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 n̄ₑ=6×10¹² cm^-3, 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 n̄ₑ=1.0×10¹³ cm^-3, without increase of the toroidal magnetic field ($B<~13$ kG, ωₚₑ²ωce²>1). Incremental current increases in Ohmically heated discharges have been observed at densities exceeding n̄ₑ=2.0×10¹³ cm^-3.