Confinement study on the reactor relevant high beta LHD plasmas

By improving the heating efficiency of the perpendicular NB injection due to the suppress the Shafranov shift by the pellet injection and the NBI power modulation, in addition to the increment of the parallel NBI power, we had the 5% beta plasma, and we had the 4.8% beta plasma in quasi-steady state with only gas-puffing and parallel NB injection. According to an analysis of the resistive g-mode by using 3 dimensional resistive MHD analyzing code for a typical high beta discharge with 4% beta and 106 magnetic Reynolds number, the radial mode width normalized by a plasma minor radius is ∼5%. This fact supports that we have never observed disruptive phenomena in the LHD high beta discharges because the predicted MHD instabilities are located at peripheral surfaces with the high magnetic shear and their radial mode structure is quite narrow. According to the transport properties in the high beta discharges, the gradual degradation of the local transport with beta comparing with GB (Gyro-Bohm) model is observed. An anomalous transport model based on a GMT (G-Mode Turbulence) model is fairly consistent with the beta dependence of the experimental thermal transport. However, for a fusion reactor with LHD like configuration, the anomalous transport based on the GMT is still important, but it would not be strong obstacle for the production of the high performance plasmas because the magnetic Reynolds number in a reactor is much less by 300-400 times than that in the present LHD high beta discharges. (author)