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Observation of Magnetohydrodynamics Instabilities in Ion Berstein Wave and Lower-Hybrid-Current Driving Synergetic Discharges on HT-7 Tokamak

The normalized performance indicated by the product of βNH89>2 was achieved by a combination of the lower hybrid current driving (LHCD) and the ion Berstein wave (IBW) heating in the HT-7 tokamak. More than 80% of the plasma current was sustained by the LHCD and the bootstrap current. Large edge pressure gradients were observed. The magnetohydrodynamic (MHD) instabilities were often driven to terminate the discharge or reduce the discharge performance, when the IBW resonant layer was near the rational surface. The resonant layer of the safety factor q = 2 is located at 0.6 a with a = 27 cm being the minor radius. The width of magnetic island (the poloidal mode number m = 2) was about 2 cm. The plasma energy was reduced quickly by 30% by MHD instabilities. The behaviour of MHD instabilities is reported. A large sawtooth activity (m = 1) was observed before inducing MHD (m = 2).