MHD study of the reactor-relevant high-beta regime in the Large Helical Device

In the Large Helical Device (LHD), the volume averaged beta value <β dia > of 5 %, which is the highest value in all of heliotron/stellarators and relevant to the reactor requirement, was achieved by optimizing the magnetic configuration from the viewpoint of magneto-hydrodynamic (MHD) characteristics, transport and heating efficiency of the neutral beam.This beta value was instantaneously obtained by pellet injection and maintained for more than 10τ E , whereas the steady state plasma with a maximum <β dia > of 4.8 % was sustained for 85τ E by the gas-puff fueling.While it is predicted theoretically that stochastization of the peripheral magnetic field structure develops with an increment of <β dia >, no serious degradation of the global confinement has been observed in the present <β dia > range.The several low-order MHD activities located in the periphery were enhanced with the beta value and sometimes affect the local profiles.The amplitude of the mode in the periphery strongly depends on the magnetic Reynolds number, which is close to that of the growth rate and/or the radial mode width of the resistive interchange instability.