To clarify the effect of the plasma-wall separation on the stability of the resistive wall modes (RWM), we have performed current-driven RWM experiments in the JT-60U ohmic plasmas. In the experiment, the plasma was placed near the outer wall, and the plasma current was ramped up so as to destabilize the current-driven RWM. An instability with a clear m/n = 3/1 mode structure was observed just before a thermal collapse. The growth time of this instability is about 10 ms, which is of the order of the skin time of the JT-60U wall. Therefore, the instability has been identified as a current-driven RWM. The plasma-wall separation was systematically changed, and it is found that the growth rates have strong dependence on the wall position. Thus, the growth rates became smaller with decreasing the plasma-wall separation. Additionally, the observed growth rates have been compared with the AEOLUS-FT code, which is based on the resistive MHD equations with a resistive wall, and the model of the RWM dispersion relation. The dependence of the observed growth rate on the wall position is in qualitative agreement with these numerical results.
Energy principle for the modes interacting with a resistive wall in toroidal systems