Suppression of disruptions with ECH on JFT‐2M

Typical MHD disruptions at surface safety factor qa=3 have been avoided by off‐central ECH in the JFT‐2M Tokamak. For the suppression, the electron cyclotron resonance layer has to be placed radially in a very narrow region of width about 1 cm near the q=2 surface. Both low‐field side heating and high‐field side heating are effective for the suppression. Fourier spectrum of the magnetic fluctuation shows that mode amplitude decreases by ECH and disruption is avoided. Without ECH, the mode frequency decreases (mode locking) and disruption occurs. Pulse feedback modulation of the ECH power by a magnetic pickup signal shows that the effect of the ECH to reduce the fluctuation is observed only when O‐point of the island is likely to be heated. The actual time scale of the suppression is much faster than the time scale of the change of the overall current profile. These observations show that ECH acts on the m=2 island. Further this heating is found to be effective to reduce the m=2 mode which appears at the d...