The major results and accomplishments of the MIT tokamak program are surveyed. These are considered to be 1) discovery of an ohmic-heating confinement law in which TE -naR 2 ; 2) reduction of anomalous ion conduction to the neoclassical value by use of pellet fueling; 3) formulation of an empirical model for confinement of impurities in ohmically-heated tokamaks; 4) seminal experiments on current drive by lower hybrid waves and production of quasi-stationary driven current discharges with n -1020 m-3 ; and 5) heating of electrons by Landau damping of lower hybrid waves withATe -I keV. The advance of nOTE is also traced from values of -1018 sec-m-3 which were typical of tokamaks at the beginning of the Alcator program to values achieved on Alcator C in excess of 6 x 1019 sec-m-3 , which is required for thermalized energy breakeven at higher temperature.

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.

Ray tracing simulations based on experimental PBX-M equilibria show a limited range of the parallel wavenumber (n, = R,c/w) along a ray trajectory. The range of nl accessible to the excited wave is shown to have both a lower and an upper bound. The ray's phase volume is projected into (n,, R, Z) space to define a domain of wave accessibility for each toroidal mode number excited by the launcher. A comparison of circular and bean shaped plasmas with a high aspect ratio ( A -5.5) shows that the rays fill a substantially larger portion of the accessible domain in bean shaped plasmas. Furthermore, in the bean shaped case the accessible domain tends to extend to higher n N .

The momentum distribution of the current carrying suprathermal electrons is determined from the measurement of X mode transmission near the fundamental of the cyclotron frequency during inductive and radio frequency current drive in the Versator II tokamak. The measurement of the differential attenuation of radiation of two beams with finite and opposite N allows the separation of the wave attenuation due to cyclotron absorption from that due to non-resonant mechanisms. It is shown that such transmission measurements can be used to determine the part of the parallel momentum distribution, which is asymmetric in p 11 , Fa(Pii) =. F(-p 1 ) -F(p 11 )). Measurements of F(p 1 j) and its time evolution generally agree with expectations, although T is found to be lower than predicted by collisional theory in the inductively driven case.

Silicon absolute extreme ultraviolet ͑AXUV͒ photodiodes have been employed in a disruption radiometer diagnostic for measurement of radiant power in the DIII-D tokamak with a 170 kHz bandwidth. This is motivated by a need to improve the understanding of radiative processes in tokamak disruptions. The diagnostic described in this article has a single line of sight though the central plasma. Accounting for the photon energy dependence of the AXUV photodiode responsivity is made possible by optical filtering, with the aid of spectra from an extreme ultraviolet survey spectrometer. The appropriate effective responsivity for interpretation of the data is lower than the nominal value typically used for the detector. In the current quench phase of disruptions, it is less than half the nominal value. Comparisons with results from a foil bolometer find good agreement.

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 ...

Application of Ion Bernstein Wave Heating (IBWH) into the Princeton Beta Experiment-Modification (PBX-M) [Phys. Fluids B 2, 1271 (1990)] tokamak stabilizes sawtooth oscillations and generates peaked density profiles. A transport barrier, spatially correlated with the IBWH power deposition profile, is observed in the core of IBWH-assisted neutral beam injection (NBI) discharges. A precursor to the fully developed barrier is seen in the soft x-ray data during edge localized mode (ELM) activity. Sustained IBWH operation is conducive to a regime where the barrier supports large ∇ne, ∇Te, ∇νφ, and ∇Ti, delimiting the confinement zone. This regime is reminiscent of the H(high) mode, but with a confinement zone moved inward. The core region has better than H-mode confinement while the peripheral region is L(low)-mode-like. The peaked profile enhances NBI core deposition and increases nuclear reactivity. An increase in central Ti results from χi reduction (compared to the H mode) and better beam penetration. Bootstrap current fractions of up to 0.32–0.35 locally and 0.28 overall were obtained when an additional NBI burst is applied to this plasma.

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