Analysis of the Ballooning Modes in High-β ST with Magnetic Fluctuation Measurement

The Ballooning mode, which occurs at a region where the plasma pressure gradient exceeds a certain limit, needs to be suppressed to achieve high β. Using a magnetic-probe system with a high spatial resolution, we measured magnetic fluctuations in a very local area to get fluctuations due to the Ballooning mode. As its result, we found that the amount of fluctuation decreased with I_tfc while I_tfc < 40 kA, but that it increased while I_tfc > 40 kA, which agreed with the earlier simulation of the ballooning mode. Analyzing the fluctuation of I_tfc > 40 kA, we found that the frequency was 0.4 - 0.8 MHz and the mode number was about 10.