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Experiments on the current rampdown phase in the STOR-M tokamak for AC operation

The current rampdown phase followed by non-disruptive current termination is studied for future alternating current (AC) operations in the STOR-M tokamak. The plasma current is smoothly reduced to zero without disruptions with the rampdown rate of around 1.5 kA ms-1. The plasma density in the current rampdown phase is controlled by gas puffing. The plasma current decreases, maintaining the relationship 1/qa approximately 0.01 nR/Bt (10-18 m2T). At current termination, Ip=0, a residual plasma density (0.58-2.9)*1018 m-3 is observed with a corresponding Murakami parameter nR/Bt=(0.41-2.1)*1018 m-2 T-1. After current termination, the low-density plasma remains for 5 to 15 ms, which would allow AC operation in the STOR-M tokamak if the radiation barrier does not affect the subsequent current evolution.