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H-mode transitions in TEXT-U limiter plasmas have been observed at and kA ( kW) with at least 300 kW of central electron-cyclotron heating (ECH). These are dithering transitions which are induced by sawtooth crashes and display the typical signatures of H-modes ( drop, spontaneous density increase, evidence of a transport barrier). However, they show only a slight improvement over L-mode energy confinement. The vessel walls are boronized and conditioned prior to experiments to achieve low-impurity influx and particle recycling. Discharges which undergo transitions are fuelled almost entirely on residual recycling. Transitions are observed when limited on a toroidally localized top or bottom limiter and, more often, when the limiter surface is `fresh', which is achieved by alternating between top and bottom limiters on successive shots. No strong dependence upon the distance from the low-field-side limiter has been found. Transitions are not yet observed when limited on the high-field-side wall tiles or in the case of TEXT-U diverted configurations. Preliminary measurements with the 2 MeV heavy-ion beam probe (HIBP) (in the core) and Langmuir probes (in the edge) indicate that the plasma potential drops outside the q = 1 radius while only small changes are observed in the density fluctuation level.

Electron cyclotron heating (ECH) is used to suppress m=2 magnetohydrodynamic (MHD) oscillations in the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)]. The location of ECH power deposition is controlled by a movable antenna. The MHD activity is suppressed when the ECH beam is directed close to the q=2 surface. The experiment is simulated using a three‐dimensional resistive MHD code in cylindrical geometry. For fixed plasma current, the saturated m=2 island width is found to depend on the value of the safety factor at the magnetic axis (q0). The simulation suggests that the observed saturated m=2 island in the pre‐ECH plasma, which typically occupies 25% of the minor radius, corresponds to q0∼1.3. The suppression of the island in the presence of ECH is attributed to current profile modification. In some discharges, the m=2 activity does not resume even after the ECH pulse is turned off.

Recent experiments on the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)] have focused on identifying the drives and transport significance of the turbulence in the interior of discharges in the neo‐Alcator confinement regime. Far‐infrared (FIR) scattering consistently observes density fluctuations indicative of electron drift waves, i.e., ωk∼ωe*+kθvE, where vE≡−Er/B (the electron diamagnetic frequency Doppler‐shifted by the E×B poloidal plasma rotation) and an amplitude that scales inversely with the density scale length Ln. Although consistent with scattering on the power‐weighted frequency ω, heavy‐ion beam probe (HIBP) data typically indicate kθρs

Analysis of the circular polarization of a spectral line emission inside plasmas provides a measurement of the poloidal magnetic field on the Texas experimental tokamak (TEXT). Three sets of different polarimeters, using, respectively, the Ti XVII 3834-A line of the intrinsic titanium impurity and the Li I 6708-A line of a monoenergetic lithium beam, were developed and operated on TEXT. The safety factor {ital q} on the magnetic axis, determined from the measured magnetic field, was found to be near unity in all cases of Ohmically heated sawtoothing and nonsawtoothing discharges, and significantly below unity in the case of electron-cyclotron-resonance heating at the plasma center for high edge {ital q} discharges.

Analysis of the circular polarization of the Lii 6708 A line emitted from a monoenergetic lithium beam provides a spatially resolved measurement of the poloidal magnetic field on the Texas experimental tokamak [Nucl. Technol. Fusion 1, 479 (1989)]. Determined from the measured magnetic field, the safety factor on the magnetic axis during Ohmically heated sawtoothing discharges is near unity.

Links between turbulent fluctuations and transport processes in tokamaks, particularly in TEXT, are discussed. In the plasma edge probes allow detailed measurements of the quantities required to directly determine the fluctuation driven fluxes. The total flux of particles is well explained by the measured electrostatic fluctuation driven flux. However, a satisfactory model to explain the origin of the fluctuations has not been identified. The processes responsible for determining the edge electron heat flux are less clear. In the confinement region experimental observations are restricted to measurements of density and potential fluctuations and their correlations. Three distinct features have been identified: (1) the ubiquitous broadband turbulence and (2) a quasi-coherent feature, both of which propagate in the electron diamagnetic drift direction, and (3) an ion feature which by definition propagates in the ion drift direction. The characteristics of the measured fluctuations are discussed and compared with the predictions of various models. Comparisons between measured particle, electron heat and ion heat fluxes, and those fluxes predicted to result from the measured fluctuations, are made.

Particle transport coefficients during TEXT discharge have been determined using a modulated‐gas feed technique. Comparisons made before and during electron cyclotron heating experiments have shown that the largest effect on transport is an increase in particle diffusion at the periphery of the plasma.

Electron Cyclotron Heating experiments have been performed on the TEXT tokamak using Varian gyrotron. Some degradation of electron energy confinement is observed for sawtoothing and non‐sawtoothing discharges. Sharp electron temperature profiles are produced in high‐q discharges by extremely localized ECH power deposition. (AIP)