Table of contents

The formation and compression of e-beam-induced reversed-field theta-pinch configurations are studied theoretically in an MHD treatment. Taking into account classical collisional resistivity plus anomalous resistivity derived from microinstabilities, it is shown that plasmas can be created with parameters suitable for liner compression experiments. Adiabatic compression of the quasi-equilibrium plasma/flux mixture is described by using Lagrangian co-ordinates. Results are given for a sample (unoptimized) initial configuration. In the final state, the ratio of plasma to total energy and the total D-T rate are 30% and 20%, respectively, of the values which would be obtained if all the compressional energy went into heating plasma,i.e. assuming negligible magnetic-field-filled volume.

Self-modulation effects can become important for the propagation of lower hybrid waves in plasma, particularly for the high power levels envisioned in r.f. heating schemes. Earlier studies in two dimensions (in the plane defined by the electric field of the pump wave and the background magnetic field) have led to non-linear propagation equations, such as the MKdV or the non-linear Schrödinger equation, which admit multiple-soliton solutions. These could physically manifest themselves by breaking up the resonance cones into filaments with intense localized electric fields and could further lead to localized heating. This problem is studied in three dimensions with the motivation of including two additional physical factors. First, the non-linear effect arising from the motion of electrons is included; this leads to an enhancement in the threshold value for the formation of solitons. Secondly, the stability of the two-dimensional solitons to perturbations in the third dimension is studied, and it is found that the third dimension introduces additional dispersive effects which render the solitons unstable to these perturbations.

This paper discusses the time evolution of a slow Reversed-Field Pinch during the sustainment phase, starting from an initial low-β configuration at the end of the setting-up phase which corresponds approximately to a Taylor Reversed-Field Bessel-function state. Calculations are made with a one-dimensional MHD (1DMHD) equilibrium and diffusion code. Because of local overheating and consequent rise of plasma pressure the central region of low shear becomes Suydam-unstable, and it is assumed that this results in local MHD turbulence, so flattening out the central density and temperature profiles and leading to a quasi-steady state which gradually evolves in time. The pinch then consists of two main zones, a central Suydam-unstable core I and an outer MHD-stable layer II which is responsible for confinement. An external zone III may exist near the wall but is not studied here. The configuration time τ_c depends on the time for which the trapped positive B_z flux ψ₊ can persist against resistive diffusion. This in turn depends on the electron temperature in zone II and hence on the anomalous electron thermal conductivity that is assumed in the model. Some information can be obtained by normalizing the phenomenological transport coefficients used in the code to the measurements made on ZETA but this does not provide a very sensitive check. Further information can be obtained from empirical tokamak scaling laws. Then the performance of the proposed device RFX is predicted. A significant difference between ZETA and RFX is that in ZETA it was the disappearance of the negative flux ψ₋ that terminated the quiescent period, while in RFX it is the positive flux ψ₊ that controls the configuration time. This leads to a substantial improvement in predicted performance.

It is shown theoretically that an electron beam injected into a plasma can substantially modify the dispersion characteristics of lower-hybrid pump-driven quasi-modes, ultimately enhancing in this way the efficiency of plasma heating.

Superthermal electrons are observed in the scrape-off layer of DIVA. The superthermal electrons discussed in this paper are divided into two groups, high-energy electrons (10 keV – 100 keV) and epithermal electrons (150 eV – 500 eV). The epithermal electrons are studied quantitatively and their loss is shown to be explained by the destruction of magnetic surfaces near the separatrix owing to non-axisymmetric perturbations. The two-dimensional path of the high-energy electrons is studied, and the effects of nonaxisymmetric perturbations on the drift surface are discussed.

A class of neighbouring resistive equilibria for a cylindrical tokamak is constructed, characterized by the invariance of the helical flux χ and of the quantity K = ∫ dV. The equilibria of this class bifurcate for critical values of the distance of the current channel from the metallic wall, while a cold plasma remains in contact with the wall. The bifurcation only exists for q < m/n in the current channel and for a peaked current, the critical distance from the wall depending solely on the degree of peaking. Any helically deformed equilibrium with a sufficiently peaked and constricted current (so as to reach the bifurcation point) is unstable with respect to a dissipative purely radial compressible mode which conserves approximately K and χ. The building up of the mode implies a decrease of the current, accompanied by a negative axial voltage near the wall. The resistive mode evolves towards a non-linear phase in which the instability exhibits an explosive behaviour. This phase, however, can only be reached if the radial mode is triggered by a helical deformation with sufficiently large amplitude as can be provided by a non-linearly saturated tearing or kink mode.

The interaction of cyclotron radiation with a relativistic Maxwellian plasma has been re-examined with the aim of developing a computational method which will be useful for the range of parameters of interest in tokamak studies. A suitable method and the resulting computer code are briefly sketched. A sampling of results is presented and compared with previously published approximate results. A discrepancy in the ordinary-wave interaction at 90° propagation is pointed out. A simple model is discussed which permits easy extension of the computational method to include runaway electrons, and illustrative results are presented.

This paper intends to review recent stellarator research in various laboratories. Efforts are continuously and actively being made to improve the confinement of plasmas in stellarator fields. Advances in confinement theory are summarized. Experimental results concerning Ohmically heated plasmas as well as currentless plasmas are reviewed. Finally, stellarator reactor studies are briefly introduced.

A stability analysis of the neoclassical point model of EBT distinguishes the sign of the self-consistent radial ambipolar field for a given type of plasma profile. Positive electric fields correspond to normal plasma profiles (peaked at the centre) and negative electric fields correspond to inverted profiles (depressed at the centre). These results provide a further macroscopic prediction against which neoclassical theory is to be tested experimentally.

The interaction of large-amplitude lower-hybrid waves with finite-frequency density perturbations is considered. The non-linear equation which governs the asymptotic state of the filamentation admits moving solitary envelope solutions for the electric field.

A formula for the β-ratio of confined low-pressure plasma in linked magnetic wells within the magnetic surface is derived.