripple transport would be mitigated. A low-energy positive-NBI system was newly installed for an increase in the direct ion heating power. As a result, the Ti exceeded 5.2keV at a density of 1.2x1019m-3 in the hydrogen plasma. The Ti-increase tends to be accompanied by a large toroidal rotation velocity of the order of 50km/s in the core region. The plasma properties in the extended operational regime are discussed with regard to a perspective to the steady-state helical fusion reactor. (author)