The dependence of the efficiency of current drive by lower-hybrid waves on the shape and intensity of the wave spectrum is discussed in the case where no d.c. electric field is present. The discussion is based on a simple formula arising from a one-dimensional kinetic treatment of the problem.

In this paper the theoretical aspects of the stochastic acceleration of photoionized pick-up ions upstream of a comet's bow shock are examined. The acceleration is due to the resonant interaction of the ions with Alfvén waves. By making physically reasonable assumptions for the turbulent fields and by including an exponential spatial factor in the injection term, it is found that, with spatial diffusion ignored, exact solutions can be found.

Resonant absorption of Alfvén waves in tokamak plasmas is studied numerically using the linearized equations of resistive magnetohydrodynamics. A numerical code based on a finite-element discretization is used for determining the stationary state of a cylindrical plasma column that is excited by an external periodic driver. The energy dissipation rate in the stationary state is calculated and the dependence of the plasma heating on electrical resistivity, the equilibrium profiles, and the wavenumbers and frequency of the external driver is investigated. Resonant absorption is extremely efficient when the plasma is excited with a frequency near that of a so-called ‘collective mode’. The heating of a plasma by driving it at the frequencies of discrete Alfvén waves is also investigated.

The two-chamber model (TCM) of Singer and Langer is employed to study the plasma transport in the scrape-off and divertor regions of a tokamak. Collisiondominated transport along the field lines is considered, with a. geometric-mean flux-limited expression for parallel electron heat conduction. An analytic method for the catastrophe-theory study of the TCM is developed. Maxwell convention for the catastrophes is adopted. Catastrophes occur when the energy flux entering the divertor chamber from the main plasma scrape-off, the recycling coefficient and the ratio of electron temperatures in the scape-off to that in the divertor exceed some threshold values. It is seen that the behaviour of the plasma during these catastrophes is in qualitative agreement with the experimentally observed features of the plasma during the H-mode transition.

Since the first paper by Barston (1964) on electrostatic oscillations in inhomogeneous cold plasmas, it has been commonly accepted that all finite layers with a continuous profile in pressure, density and magnetic field cannot support normal surface waves but instead the waves always decay through phase mixing (also called resonant absorption). Here we reanalyse the problem by studying a compressible current sheet of a general structure with rotation of the magnetic field included. We find that all inhomogeneous layers considered in the high-β plasma limit do not support normal modes. However, in the limit of a low-β plasma there are some cases when normal-mode solutions are recovered. The latter means that the process of resonant absorption is not common for all inhomogeneous layers.

We study plasma-beam injection into transverse magnetic fields using both electrostatic and electromagnetic particle-in-cell (PIC) codes. In the case of small beam momentum or energy (low drift kinetic β) we study both large- and small-ion-gyroradius beams. Large-ion-gyroradius beams with a large dielectric constant ε ≫ (M/m)½ are found to propagate across the magnetic field via E × B drifts at nearly the initial injection velocity, where and M/m is the ion-to-electron mass ratio. Beam degradation and undulations are observed, in agreement with previous experimental and analytical results. When ε is of order (M/m)½ the plasma beam propagates across field lines at only half its initial velocity and loses its coherent structure. When ε is much less than (M/m)½ the beam particles decouple at the magnetic field boundary, scattering the electrons and slightly deflecting the ions. For small-ion-gyroradius beam injection a flute-type instability is observed at the beam-magnetic-field interface. In the case of large beam momentum or energy (high drift kinetic β) we observe good penetration of a plasma beam by shielding the magnetic field from the interior of the beam (diamagnetism). However, we observe anomalously fast penetration of the magnetic field into the beam and find that the diffusion rate depends on the electron gyroradius of the beam.

We investigate the effects of compressibility on magnetic reconnection, using as a basis the incompressible models of Priest & Forbes and Jardine & Priest. Our results show that compressibility modifies the reconnection process, without changing its essential character. In the region of inflowing plasma, compressibility tends to increase the convergence or divergence of the flow. Also, for regimes with a compression in the inflow the maximum rate of reconnection is increased, while for regimes with an expansion in the inflow the magnetic Mach number at the entrance to the diffusion region is increased. In the region of outflowing plasma the main effects of compressibility are to produce faster and narrower outflow jets, with a lower magnetic field strength.

A numerical parametric study of the radial ambipolar electric field in a stellarator reactor has been undertaken. With the numerical neoclassical code FLOCS (Flow Code for Stellarators), which is capable of handling both ions and electrons of all relevant kinetic energies, the radial ambipolar field (Er)AMB is determined from the algebraic condition that ion and electron fluxes are equal. As expected, the potential is of the same order of magnitude as the temperature. Somewhat surprisingly at first sight, however, the potential does not change much with the temperature (in the parameter range under consideration), being somewhat insensitive to moderate variations of T. An explanation for this behaviour is presented. Finally, the radial particle fluxes, consistent with the obtained (Er)AMB, and the particle confinement time are computed.

We have studied wave propagation in a cold plasma, in the presence of a spatially rotating magnetic field of constant magnitude. New features introduced by this variation include streaming velocities and a plasma current in equilibrium and density fluctuations. We present only the case of wave propagation along the axis of rotation of the magnetic field. A set of ordinary differential equations for the electric field components is obtained, which may be combined into a single fourth-order ordinary differential equation with periodic coefficients. Solutions are obtained in closed form and their nature is determined in terms of the physical parameters of the System, magnetic field strength, number density and wave frequency.

Electromagnetic-wave propagation in inhomogeneous magnetized plasmas is studied. Two different approaches to the subject are discussed and compared. Explicit expressions for the dielectric tensor components are derived following an established procedure that takes into account the effects of the gradients of plasma parameters, and are shown to possess non-resonant contributions to the anti-Hermitian parts. General and explicit expressions are also derived by following a different approach that has recently appeared in the literature, and are shown to possess satisfactory symmetry properties leading to anti-Hermitian parts comprising only resonant terms. The simple case of high-frequency waves in the ordinary mode propagating perpendicularly to the ambient magnetic field is presented as an example in order to show that the use of the dielectric tensor derived using the second method correctly describes wave absorption and/or amplification, eliminating the feature of non-resonant absorption that arises from the use of the first method.

The effect of non-uniformities on the flow of electric current in weakly ionized plasmas is investigated by taking into account the ion slip as well as the Hall current. An Ohm's law for a non-uniform plasma is derived, from which the formula previously obtained by Numano, i.e. an extension of Rosa's equation, is obtainable as a special case. Making use of this new Ohm's law, the effective electrical conductivity and the effective Hall parameter are determined for isotropically turbulent plasmas. It is found that when the ion-slip effect is absent they are in good agreement with the results obtained previously.

We consider the resonant interaction between two transverse waves and one longitudinal wave in a plasma. In particular, we discuss coupling phenomena involving long-wavelength modes that have been overlooked by previous authors.