Table of contents

Interest in attaining high beta has stimulated the study of strongly shaped tokamak configurations. The influence of strong shaping on neoclassical transport, specifically on the ion thermal flux and the diffusion induced (or 'bootstrap') current, has been investigated. Axisymmetric magnetohydrodynamic equilibria have been computed for elliptical, D, bean and crescent shapes. The geometric data have been used in the Drift Kinetic Equation Solver code of the Oak Ridge National Laboratory to compute the resulting fluxes under a range of conditions. It is found that neoclassical transport generally improves with advanced shaping, as the larger plasma current reduces the ion thermal flux. Optimization of both neoclassical transport and beta presents conflicting demands, however, because 'omnigenous' reductions in neoclassical fluxes can be obtained only at the expense of reducing the local shear, which causes a deterioration in the critical beta.

The results of runaway electron confinement experiments from ASDEX are analysed to elucidate the structure of electromagnetic turbulence which may cause anomalous electron heat transport in the L-mode confinement regime. From a simple model, the radial correlation length (W) of the magnetic turbulence is determined to be about 1 mm. Using this value and that of the experimentally deduced electron thermal diffusivity, the authors determine the radial magnetic fluctuation level at the plasma edge in the L-mode to be (_r/B₀) ∼ 2 × 10⁻⁴. Scalings of W and _r/B₀ are deduced from parameter scans. From a comparison of these results with the predictions of various theoretical models, it is concluded that skin depth turbulence, electromagnetic drift wave turbulence, rippling modes, and microtearing modes are inferior candidates and that resistive ballooning modes offer the best possibility for a consistent interpretation of the data.

In relating the impurity influx Φ₁⁰ [atoms per second] into a plasma from the edge to the central impurity ion density n₁(0) [ions-m~3], it is necessary to know the value of τ₁^SOL, the average dwell time of impurity ions in the scrape-off layer. It is usually assumed that τ₁^SOL = L_c/c_s, the hydrogenic dwell time, where L_c is the limiter connection length and c_s is the hydrogenic ion acoustic speed. Monte Carlo ion transport results are reported here which show that, for a wall (uniform) influx, τ₁^SOL is longer than L_c/c_s, while for a limiter influx it is shorter. Thus for a limiter influx n¹(0) is predicted to be smaller than the reference value. Impurities released from the limiter form ever large 'clouds' of successively higher ionization stages. These are reproduced by the Monte Carlo code as are the cloud shapes for a localized impurity injection far from the limiter.

At low q_ψ (2.3 ≤ q_ψ ≤ 4.5), in the Princeton Beta Experiment, the discharges are limited by a hard disruption following the growth and sawtooth-like 'crash' of a ≤25 kHz precursor oscillation. The disruption, which occurs even in discharges with ⟨β_t⟩ well below the first stability regime boundary (≅2.5 μ₀I_p/aB_t), follows the crash of this precursor mode either immediately or with a delay of several milliseconds, with the immediate disruptions primarily occurring in the discharges with ⟨β_t⟩ close to the first regime limit. The highest ⟨β_t⟩ discharges also exhibit the fastest growth times and the highest level of edge MHD activity. Associated with the precursor mode crash is a loss of up to 30% of the plasma energy; thus, for non-zero delay shots, it is the crash and not the actual disruption that is the ⟨β_t⟩ limiting process. The delay period is interpreted as a period during which a locked mode, consisting of several toroidal components of comparable amplitude, grows. Because of the energy loss associated with the crash, the plasma goes vertically unstable during the delay period. The results of this study indicate that even within the relatively narrow low-q_ψ operating space, there is a continuum in the characteristics of the low-q^ disruptions with a primary dependence on the value of ⟨β_t⟩. While the ideal external kink instability may give rise to the growing oscillations that lead up to the ultimate disruption, the instabilities are weighted towards the edge only at the lowest q_ψ (≤3) and highest ⟨β_t⟩. The results of this study indicate that effects outside the scope of ideal MHD theory may play a significant role in low-q_ψ disruptions.

The strength of the photoneutron emission accompanying major plasma disruptions in JET has been studied for the period from startup to December 1986. The observed photoneutron yields cover the range from 10¹⁰ to 10¹⁵ neutrons, with instantaneous intensities reaching 5 × 10^l7 neutrons⋅s⁻¹. It is shown that the energies attained by runaway electrons can be very high, perhaps exceeding 100 MeV, so that the runaway currents of high energy electrons are significant, sometimes exceeding 1 MA, and, on average, the fraction of the available magnetic energy converted to high energy runaway electron production is f = 2.9 × 10⁻² I_p^0.6, with the plasma current I_p in MA.

A joint Euratom-CEA, Euratom-FOM electron cyclotron resonance heating (ECRH) programme was conducted on the TFR tokamak during 1985 and 1986, with 600 kW of ECR power at 60 GHz. The paper describes the experiments performed during this period and analyses the results. In spite of considerable electron heating, a degradation of the global energy confinement time has been observed in all cases, with incremental confinement times ranging from 1 to 2 ms, depending on the experimental conditions. The electron thermal conductivity has been determined in three different independent ways and the results are in reasonable agreement, but only 1/4-1/3 of the gyrotron power is found in the rate of change of the plasma energy. Other topics addressed are: impurity content and transport, sawtooth behaviour, MHD stabilization, suprathermal electron population and pellet ablation during ECRH.

Ion cyclotron radiofrequency heating experiments were performed with a ³He minority ion species in a ⁴He majority plasma in the Princeton Large Torus. The energetic ³He ion 'tail' was measured directly with a charge exchange neutral analyser for the first time. Comparisons with bounce averaged quasi-linear calculations suggest a modestly peaked radiofrequency power deposition profile. The double charge exchange process ³He⁺⁺ + ⁴He⁰ → ³He⁰ + ⁴He⁺⁺ demonstrated in these measurements may be useful as part of an alpha particle diagnostic in a fusion reactor experiment.

The review is devoted to field reversed configurations and to the related field reversed mirrors; both are compact toroids with little or no toroidal magnetic field. Experimental and theoretical results on the formation, equilibrium, stability and confinement properties of these plasmas are presented. Although they have been known for about three decades, field reversed configurations have been studied intensively only in recent years. This renewed interest is due to the unusual fusion reactor potential of these high beta plasmas and also to their surprising macroscopic stability. At the present time, field reversed configurations appear to be completely free of gross instabilities and show relatively good confinement. The primary research goal for the near future is to retain these favourable properties in a less kinetic regime. Other important issues include the development of techniques for slow formation and stability, and a clearer assessment of the confinement scaling laws.

The radial transport rate of bulk ions in the Tara central cell, in a globally axisymmetric configuration, is determined by comparing the ionization source profile deduced from axial two-dimensional measurements of H_α emission with the axial electron end-loss profile. The radial transport of an impurity ion is also calculated, from the departure of the radial profile of O V from coronal equilibrium. The two techniques yield comparable transport coefficients: D ∼ 3 × 10⁴cm² · s⁻¹ and v ∼ – 1000r cm · s⁻¹, near the plasma core. A separate technique is used to deduce a lower bound for the edge bulk ion radial transport rate of 3 × 10⁵ cm² · s⁻¹. Azimuthal electric fields are proposed as the likely source of particle transport.

The 16th IQEC (IQEC '88) was held from 18 to 21 July 1988 at the Keio Plaza Hotel, Tokyo, Japan. It was chaired by K. Shimoda, Professor Emeritus of the University of Tokyo, and organized by the Japan Society of Applied Physics. The International Joint Council on Quantum Electronics, composed of Japanese, North American, Australasian, Chinese, European and Soviet subcommittees, was organized to select papers to be presented at IQEC '88. Altogether, 378 papers were presented, including 4 plenary lectures, 78 invited presentations and 32 post-deadline papers. Oral presentations were given in four to five parallel sessions, and two sessions were organized for poster presentations. The conference was attended by 869 participants from 26 countries.

The 15th Symposium on Fusion Technology (SOFT) was held in the Congress Centre of the Jaarbeurs in Utrecht from 19 to 23 September 1988. The Symposium was hosted by the Association Euratom-FOM and the Netherlands Energy Research Foundation (ECN). More than 500 scientists from 21 countries assembled in this luxurious setting to exchange information on the technological aspects of fusion research. About 15% of the participants came from outside Europe and about 15% of the participants are involved in industrial research. Parallel to the Symposium, an industrial exhibition was organized by Dutch Scientific. Twenty-two invited papers and 298 contributed papers were presented, covering the topics shown in Table I. Compared with the previous SOFT, the number of papers is 8% higher. The largest increase is for papers on the topic "materials for fusion devices", which indicates the approach of the first reactor-like devices; there is an almost equal and surprising increase for papers on the topic "experimental systems". The number of papers on the topic "magnet engineering and power supply technology" declined strongly.