Extremely large-amplitude Langmuir waves are investigated on the basis of relativistic kinetic theory. Analytical formulae for the electron density and the electric field of the wave differ essentially from earlier results obtained using the cold-hydrodynamical equations.

Reflection and excitation of ion-acoustic waves by a bipolar potential structure consisting of a grid and a metallic plate have been investigated experimentally in a multi-component plasma with negative ions. Reflection is observed owing to the presence of the negative ions, even if both the grid and the plate are biased negatively with respect to the plasma potential so as to collect positive ions. Reflection coefficients and excitation efficiencies are measured as a function of the bias voltage of the electrodes. The reflection coefficient becomes larger for a negative bias of the grid, and is almost constant for a positive bias of the grid as the partial pressure of SF6 gas is increased.

Parametric decay of an upper-hybrid pump into another upper-hybrid wave and a low-frequency lower-hybrid mode is considered in a two-electron temperature plasma. Expressions for the nonlinear dispersion relation and growth rate are obtained. It is found that the growth rate is quite sensitive to the hot-electron temperature and the density ratio of the hot and the cold components only when the side-band frequency is close to the second or third harmonic of the cyclotron frequency. The relevance of our investigation to Q machines and the ELMO bumpy torus is pointed out.

MHD instabilities are governed by the transport-determined plasma profiles, and the transport process is affected in turn by the instabilities. Using a one-dimensional code, we have investigated the inter-relationship between instabilities, transport and plasma profile in a tokamak discharge. The results show that the global energy confinement becomes strongly dependent on the boundary transport condition owing to strong coupling between them, and a higher edge temperature would ensure a higher core temperature and hence greater global energy confinement.

The characteristics of the specularly reflected laser light (1.06 μm) from a copper plasma have been studied experimentally. The diffusivity and depolarization of specularly reflected radiation can be explained by the density contours and self-generated magnetic field at the radiation turning point in the plasma layer.

The theory of linear magneto-acoustic surface waves is investigated for current sheets across which the magnetic field has an arbitrary change of direction: in the first place discontinuously, and in the second place via a narrow transition region in which the magnetic field rotates with constant amplitude, so that the gas pressure remains constant. It is found that the effect of non-zero pressure is to eliminate the surface wave for certain angles of propagation and to allow the existence of an additional, slower, surface wave for other angles of propagation. The resonance damping of the surface waves when the current sheet is of small non-zero width is considered, and it is found that Alfvénresonance damping always occurs, as well as (for high β and certain angles of propagation) compressive- or cusp-resonance damping.

We present the results of a theoretical investigation of the conditions for the existence of a stationary, strong, monotonic double layer. A model that includes finite temperatures both of the particle species accelerated by the double layer and of those reflected by it is developed on the basis of the general theory of double layers due to Andrews and Allen. A numerical solution of the model equations is presented and analysed. Explicit approximate formulae for the electron-to-ion current ratio (i.e. the Langmuir factor) and for the initial velocities (i.e. directional energies of the accelerated particles at the double-layer boundaries before acceleration) are also derived that fit almost exactly the exact numerical solution over a wide range of double-layer potentials. It is shown that in the cases in which the standard results of Langmuir and Bohm are an oversimplification, approximate formulae may be used instead of exact numerical solutions to obtain quite accurate results in a simple manner.

A lower bound for the total variation Bmax/Bmin of the field strength B on a toroidal vacuum magnetic surface with finite twist (rotational transform) of the field is given as a necessary (not sufficient) condition for the field lines to be geodesics on it. If such a surface existed, it would be omnigenous with respect to the local gradient-B and centrifugal guiding-centre drift approximation, which means that the normal components of these drift velocities would be zero. It is found that the lower bound on Bmax/Bmin depends on the maximum of – (r/B) ∂B/∂r within the surface. Here r is the distance from the origin of the torus. The lower bound is large if this maximum is small, and vice versa. For some more recent stellarator configurations the condition is found to be satisfied at outer magnetic surfaces. It is obtained without the use of expansion techniques, under the assumption that there is a nested system of magnetic surfaces within the omnigenous one.

Dust particles, assumed to be of one size and to exhibit a discrete distribution of electric charges, are treated as heavy ions with a large number of ionization levels. The average of the discrete particle effects on the kinetic equations is approximated by the Lénard–Balescu collision term and by detailed counting to describe transport in velocity space and transitions between the differentionization levels respectively. We estimate analytically and numerically the relaxation times for the dust particles both towards a Maxwellian velocity distribution and towards an equilibrium distribution for the ionization levels. We sum over the ionization levels to obtain a hierarchy of ‘charge-moment’ equations for the single dust density function, and estimate the importance of terms originating from the ionization distribution. Similar terms are also present in the hydrodynamic equations for a dust plasma, and we briefly discuss these.

In this paper we consider a set of nonlinear MHD equations that admits in a linear approximation a solution in the form of a slow sausage surface wave travelling along an isolated magnetic slab. For a wave of small but finite amplitude, we investigate how a slowly varying amplitude is modulated by nonlinear self-interactions. A stretching transformation shows that, at the lowest order of an asymptotic expansion, the original set of equations with appropriate boundary conditions (free interfaces) can be reduced to the cubic nonlinear Schrödinger equation, which determines the amplitude modulation. We study analytically and numerically the evolution of impulsively generated waves, showing a transition of the initial states into a train of solitons and periodic waves. The possibility of the existence of solitary waves in the solar atmosphere is also briefly discussed.

The theory of flute-mode stability is developed for a two-energy-component plasma partially terminated by a conducting limiter. The formalism is developed as a preliminary study of the effect of end loss in open-ended mirror machines, where large-Larmor-radius effects are important.

We present a new theory for the relaxation of two-dimensional collisionless quasi-neutral plasmas by mixing in phase space. In general, mixing in phase space does not lead to equilibrium states in the framework of exact, collisionless theory. However, filamentation on smaller and smaller scales can result in stationary states from a macroscopic (coarse-grained) point of view. In a first step we discuss a general technique to calculate suitably stable equilibria of collisionless plasmas. In a second step we develop a model relaxation that yields a simple transition to a description on macroscopic scales without following the details of the complicated mixing dynamics. Combination of both results allows calculation of final states of collisionless relaxation. We illustrate our approach with the help of a one-dimensional sheet configuration as a simple example.