Current drive on the Versator-II tokamak with a slotted waveguide fast wave coupler

A dielectric-free slotted waveguide fast wave coupler (SWFWC), operating in the lower hybrid range of frequencies, has been used to drive current on the Versator-II tokamak. Up to 35 kW of radiofrequency at 2.45 GHz has been radiated into plasmas with electron densities of 2 x 1012 cm-3 ≤ n-bare ≤ 1.2 x 1013 cm-3 and a magnetic field of Btor = 1.0 T. When the coupler was flush with or beyond the limiter, the radiating efficiency improved linearly with increasing n-bare. For electron densities n-bare ≥ 5 x 1012 cm-3, the current drive efficiency was similar to that of the slow lower hybrid wave; however, at lower densities the efficiency of the SWFWC was reduced and it drove no current when n-bare ≤ 2 x 1012 cm-3. During bench tests, the antenna radiated >90% of its power in the fast wave polarization. These observations suggest that the SWFWC launched primarily fast waves. However, we do not exclude the possibility that, owing to weak single-pass absorption, a substantial fraction of the power may have been converted to slow waves within the plasma, and these waves may have been responsible for most of the driven current. (author). Letter-to-the-editor. 32 refs, 11 figs