Intermittent sawtooth-like crash in the soft x-ray emissions was observed during the plasma current ramp-up in the initial operation phase of JT-60SA. In particular, this behavior was seen in plasmas with electron cyclotron (EC) heating which has a resonance layer at off-axis position. These events are thought to be magnetic reconnection between multiple rational surfaces with the same value of the safety factor q, which is known as the double tearing modes (DTMs). From the experimental viewpoint, this hypothesis is supported by the fact that the annular crashes were observed only in plasmas with off-axis EC heating, which facilitates the formation of a local off-axis minimum in q due to reduced ohmic current penetration through locally increased electron temperature . This hypothesis is supported by numerical simulations using the integrated transport code TOPICS and the reduced two-fluid code R2F. It has also been shown that the period between the crashes mainly consists of the recovery of the profile by both EC and ohmic heating, and the recovery of the current profile by the resistive current diffusion. Above discussions imply that the annular crashes related to the DTMs, which can reduce plasma performance, should be taken into account in cases where the off-axis EC heating power is used to assist with the plasma current ramp-up.
Disruption avoidance by means of electron cyclotron waves