Stabilization and Destabilization of Sawtooth Oscillations by Lower Hybrid Current Drive in the WT-3 Tokamak

Sawtooth oscillations appearing in Ohmically heated (OH) plasmas are suppressed by the lower hybrid current drive (LHCD) in the WT-3 tokamak ( R =65 cm, a =20 cm, I p ≃100 kA, n e =1-2×10 13 cm -3 , q a ≃4.5, and \(PLH{}100\) kW). After the suppression, sawteeth reappear following m =1 oscillations in the LHCD plasmas. The latter sawteeth have a large amplitude and long period ( T s =2-5 msec), compared with the former one ( T s =0.5-1.5 msec). Furthermore, they do not acccompany a precursor and crash very fast.