Table of contents

The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

Lower-hybrid current drive requires the generation of a high-energy electron tail anisotropic in velocity. Measurements of bremsstrahlung emission produced by this tail are compared with the calculated emission from reasonable model distributions. The physical basis and the sensitivity of this modelling process are described, and the plasma properties of current-driven discharges which can be derived from the model are discussed.

Numerical studies of the beta limit in the first region of stability for n = ∞ ballooning modes in advanced-shaped tokamaks are presented. A higher beta value than corresponds to the present conventional beta scaling is expected for advanced-shaped tokamaks with sufficient triangularity/indentation. Extremely elongated configurations without appropriate triangularity do not lead to an increase in critical beta. Dee or bean shapes with sharply tipped cross-sections are more favourable for achieving high beta values than those with round-tip cross-sections having nearly the same safety factor profile. A new beta scaling for elongated ellipse, Dee- and bean-shaped tokamaks is derived.

The H-mode transition can lead to a rapid increase in tokamak plasma confinement. A semiempirical transport model was derived from global OH and L-mode confinement scalings and then applied to simulation of H-mode discharges. The radial diffusivities in the model depend on local density and pressure gradients and satisfy an appropriate dimensional constraint. Examples are shown of the application of this model and of similar models to the detailed simulation of two discharges which exhibit an H-mode transition. The models reproduce essential features of plasma confinement in the Ohmic heating and the low- and highconfinement phases of these discharges. In particular, the evolution of plasma energy content through the H-mode transition can be reproduced without any sudden or ad hoc modification of the plasma transport formulation.

Resistive ballooning mode stability is examined using one-fluid, two-fluid, and kinetic theories to determine the domain of validity of the resistive-fluid theories in the high-beta Doublet III tokamak. In general, it is found that the requirements for the validity of the resistive-fluid theories in Doublet III are only satisfied on the cold, outermost edge of the plasma with temperatures T ≤ 25 eV. In the domain of validity, the two-fluid and kinetic theories are qualitatively the same and indicate that the dominant effect is parallel resistivity with small corrections from perpendicular resistivity, fluid compression, thermal conduction, and ion collisional viscosity. The generic role of resistive ballooning modes in tokamaks and the implications for anomalous transport are also discussed.

Impurity radiation losses in net-current-free neutral-beam-heated plasmas in the Wendelstein W VII-A stellarator are the combined effect of particularly strong impurity sources and improved particle confinement as compared with ohmically heated tokamak-like plasma discharges. Experiments are described and conclusions are drawn about the impurity species, their origin and their transport behaviour. The impurity transport is modelled by a 1-D impurity transport and radiation code. The evolution of the total radiation in time and space deduced from soft-X-ray and bolometer measurements can be fairly well simulated by the code. Experimentally, oxygen was found to make the main contribution to the radiation losses. In the calculations, an influx of cold oxygen desorbed from the walls of the order of 10¹³–10¹⁴ cm⁻²·s⁻¹ and a rate of fast injected oxygen corresponding to a 1% impurity content of the neutral beams in combination with neoclassical impurity transport leads to quantitative agreement between the simulation and the observed radiation. The transport of Al trace impurities injected by the laser blow-off technique was experimentally studied by soft-X-ray measurements using a differential method allowing extraction of the time evolution of Al XII, XIII radial profiles. These are compared with code predictions, together with additional spectroscopic measurements. The main features of the impurity transport are consistent with neoclassical predictions, which explain particularly the central impurity accumulation. Some details, however, seem to require additional 'anomalous' transport. Such an enhancement is correlated with distortions of the magnetic configuration around resonant magnetic surfaces.

An efficient method is given to reconstruct the current profile parameters, the plasma shape, and a current profile consistent with the magnetohydrodynamic equilibrium constraint from external magnetic measurements, based on a Picard iteration approach which approximately conserves the measurements. Computational efforts are reduced by parametrizing the current profile linearly in terms of a number of physical parameters. Results of detailed comparative calculations and a sensitivity study are described. Illustrative calculations to reconstruct the current profiles and plasma shapes in ohmically and auxiliarily heated Doublet III plasmas are given which show many interesting features of the current profiles.

Current drive by lower hybrid waves that have a phase velocity five times the electron thermal velocity has been investigated in the JFT-2M tokamak. Two different modes of tokamak operation, the automatic current regulation (ACR) and the automatic voltage regulation (AVR) modes, were employed to study the plasma control during the current drive. In the ACR mode, which is the ordinary tokamak operation in JFT-2M, the current of the poloidal-field coils which produce the main magnetic flux outside the torus is controlled by thyristor power supplies, while in the AVR mode their voltage is controlled. The results indicate that control of the poloidal field determines the time evolution of the plasma current during current drive whereas operation in the AVR mode suggests the sustainment of a steady-state plasma current through recharging of the Ohmic transformer. A high current drive efficiency was obtained which increased with the plasma current as up to I_p = 150 kA. The dependence of the current drive efficiency on the plasma current is related to the electron temperature and to propagation effects of the lower hybrid waves. The interaction of the excited lower hybrid waves with the electrons was investigated by pulse height analysis of the soft-X-ray emission. It was shown that the measured soft-X-ray energy spectrum is consistent with the theoretical spectrum of the launcher. The characteristic times for the modification of the electron distribution function and for the drop in loop voltage are proportional to the electron density.

An experiment to test beryllium as a limiter material has been performed in the ISX-B tokamak. One facet of the studies centred on characterizing impurity influxes and concentrations. The radiation from both low-Z (Be, C, N, O) and intermediate-Z (Ti, Cr) contaminants was measured at the limiter surface and at a wall location 90° away from the limiter to assess the relative contributions of metallic impurities from the two different sources. The effect of limiter melting, with concomitant reduction of both low-Z and intermediate-Z elements, was also documented. The analysis, including atomic rate coefficients, for interpreting spectral radiation in terms of production rates is discussed in detail.

A zero-dimensional time-dependent energy balance model is used to explore the energy loss mechanisms of the CTX spheromak experiment at Los Alamos National Laboratory. A coupled set of model equations representing electron, ion, neutral, and impurity particle balance, electron and ion temperature, and magnetic field decay, are solved from initial values and the results compared to the time behaviour of experimentally measured average densities, temperature, and magnetic field. The energy balance model considers all the major atomic physics processes, especially the effects of radiation from a non-equilibrium distribution of impurity charge states evolving in time. The model includes the effects of a strong neutral-particle source which replaces by ionization the plasma being lost by a short particle confinement time. The neutral source is required in experiments to prevent the sudden termination of the discharge associated with low densities. A major new conclusion is that all the data from resistively decaying spheromaks can be effectively modelled, when the flux loss effects of a resistive flux conserver are also included, using plasma resistivity increased by a factor of 3.2 ± 0.6 over the Spitzer-Härm value evaluated with the volume-average temperature. This factor appears to be constant for all discharges at all times. The analysis has determined that the power density associated with the particle replacement is the most important loss in the warm, non-radiation-dominated CTX spheromaks.

An analysis is made of the regeneration process of toroidal magnetic flux in an RFP plasma based on Sato's nonlinear driven reconnection model. The expression for the magnetic field perturbation due to a helical kink instability indicates that a global helical kink mode can create the required configuration for this model. The approximate saturation level of the generated toroidal magnetic flux is estimated from the limit of the magnetic island width.

Electron cyclotron wave absorption by moderately relativistic electrons in the low-density regime of the PLT tokamak is investigated. Appreciable wave damping is found for vertical propagation at frequencies of 50, 60, and 70 GHz when the spatially constant cyclotron frequency is 89 GHz. The perpendicular temperature T_⊥ (v_||) of the fast tail is estimated from emission of radiation in the same direction. The results obtained are in satisfactory agreement with the theory of wave emission and absorption.

The Ergodic Magnetic Layer Experiment on ohmically heated Text demonstrates that a small resonant helical field (m/n = 7/2 or 7/3) with fractional amplitude _r/B_T of about 10⁻³ creates a stable ergodic magnetic layer and substantially modifies the boundary heat flow. Field line tracings accurately map the observed perturbed limiter heat load patterns. The level of the intrinsic impurities is reduced, consistent with a lower edge electron temperature, as measured by the electron cyclotron emission detector.

The IAEA Technical Committee Meeting on Negative Ion Beam Heating, held in Grenoble (France) from 20–22 March 1985 was initiated by the International Fusion Research Council and the International Atomic Energy Agency. This meeting was organized by the Département de recherches sur la fusion contrôlée (Euratom-CEA Association) of the Commissariat à l'énergie atomique with the support of the Centre d'études nucléaires de Grenoble.