Table of contents

The stability of axisymmetric modes is studied by using the general ideal-MHD code ERATO. The parameters investigated are the triangularity and the elongation of the tokamak meridian cross-section and the distance between the plasma and an external conducting wall. Both aspect ratio, R/a = 3, and safety factor on the axis, q₀ = 1, are kept fixed. The validity of simpler models is tested. Wall stabilization is found to be effective for axisymmetric modes.

The radiation loss in DIVA is measured quantitatively by a pyroelectric detector calibrated by heavily contaminated resistive discharges. The results are: (1) radiation loss power due to low-Z impurities does not play an important role; (2) radiation loss power due to the pseudo-continuum from high-Z impurities affects the power balance of the confined plasma; (3) the time dependence of energy loss channels is clarified in the gross power balance, a fact which attests to the reliability of the radiation measurements carried out by the pyroelectric detector.

A pulse of methane gas and a short burst of aluminium have been injected into DIVA plasma at the point farthest from the divertor. For the same amount of injected impurity, the accumulation of injected impurity in the main plasma is reduced by a factor of 2 to 4 by the action of the divertor. The impurity shielded by the scrape-off plasma is rapidly guided into the burial chamber. The time needed for the poloidal excursion of the injected impurity ions in the scrape-off layer is roughly equal to that of the scrape-off plasma. Sweeping-out of the impurity ions diffusing from the main plasma into the burial chamber is also observed.

It is shown that the presence of a sufficient number of accelerated electrons in a plasma leads to anomalous diffusion of the electrons across the magnetic field. The diffusion of the particles is associated with the presence of waves driven unstable by the runaway electrons via anomalous and Doppler effect. Expressions are obtained for the diffusion coefficient, the oscillation spectrum and the distribution function of the accelerated electron beam.

The growth rates and eigenfunctions of the leading internal modes are calculated for some high-beta tokamak equilibria with force-free currents for beta values ranging from 5 to 20%. The force-free currents flowing between the main plasma and the conducting wall stabilize the internal hydromagnetic instabilities including the ballooning modes. Modes with low toroidal mode numbers (n ⩽ 3) are stabilized at a plasma beta of about 10% when the current density in the force-free region is comparable to that in the main column.

An alternative approach to the analysis of radial eigenmodes of finite-β-modified drift waves in a sheared magnetic field is presented. As an illustration of this approach, it is applied to the universal drift mode. For the lowest (n = 0) radial eigenmode, the previous result [1] that finite-β effects are further stabilizing is recovered. For the next (n = 1) radial eigenmode, however, it is found that finite-β effects play a de-stabilizing role. The corresponding electromagnetic eigenmode structures are also obtained.

By means of a 2-d, multi-species, Fokker-Planck code, it is found for conventional mirror reactors that heating of the electrons from an external power source leads to a decrease in Q. Likewise, for mirror plasmas such as 2XIIB, electron heating does not improve the total energy confinement because of additional losses associated with quasi-linear stabilization requirements.

A study of the possible non-linear saturation by mode coupling of the dissipative trappedion mode is presented in which both radial and poloidal variations are considered. The saturation mechanism consists of, the non-linear coupling via convection of energy from linearly unstable modes to stable modes. Stabilization is provided at short poloidal wavelengths by Landau damping from trapped and circulating ions for η_i ≡ d ln(T_i)/d ln(n₀) < 2/3, at short radial wavelengths by effects associated with the finite ion banana excursions and at long wavelengths by ion collisions. The authors' analysis applies only to the case η_i < 2/3. A one-dimensional, non-linear partial differential equation for the electrostatic potential derived in earlier work is extended to two dimensions and to third order in amplitude. Included approximately in a systematic way are kinetic effects, e.g. Landau damping and its spatial dependence due to magnetic shear. The stability and accessibility of equilibria are considered for cases far from as well as close to marginal stability. In the first case, three-wave interactions are found to be important when the spectrum of unstable modes is sufficiently narrow. In the latter case, it is found that, for a single unstable mode, a four-wave self-interaction can provide a saturation mechanism. Accessibility of equilibria is determined in both cases. In the case where three-wave interactions are dominant, the stability analysis is inconclusive. However, plasma parameters are found for which equilibria established by means of the four-wave self-interaction are stable to linear perturbation. Cross-field transport is calculated, and the scaling of results is considered for tokamak parameters.

Chord observations of radiation from oxygen impurity ions (O III, O IV and O V) in the LT-3 hydrogen plasma have been made simultaneously through a number of optical ports. These showed a spatially structured entry of oxygen into the plasma just before the disruption and, at the disruption, an expansion of the central plasma into the outer regions with velocities of 3–8 km · s⁻¹. The plasma column appeared to be kinked with a stationary modal structure of m = n= 1 on which were superimposed a moving central m = 1 perturbation and, at larger radii, moving perturbations with m > 1.

A two-dimensional simulation of the spherical expansion of a laser-produced plasma is performed for an SiO₂ microballoon irradiated by CO₂ laser light. Important implications of the results are discussed with regard to (a) absorption measurements, (b) unifying previous calculations and (c) symmetry of the expanding plasma.

A stability principle is derived and applied to simple tokamak configurations in order to study feedback stabilization of unstable, vertically elongated tokamak plasmas. For practical applicability it is assumed that the fast instabilities are slowed down by passive conductors so that only slow motions have to be considered. Numerical results are presented for a surface current model of plasma with one conjugate pair of axisymmetric feedback loops. Stabilization is possible, except in a limited region of loop positions. The optimum loop position in the region with possible stabilization is determined.

Disruptive instabilities in PLT have been analysed by using an array of surface barrier X-ray detectors viewing the plasma cross-section along 20 chords. A wide variety of phenomena has been observed. A system of classification has been attempted, based upon: (1) the severity of the disruption, (2) the dominant precursor oscillations, and (3) the location of the onset of the disruption. Minor disruptions, in which the disruption does not appear at the location of the island of the precursor oscillation, have been observed, sometimes accompanied by seemingly independent higher-frequency oscillations of different helicity localized near the point of the disruption. Major disruptions exhibit flatter central q-profiles, slowing of the oscillations, asymmetry with respect to the centre of the discharge, and a correlation with high-Z impurity radiation.

Conventional stellarators utilize low-multipole-order helical windings, ℓ = 2 or 3, because a high-multipole-order helical winding results in localization of the rotational transform near the plasma surface. If, however, a carefully chosen axisymmetric poloidal magnetic field is added, high-shear, high-multipole-order (ℓ ≈ 6–9) stellarator configurations become possible without a vanishing rotational transform in the plasma interior. Because of the well-known potential advantage of non-circular cross-section plasmas, such as the Doublet configuration over the circular cross-section tokamak, the Doublestar was chosen for a numerical study. Included in this study was the question of the sensitivity of the Doublestar configuration to error fields. As expected, it was found that the size of islands produced by error fields is relatively small because of the large shear of the Doublestar.

A simple equation is given for the impurity radiation in fusion plasmas. The equation is an empirical fit to corona-model calculations found in the literature and is valid for all Z > 6 and electron temperatures between 1 and 100 keV.

A fusion power reactor can be fuelled practicably by injection of pellets accelerated by laser-produced ablation. The fuelling requirements of a demonstration fusion reactor can probably be met by present laser technology.