Recent Technological Progress for Advanced Tokamak Research in JT-60U and JFT-2M

Abstract

For steady state advanced tokamak research with long pulse operations, JT-60U tokamak discharge, NBI and RF heating injection durations have been extended from 15 s to 65 s and from 10 s to 30 s respectively mainly by means of modifying their control systems and using derated power levels. In addition, technological issues for their long pulse injections with the heating systems have been solved as follows. The ion source of the negative ion NBI system was modified to increase gas conductance in the accelerator, which reduced the heat load to the grounded grid due to stripping loss to a level that enables operations of 2 MW for 30 s. A new method of controlling the anode voltage has been developed for sustaining the oscillation condition of a gyrotron in the electron cyclotron (EC) system. With this method, the EC injection duration has reached 16 s at 0.4 MW. To avoid serious damage of the LH launcher, a heat-resistant carbon grill LH antenna was implemented on the original stainless steel grill. To date, the advanced tokamak operations have been extended to β_N = 2.1 for 20 s. In JFT-2M, high β_N plasmas had been investigated with the vacuum vessel covered with ferritic steels. β_N of ~3.5 was obtained with r_wall/a~1.3-1.6 without serious influence of ferromagnetic walls (r_wall is distance of the wall from a plasma center and a is minor radius of a plasma). This encourages the utilization of ferric steel as a structural material for future reactors.