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Experimental verification of lower-hybrid RF current drive in the Versator II tokamak is presented. The experiments show that efficient current drive exists only in low density discharges (ffe < 6 X 101 2 cm-3) in the "slide-away" regime where a preformed supra-thermal electron tail exists prior to the application of the RF power.

Measurements of low power (≃ 1 mW) antenna loading are used to study the coupling of a compact loop antenna structure to plasmas in the divertor configuration in DIII-D heated by neutral beam injection (NBI) or electron cyclotron heating (ECH). When a transition to the H-mode regime occurs during NBI, the antenna loading resistance drops by approximately a factor of two. This coupling decrease is due to a steepening of the edge density profile near the separatrix, accompanied by a reduction in edge density in the scrape-off layer. During edge localized modes, the opposite effects occur, and the antenna coupling increases transiently. The loading measurements are compared with theoretical calculations which take into account the measured density profiles as well as the conducting side-walls of the recessed antenna housing. Absolute agreement between the theoretical and the experimental results is obtained, including the correct dependence on the density, antenna position, RF frequency and antenna geometry. The theoretical interpretation of the results is discussed, together with the technological implications for future high power experiments.

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 ...

ABSTRACT. Significant levels of parametric decay activity and correlated edge ion heating were observed during injection of high power ion Bernstein waves (IBWs) in DIII-D. Both minority hydrogen ions and majority deuterium ions showed the formation of a high energy perpendicular tail; no parallel heating was obeerved. The edge ion heating and the parametric decay activity were both strongest when an ion cyclotron harmonic was present at the plasma edge. Ion tail formation had a power threshold of several hundred kilowatts, above which the tail &e increased with antenna power; a comparable power threshold for parametric decay instability (PDI) was observed. Both the PDI and the associated edge deuterium heating were found to be sensitive to the hydrogen-to-deuterium ratio.

Our data indicate that the L-mode to H-mode transition in the DIII-D tokamak is associated with the sudden reduction in anomalous, fluctuation-connected transport across the outer midplane of the plasma. In addition to the reduction in edge density and magnetic fluctuations observed at the transition, the edge radial electric field becomes more negative after the transition. We have determined the scaling of the H-mode power threshold with various plasma parameters; the roughly linear increase with plasma density and toroidal field are particularly significant. Control of the ELM frequency and duration by adjusting neutral beam input power has allowed us to produce H-mode plasmas with constant impurity levels and durations up to 5 s. Energy confinement time in Ohmic H-mode plasmas and in deuterium H-mode plasmas with deuterium beam injection can exceed saturated Ohmic confinement times by at least a factor of two. Energy confinement times above 0.3 s have been achieved in these beam-heated plasmas with plasma currents in the range of 2.0 to 2.5 MA. Local transport studies have shown that electron and ion thermal diffusivities and angular momentum diffusivity are comparable in magnitude and all decrease with increasing plasma current.

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Antenna loading measurements carried out during high power ion Bernstein wave (IBW) heating experiments on the DIII-D tokamak indicate that efficient, direct coupling to the IBW at ω ≲ 2ωci as predicted by linear coupling theory did not occur. Whereas strong peaking in the loading resistance near cyclotron harmonics is predicted for high edge densities (ω < ωLH|edge), the observed loading resistance is nearly independent of the toroidal magnetic field. The loading anomaly can be explained by invoking the ponderomotive force which decreases the edge density immediately in front of the antenna, thus allowing coupling to the cold plasma lower hybrid wave (LHW). A linear LHW coupling code including the modified density profile due to the ponderomotive force reproduces the measured dependence of antenna loading on toroidal magnetic field, edge density, antenna frequency and antenna phasing. Further evidence for the ponderomotive force is obtained from reactive loading measurements which indicate that the plasma is pushed away from the antenna as the radiofrequency power level is increased. The results indicate that the lack of central ion heating observed during DIII-D IBW experiments may be due to a lack of efficient mode transformation from the coupled LHW to a centrally propagating IBW, possibly as a result of nonlinear mechanism(s)