Effect of Toroidal Current on Rotational Transform Profile by MHD Activity Measurement in Heliotron J

The effect of toroidal current on the rotational transform was investigated in Heliotron J by measuring magnetohydrodynamic (MHD) activities at two configurations with a rotational transform (ι/2π) close to 0.5. The resonant m/n = 2/1 mode was observed in an ECH + co-NBI plasma in the configuration with ι/2π = 0.48 at ρ = 0.7. Here, m and n are the poloidal and toroidal mode numbers, respectively. ρ is the normalized minor radius. The rotational transform is increased presumably due to the toroidal current. The location of the ι/2π = 0.50 rational surface was determined to be ρ = 0.8-0.9 by soft X-ray (SX) fluctuations related to the MHD mode. An equilibrium calculation considering the toroidal current showed that the increase in the rotational transform due to the toroidal current was consistent with experimental results. The resonant mode structure was also investigated in an ECH + counter-NBI plasma at the ι/2π = 0.50 configuration. The location of the ι/2π = 0.50 rational surface, as determined by SX signals, did not change significantly compared with that obtained under a vacuum configuration. There is no significant difference between the rotational transform profile that considers toroidal currents by equilibrium calculation and that of the vacuum configuration. These results suggest that the change in the rotational transform profile caused by the toroidal current was small, owing to the balance between the bootstrap current and counter-flowing NB driven current.