Toroidal Plasma Current Sustainment by Lower Hybrid Waves in the WT-2 Tokamak

By injecting rf power $P_{\mathrm{rf}}$ near the lower hybrid frequency into an Ohmically heated plasma near the end of tokamak discharge in the WT-2, an rf-driven current-sustained plasma is produced and continues to exist during the rf injection period, the toroidal current ($I_p\sim 10$ kA) being generated by the lower hybrid wave only without the Ohmic power. The efficiency of rf-driven current generation is high and attains $\frac{I_p}{P_{\mathrm{rf}}}\approx 0.9$ kA/kW at the plasma density $n_e\simeq 1.4\times {10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$, with somewhat lower efficiency at higher density.