m=1/n=1 density fluctuations in currentless high-β plasma in heliotron E

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Abstract
High‐frequency density fluctuations with m=1/n=1 have been observed in a currentless high‐β plasma. The line density was measured with a 5‐ch FIR laser interferometer. A high‐β plasma with 〈β〉∼2% was obtained with about 2‐MW neutral beam injection into a currentless target plasma produced by second‐harmonic ECRH. With weak gas puffing [β(0)≳1%], sawteeth‐like oscillations accompanied by high‐frequency fluctuations of about 2–5 kHz were observed on the line density. These fluctuations coincide with the soft x‐ray fluctuations. From both signals the mode number of this instability is estimated to be m=1 and n=1. During the fluctuations, rapid enhancement (10%–20%) in the edge ion saturation current was also measured, which indicates that this instability causes particle loss from the bulk to the edge. The instability is quenched by a change in density profile from peaked to broad with intense gas puffing. This behavior is consistent with a theoretical analysis.

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