Table of contents

Rotation of a helical magnetic limiter in the poloidal and toroidal directions is shown to be a good method of avoiding the non-uniform wall loading that is obtained when a static local helical field is applied. Mechanisms of the static and dynamic modification of the plasma potential at the tokamak edge are discussed in terms of magnetic field structure, ambipolarity, electron transport along the stochastic magnetic fields and cross-field drift motion of plasma particles. The magnetic structure is studied by Poincaré mapping of field lines traced numerically, using a code that accounts for the exact configuration of local helical coils.

The paper deals with the simulation of neutral particle spectra obtained in JET with an array of four passive neutral particle analysers during discharges with applied stationary or modulated radiofrequency power in the ion cyclotron range of frequencies for a deuterium plasma with hydrogen minority. The distribution function of the hydrogen ions has been computed using a one-dimensional Fokker-Planck equation describing wave-particle interaction by a quasi-linear diffusion term in velocity space. Both stationary and oscillating solutions of the Fokker-Planck equation have been worked out and the computed hydrogen fluxes have been found to be in very good agreement with the measured ones. In this context, the problems arising in the deduction of the power deposition profiles in modulated discharges are discussed. The profiles of power delivered to the electrons and the bulk ions have been obtained. It is shown that no adjustable parameters need to be introduced and that the main source of error lies in the uncertainty in the knowledge of the ion density.

To produce a field reversed configuration (FRC) without using the very high voltage techniques required in a conventional theta pinch, the author has studied the steady head-on collision of counter plasma streams, flowing along the magnetic field, as ejected from two identical co-axial plasma sources mounted at each end of the apparatus. The study was motivated by the fact that such a flow can be regarded as a steady state theta pinch. Ideal Poisson and shock adiabatic flow models were used to analyse the steady colliding process. It was demonstrated that an FRC involving a large number of particles can only be produced by the weak shock mode, which is achieved when energetic plasma flow is decelerated and heated up exclusively through the Poisson adiabatic process before the streams collide.

Coupling structures for lower hybrid current drive experiments have, until now, been smaller than a free space wavelength and have had a correspondingly broad wavenumber spectrum. The paper reports the results of experiments on the PLT tokamak using a 16-waveguide grill (2.2 wavelengths) which produces a very narrow n_∥ = k_∥c/ω spectrum. Experimental results from the 16-waveguide grill are compared with results from three other PLT grills with less sharply defined n₁ spectra. The current drive figure of merit, , is approximately 40% higher for the experiments with the 16-waveguide coupler than for previously reported experiments on PLT, in spite of the larger 'spectral gap'. The experimental results are consistent with the first-pass damping of a large fraction of the launched spectrum.

The paper describes non-linear numerical simulations of ideal internal kink modes in cylindrical geometry with a flat central q-profile as a model for the rapid sawtooth collapse seen in large tokamaks. The resulting plasma motion is consistent with that obtained experimentally from X-ray tomographic reconstruction. Attention is drawn to the effects of limited poloidal resolution in this diagnostic.

The paper presents a survey of possible parametric instabilities in the upcoming Microwave Tokamak Experiment (MTX) at the Lawrence Livermore National Laboratory in which an intense pulsed freeelectron laser will be used to carry out electron cyclotron heating in the Alcator C tokamak. Potentially the most dangerous instability is stimulated Brillouin backscatter by electrostatic ion cyclotron modes or quasi-modes for ordinary-mode heating at the fundamental harmonic or extraordinary-mode heating at the second electron cyclotron harmonic. Large reflectances may occur for coherent microwave sources. In addition, extraordinary waves at the amplitudes considered here can be non-linearly absorbed by parametric decay processes at the resonance layers where the pump frequency equals the second electron cyclotron harmonic and where it equals twice the local upper hybrid frequency. Finite-bandwidth effects can significantly inhibit parametric instabilities. Density fluctuations induced by drift wave turbulence at the plasma surface, with amplitudes as observed in earlier Alcator C experiments, are expected to spread the electron cyclotron wave packets but may be of insufficient amplitude to suppress Brillouin backscatter instability.

Small aspect ratio tokamak configurations are produced by adding a toroidal magnetic field to the rotamak. Configurations with toroidal fields ranging up to ∼ 200 G at the magnetic axis are studied. Typical parameters are I_tor ∼ 2 kA, B_poi ∼ 50 G, q₀ ∼ 1-3, n_e (peak) ∼ 10¹⁹ m⁻³, T_e ∼ 12 eV, τ_p ∼ 15 μs and τ_e ∼ 5μs for an RF input power of ∼ 40 kW. Most features of these configurations can be modelled by an MHD equilibrium code. The presence of the toroidal field is observed to have a significant influence on the rotating magnetic field current drive mechanism.

The current drive by HF waves in a tokamak with longitudinal electric field is studied on the basis of the two-dimensional kinetic equation for electrons. Analytical formulas for current and current drive efficiency are obtained for a low level of absorbed power. Numerical methods are used to solve the kinetic equation at high HF power level. It is shown that the dependence of the efficiency on the effective charge of the plasma changes with increasing power absorption. The existence of trapped particles reduces current drive efficiency. An analysis of the calculation results leads to simple approximate expressions for current and efficiency, convenient for comparisons with the experiments.

The power deposition profiles during minority ion cyclotron heating are analysed in large tokamaks by using the global, toroidal wave code LION. For tokamaks with large aspect ratio and with circular cross-section, the wave is focused on the magnetic axis and can be absorbed there by cyclotron absorption when the cyclotron resonance passes through the magnetic axis. The power deposition profile is then essentially determined by the Doppler broadening of the ion cyclotron resonance. For equilibria either non-circular or with a small aspect ratio the power deposition profile depends also on the strength of the damping. In this case the power deposition profile can be expressed as a sum of two power deposition profiles. One is related to the power absorbed in a single pass, and its shape is similar to that obtained for large aspect ratio and circular cross-section. The other profile is obtained by calculating the power deposition in the limit of weak damping, in which case the wave electric field is almost constant along the cyclotron resonance layer. A heuristic formula for the power deposition is given. The formula includes a number of calibration curves and functions which has been calculated with the LION code for JET relevant equilibria. The formula enables calculation of the power deposition profile in a simple way when the launched wave spectrum and damping coefficients are known.

The condition for stability of the ideal m = 1 mode in toroidal tokamak geometry is calculated for safety factor profiles which are flattened inside the q = 1 surface. It is shown that for such profiles the critical β_p for stability goes to zero as the safety factor on axis approaches unity. The critical β_p may therefore be small enough to explain sawtooth observations on JET.

Transitions of L-mode plasmas to the H-mode have been induced by an electron cyclotron heating (ECH) pulse. The transitions occur when ECH is applied to plasmas preheated either by a neutral beam or by waves in the ion cyclotron range of frequency with power levels well below their own threshold power for the H-mode transition. The position of the electron cyclotron resonance layer has been scanned and it has been shown that edge heating rather than central heating is effective in inducing the transition to the H-mode.

The interchange (Mercier) stability of toroidal reversed field pinch plasmas with non-circular cross-section has been evaluated numerically. Marginally stable pressure profiles and beta values have been produced. Most shapes, such as indented or vertically elongated, reduce the stability by making the net magnetic curvature of the poloidal field dominated plasmas yet worse than that of circular configurations. The stability of horizontally elongated plasmas is slightly enhanced compared with that of circular plasmas as a result of increased shear produced by toroidicity. This shear enhancement by toroidal shift of magnetic surfaces might be exploited in future, more comprehensive studies.

The loop voltage of REPUTE-1 RFP is decreased by reducing the toroidal field ripple. The low frequency part of the toroidal field fluctuations, which is mainly composed of the m = 1 mode and has good coherence in space and time, is found to be reduced, especially outside the reversal surface, when a trimming field is applied. High frequency fluctuations, which have little coherence, are not influenced effectively.

The characteristics of ohmically and high power neutral beam heated (P_NBI ∼ 20 MW) hydrogen divertor plasmas in JT-60 are analysed on the basis of a plasma fluid model. The calculated radiation loss of the divertor plasma and the density dependence of the neutral pressure in the divertor chamber are qualitatively in good agreement with the experimental results. The divertor radiation loss in ohmically heated discharges can be explained by hydrogen line radiation and impurity radiation. The higher neutral pressure in neutral beam heated discharges can be explained by the decrease of the global particle confinement time in comparison with that in ohmically heated discharges.

The IAEA Technical Committee Meeting on Negative Ion Beam Heating was hosted by the Culham Laboratory (United Kingdom Atomic Energy Authority) at the instigation of the International Atomic Energy Agency. The meeting was attended by 38 participants from nine countries; 24 papers were presented and two panel sessions held.

The Energy Independence Conference on Fusion Energy and Plasma Physics was held in Rio de Janeiro (Brazil). This conference was sponsored by the International Union of Pure and Applied Physics (IUPAP). One of the prime purposes of the meeting was to assess and promote the role of developing countries in fusion research. Some one hundred people attended the meeting, with a good representation from the Latin American countries, and participants from Austria, Bangladesh, China, Egypt, India, Israel, Kenya, Malaysia, Poland and Portugal, and with a good selection of specialists from the developed countries.