Study of toroidal current effect on rotational transform profile by MHD activity measurement in Heliotron J

Effect of toroidal current on rotational transform has been investigated in Heliotron J by measuring magnetohydrodynamic (MHD) activities at two configurations with rotational transform (ι/2π) close to 0.5. The resonant mode has been observed in ECH + co-NBI plasma in the configuration with ι/2π=0.48 at ρ=0.7. This result shows that the rotational transform is probably increased due to the toroidal current. The location of the rational surface is determined at ρ=0.8-0.9 by soft X-ray (SX) fluctuations related to the MHD mode. Equilibrium calculation considering the toroidal current shows that the increase of the rotational transform by the toroidal current is consistent with experimental results. The resonant mode structure has been investigated also in ECH + counter-NBI plasma at ι/2π=0.50 configuration, and the location of the rational surface determined by SX signals is not significantly changed compared to vacuum condition, suggesting that the change in the rotational transform profile by the toroidal current is weak due to the balance between bootstrap current and counter-flowing NB current. (author)