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The motional Stark effect (MSE) has become the leading diagnostic technique for measurement of the internal poloidal field profile in neutral beam heated tokamak plasmas. The technique relies upon the measurement of the polarization angle of Stark broadened neutral beam Dalpha emission due to the strong E= nu b*B motional electric field. In many recently discovered enhanced confinement regimes in tokamaks, a strong radial electric field Er is also present in the plasma. It is shown that in these cases, the effect of Er on the interpretation of MSE measurements is significant and cannot be neglected. The importance of this effect is illustrated with the analysis of the q profile in a high performance discharge from the DIII-D tokamak

Core transport barriers can be reliably formed in DIII-D by tailoring the evolution of the current density profile. This paper reports studies of the relative role of magnetic and ExB shear in creating core transport barriers in the DIII-D tokamak and considers the detailed dynamics of the barrier formation. The core barriers seen in DIII-D negative shear discharges form in a stepwise fashion during the initial current ramp. The reasons for the stepwise formation is not known; these steps do not correlate with integer values of q(O) or minimum q. The data from DIII-D is consistent with previous results that negative magnetic shear facilitates the formation of core transport barriers in the ion channel but is not necessary. However, strongly negative magnetic shear does allow formation of transport barriers in particle, electron thermal, ion thermal and angular momentum transport channels. Shots with strong negative magnetic shear have produced the steepest ion temperature and toroidal rotation profiles seen yet in DIII-D. In addition, the ExB shearing rates seen in these shots exceed the previous DIII-D record value by a factor of four.

The Pfirsch-Schluter theory of diffusion is invoked to calculate the poloidal asymmetry in the impurity ion distribution in a tokamak. The results predict the sign of the asymmetry and the same kind of parametric dependences as are observed in Alcator. In addition, the theory is capable of explaining the change in the sign of the asymmetry observed during discharges in the Princeton Large Torus.

A theoretical and computational study of a tokamak boundary in contact with a limiter is given. Parallel field flows of plasma and heat induced by the sheath conditions at the limiter are treated with model sources and sinks in the cross-field transport equations. A charge-exchange neutral code is used for the re-cycled neutrals. It is found that the boundary conditions at the limiter radius have little effect on the energy flow to the boundary unless the ionization zone is well attached to the limiter. Exemplary heuristic boundary conditions useful for transport codes are given. The partitions of particle and energy flow to the limiter and wall are discussed.

Pre-ionization experiments have been performed on a tokamak by injecting about 80 kW of microwave power at 35 GHz for up to 15 ms. Microwave absorption occurs at the electron cyclotron and upper hybrid resonance frequencies as predicted by theory. Pre-ionization causes substantial (40%) reductions in loop voltage during the initial phase of the tokamak shot. Flux (volt-second) savings with pre-ionization are about 30% in the first 2 ms or about 2% of the total flux expenditure in a tokamak shot. The plasma current begins 200 μs earlier and rises 1.4 times more rapidly in the pre-ionized case. Electron densities of 5 × 1012 cm−3 can be sustained throughout the microwave pulse with only a toroidal magnetic field during microwave injection. The bulk electron temperature in the pre-ionized plasma is about 10 eV although there are indications of higher electron temperatures (50 eV) in the upper hybrid resonance layer. Although questions exist concerning the quiescent behaviour of the pre-ionized plasma, the observed parameters are shown to be consistent with a theory which employs classical models of energy and particle balance. During the early stages of Ohmic heating, the pre-ionization is effective in decreasing the peak of the radiated power.

Using a neutral-beam injection power of 3.4 M W, volume-averaged toroidal betas of up to βT = 4.5% have been obtained in low-toroidal-field, low-qψ, vertically elongated discharges in the Doublet III tokamak. This level of βT is above the minimum level required for a tokamak reactor, thus demonstrating that reactor level values of βT are possible in a tokamak device. The observed enhancement of βT with vertical elongation lends confidence in the design of future devices which rely on vertical elongation.

Bolometric, thermocouple, Langmuir probe and infrared camera data are used to deduce the power flow in neutral beam-heated ( P input ⩽ 5 MW ), dee-shaped, limiter discharges. Power loss measurements typically account for ⩽ 60% of the total input power, with the majority either deposited on the primary limiter (20–40%) or radiated away (10–25%). Infrared camera measurements indicate that the heat flux to the ion drift side of the primary limiter is significantly greater than that to the electron drift side. Charge exchange neutral analysis, diamagnetic loop data and photodiode observations are consistent with a non-thermal flux of fast, beam produced particles to the limiter. If such an effect is indeed present, it could be partially responsible for the degradation in energy confinement observed during beam heating. Analysis of data comparing neutral beam injection from the two beamlines indicates that there is a variance in the energy cofinement time which is beamline specific; this variance may be correlated to the limiter location relative to the beamlines.

Impurity transport in a tokamak plasma in the presence of an externally imposed momentum source is considered. Including a model collision term to damp toroidal angular momentum and imposing the conventional ambipolar transport condition, the authors obtain expressions demonstrating that properly chosen momentum sources (e.g. radio-frequency fields) can cause outward the impurity transport. Estimates of the RF source strength needed to stop inward impurity diffusion show that the power absorbed by the plasma is small.

Summary The first Neutral Injection System Beamline became operational on the Doublet III tokamak in September 1981. Power delivery tests and system debugging proceeded through December and initial experiments on plasma heating were started in January 1982. This first beamline has been running well and as of February 1982 is routinely delivering about 2.0 MW of neutral hydrogen power to various circular and elongated dee-shaped deuterium plasmas. The second beamline is now being brought on-line and we expect to have 5–6 MW of heating power available for definitive β studies in mid 1982. We find that the change in the total plasma stored energy caused by the beams is essentially independent of toroidal field and plasma current, but increases weakly with density. The total stored energy increases with current and density, but is independent of toroidal field. A volume-average βT= 2.2% is obtained in low toroidal field discharges.