Frequency Scaling of the Lower-Hybrid-Current-Drive Density Limit in Tokamak Plasmas

It is shown experimentally that the lower-hybrid-current-drive density limit is a function of the rf source frequency. While in previous 800-MHz experiments this limit occurred at ${\overline{n}}_e=6\mathrm{}\times {10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$, with a newly installed 2.45-GHz, 100-kW rf system on the Versator II tokamak, fully rf-driven discharges have been achieved at densities up to ${\overline{n}}_e=1.0\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$, without increase of the toroidal magnetic field ($B<~13$ kG, $\frac{{\omega }_{\mathrm{pe}}^2}{{\omega }_{\mathrm{ce}}^2}>1\mathrm{}$). Incremental current increases in Ohmically heated discharges have been observed at densities exceeding ${\overline{n}}_e=2.0\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$.