It is shown experimentally that the lower-hybrid current drive "density limit" is a function of the rf source frequency. While in previous 800MHz experiments this limit occurred at ie = 6 x 1012 cm-3. with a newly installed 2.45GHz, 100kW rf system on the Versator II tokamak fully rf-driven discharges have been achieved at densities up to i = 1.Ox 1013 cm-3 , without increasing the toroidal magnetic field (B < 13kG, We/W 2e > 1). Incremental current increases in ohmically heated discharges have been observed at densities exceeding ie 2.0 x 1013 cm-3.± Present Address: Department of Electrical Engineering, Stanford University, Stanford, CA 94305. $ Present Address: Instituto Tecnol6gico Estudios Superiores de Monterrey, 50000Toluca, M6xico.1 Lower-hybrid current drive experiments in recent years have demonstrated quasisteady-state sustainment of toroidal plasma currents in tokamaks with no assist from the ohmic heating (OH) transformer." The rf-driven currents are generated when momentum is transferred to resonant superthermal electrons from unidirectionally traveling slow waves. Because such waves can be launched from phased arrays of waveguides, lowerhybrid current drive is attractive for applications to toroidal reactor devices. However, before useful extrapolation of present-day results can be carried out, certain discrepancies between experimental observations and theory must be resolved. In particular, the steady state current drive efficiency, r7 = nIR/P, where n is the density, I is the rf-generated current, R is the major radius, and P is the injected rf power, is predicted to scale approximately independently of density.' While this scaling has been confirmed experimentally over substantial density intervals, 2 nearly every experiment to date has encountered a "density limit": namely, above a critical density the current drive efficiency suddenly decreases and current drive effects disappear. 4 To summarize, efficient lower-hybrid current drive is observed only when L' olLH > 2, where WLHIn previous OH-assisted 800 MHz experiments on Versator, 4 the current drive density limit occurred at Ke ~ 6 x 1012 cm-3 . The PLT 800 MHz density limit in H 2 occurs at a similar value,' namely h ~ 8 x 1012 cm-3 . Recently, higher frequency lower-hybrid experiments have demonstrated quasi-steady-state current drive at higher densities. 2 ' 6 However, in these experiments the toroidal magnetic field tends to increase with frequency and density so that the dielectric constant remains relatively low (W 2/wje ~ 0.2), and low-n waves remain accessible to the plasma core. Consequently, the variation of the current 2 drive density limit with frequency for a given device and magnetic field has not been explored. Furthermore, the physical mechanism responsible for the "density limit" is not well understood. 6 In this letter, we report the first direct experimental comparison of the lower-hybrid current drive density limit at two different frequencies, namely 800MHz and 2.45 GHz. The experiments were carried out on the ...
