Detailed measurements of the trapped magnetic fields and currents in plasma structures generated by conical theta-pinches are reported. Studies of these structures interacting with a magnetic barrier, and with each other in a collision at the centre of a magnetic mirror, are reported. The magnetic well formed by the collision has been studied by simultaneous use of several diagnostic techniques. The measurements are in agreement with a force-free, collinear magnetic field configuration (Wells 1972). Arguments relating superposability and collinearity of flow fields to these observations are given.

The results of the nonlinear computer investigationof the whistler mode instability with the aid of particle-in-cell simulation methods are presented. The electron plasma considered is hot (kT‖ = 20 keV), anisothermal (T⊥/T‖ ≃ 2) and embedded in a static magnetic field such that β‖ = 0.8. A detailed Fourier analysis of the electromagnetic activity developed under the above stated conditions is carried out: the waves are shown to be electron-like and excellent agreement with the linear stability analysis for the first stages of evolution is found. The feed-back effect of the waves on the particles is shown to result in a continuous decrease of the thermal anisotropy ratio T⊥/T‖ corresponding changes in the Fourier spectra of the electromagnetic activity are observed; additional changes in the wave spectrum are introduced by the interaction between various instability modes. At the end of the run (ωpt ⋍600), the state of the system resembles a quasi-stable equilibrium, in which the electromagnetic energy achieves its maximum value: in this state, unlike the equilibrium one usually considered in the linear stability analysis, a thermally anisotropic plasma with T⊥/T‖⋍1·35 appears to be quasi-stable against the whistler mode instability. This last result is relevant for the geostationary magnetospheric conditions (in the equatorial region) where quasi-stationary states with T⊥/T‖ about 1·3 are observed.

A parametric investigation of parallel right-hand electromagnetic waves below the electron cyclotron frequency has been carried out; this wave is unstable for anisotropic temperatures if T⊥ <T1. Simple analytic expressions for the maximum growth rate have been obtained for the full range of the parameters β1, _ and A_ ≡ (T⊥/T1)_ -1 as they range from zero to infinity. It is shown that the parameter P ≡ β1_A_(A_+1)2 has an important role: (i) for P ≪1, the results of Kennel & Petschek (1966), for large resonant velocities, hold; (ii) for P ≫ 1, the results of Sudan (1963, 1965) for small resonant velocities are recovered; (iii) for P ≫ 1 and A ≫ 1, the growth rate approaches an upper limit (near the plasma frequency for hot plasmas) identical to the zero field instability of Weibel (1959). For the case P ≫ l and moderate A values (i.e. Sudan's regime), analytic expressions for the rates of growth are calculated without restriction on their magnitude.

The Charybdis experiment is designed to produce shock waves propagating into a magnetized hydrogen plasma in directions parallel and almost parallel to the initial field and, in particular, the ‘switch-on’ shock. An initial plasma

is produced in a pyrex chamber (0.46 m dia., 125 m long) containing an axial magnetic field (Bz up to 0.28 T): a magnetic piston is produced by a fast-rising radial discharge between a short central and an annular outer electrode at one end of the chamber; the Lorentz force on the current sheet causes axial propagation, and under certain conditions a curved shock front propagates ahead of the piston. The Alfvén Mach number of the flow is varied by changing the axial field strength: at high Mach number, where a parallel shock should be gas dynamic, no separated shock is found; at low Mach number, where theory predicts‘switchon’ shock behaviour, a clearly separated curved shock front is found which passes through a point of parallel propagation. Magnetic and electric probe measurements at the point of parallel propagation show the presence of a ‘switchon’ shock. It is suggested that the ‘switch-on’ and adjacent oblique shocks are matched together by an intermediate wave propagating behind the latter and merging with the former.

Shielding of test charges in warm, isotropic electron and electron–ion (Te ≫ Ti) plasmas is studied analytically and numerically. For a plasma with hot Maxwellian electrons and cold mobile ions, the potential due to a charge moving faster than the ion acoustic velocity has an ion acoustic Cerenkov cone. Ahead of the particle, the shielding is the usual electron Debye type with a modified longer shielding length. Potential wells with γ−1 dependence exists inside the cone. The potential falls off as along the cone surface. Outside the cone, the potential decays exponentially. A charge moving slower than the ion acoustic velocity also creates a cone, with potential decay as γ−3 outside the cone, potential wells decaying as γ−1 inside the cone, and potential wells falling off as along the cone surface. In both cases a radial logarithmic singularity exists along the trailing axis. Using a mono-energetic ion distribution, the singularity is removed and an ion thermal Cerenkov cone appears. For a monoenergetic electron plasma, assuming immobile ions, a test charge moving faster than the electron thermal velocity excites a thermal Cerenkov cone. Outside the cone, the far-field potential falls off in quadrupole form as γ−3. Inside the cone, potential wells decay as γ−1.

The instability of the ordinary electromagnetic mode propagating perpendicular to an external magnetic field is investigated for two ion beams with anisotropic temperatures, counter-streaming along the magnetic field. A necessary and sufficient condition for instability is derived. The growth rate is calculated, and found to be typically several times the ion cyclotron frequency Ωi, although in extreme cases γmax may be as large as (Mi/me)½Ωi.

This is part 1 of a two-part paper. In this part, relaxation of the population distribution of electronic states is studied theoretically for a highly-ionized nitrogen- hydrogen mixture expanding through a nozzle, wherein the hydrogen content is less than 0.1 %. The analysis incorporates quantum-mechanical excitation rate coefficients, and considers the effects of wall cooling and absorption of radiation. Calculations are carried out for a condition produced experimentally, the experiment being the subject of part 2. The electronic excitation temperatures are deduced from the computed population distributions along the nozzle, and are compared with the calculated electron temperatures; this shows a large discrepancy between the two.

The shock conditions for a perfect gas in aligned-fields magnetogasdynamics are solved to give all variables downstream as rational functions of the density ratio. The thermodynamically admissible, evolutionary shock solutions are exhaustively classified into seven shock polar types, including a new type of supersonic, sub -Alfvénic polar. Example polars of each type are presented in the magnetograph (dimensionless magnetic induction vector plane) and in the Taniuti–Resler diagram (M, A plane) in order to display, respectively, the shock geometry and the state of the gas. A short discussion of the relation of the shock properties to flow over bodies is presented.

Resonant interaction between three ordinary modes, modified by electrons streaming parallel or antiparallel to the static magnetic field of a homogeneous plasma, is investigated. The modes propagate perpendicular to the magnetic field. If the unperturbed current density is zero, the scattering matrix for three-wave resonant interaction depends only on the cube of the electron drift velocity. If two electron beams of equal density are counter-streaming, the scattering matrix vanishes completely. If the unperturbed current density is non-zero, the matrix depends on the linear and cubic terms in the electron drift velocity. In the present investigation, the Hamiltonian approach, which is microscopic in nature, has been used, and therefore the results are valid for arbitrary ratios of Larmor radius to wavelength.

Multi-fluid equations derived in a previous paper are used to study small- amplitude wave motion in a partially ionized chemically-reacting plasma. The plasma is assumed to be infinite and without external fields. The dispersion equations are derived and solved both numerically and analytically for several limiting cases. The effects of chemical reactions have been explicitly obtained. It is found that at the long-wavelength limit, the ionization and recombination terms play the dominant role for the damping of certain longitudinal waves even for a nearly frozen plasma.

We investigate electrostatic surface-wave damping in strongly inhomogeneous magneto-active plasma with collisions. We demonstrate that such damping occurs at wave frequencies close to the natural (hybrid) frequencies of the plasma oscillations. We obtain dispersion equations and expressions for the damping coefficients of surface waves propagating along (i) the interface of semi-infinite plasmas, (ii) a plane plasma layer, and (iii) a vacuum layer in the plasma. We also study the linear transformation of surface waves into longitudinal oscillations (concentrated in the transition layer and directed along the gradient of plasma density) caused by the initial perturbation.

This is part 2 of a two-part paper. In this part, visible and infrared line radiations from nitrogen and hydrogen are measured with a spectrograph in an ionized flow expanding through a supersonic nozzle, wherein the hydrogen content is less than 0·1 %. The geometry and stagnation conditions are those for which the calculations were made in part 1. The experiment confirms quantitatively the predictions of part 1 that the electronic excitation temperatures of hydrogen aud (3P) core states of nitrogen are higher than the electron temperature, and that (3P) excitation temperatures exhibit maxima within the nozzle. The measured excitation temperatures of states with (1D) core are found to agree with the calculated electron temperatures.

The problem of the stability of inhomogeneous, electrostatic, multiple water-bag plasmas is considered. Equations are derived for general stationary water-bag equilibria, as well as for the corresponding perturbations. Particular attention is directed to systems with trapped particles in periodic equilibria, and special boundary conditions for the perturbation equations at the trapped-particle turning points are introduced. A normal-mode analysis is carried out for a configuration involving trapped particles occupying a finite region in the vicinity of the trough of an equilibrium wave (BGK mode). The results confirm the validity of the bunched-beam approximation.

Uncommonly large electron density fluctuations, accompanied by large skewness and flatness factors, have been observed in the transitional zone of a plasma jet. This paper investigates these further by a digital analysis of Langmuir probe signals from this zone. The electron aggregate is found split into a decaying laminar-fluid remnant and a growing population of turbulent fluid. The uncommon statistics are caused by the double-humped histogram, while those of each population separately are normal. The mean electron density of the aggregate displays no simple chemical decay trend, but that of the laminar population fits known argon recombination schemes.

The reconnexion rate of magnetic fields is crucial in understanding the fields found in turbulent flows in the solar photosphere and in the galaxy, and in flare phenomena. This paper examines the behaviour of magnetic fields in the neighbourhood of an X-type neutral point. The treatment is kinematical, specifying the velocity field v and constructing solutions to the hydromagnetic equation for B. The calculations demonstrate that the reconnexion rate is controlled by the diffusion in the near neighbourhood of the neutral point, and is not arbitrarily large, as has been suggested by similarity solutions of the complete field and fluid equations for vanishing diffusion

The dependence of the asymptotic behaviour of small ion-acoustic waves on the number of resonant electrons is investigated by assuming an electron equation of state corresponding to the observed flat-topped electron distribution functions. The result is a modified Korteweg-de Vries equation with a stronger nonlinearity.

The problem of stability of a vortex sheet between two collisionless plasma media described by Chew et al. (1956) equations is examined. The relevance of these investigations to the internal structure of the solar wind and to the stability of the magnetosphere–solar wind boundary is discussed.

A technique involving the use of a trial function containing several adjustable parameters has been developed for the purpose of deducing the electrical conductivity from measurements on the electric arc. It is used to re-analyze experiments in argon, xenon and nitrogen. Some improvement in agreement between theory and experiment is noted.

The nature of the fluid approximation for plasmas in the high collision-frequency limit is examined in an effort to understand some of the limits of validity of the commonly used ideal magnetohydrodynamic equations. The plane wave dispersion relation for a homogeneous medium is evaluated. The dependence of the damping on the various dissipative terms is examined for each type of wave predicted by the dispersion relation. The effects of these terms are then compared, using the standard values for transport quantities with collision frequency larger than gyro-frequency. When the total dissipation is considered as a function of collision frequency, it is found that there is a minimum in the region where the collision and gyro-frequencies are about equal. This is the régime for which ideal magnetohydrodynamics is a good approximation. As collision frequency is increased, dissipation reaches a maximum when the wave is in resonance with the resistive skin depth, and then decreases into a régime where ordinary sound and light waves propagate. Lower collision frequencies, where thermal conduction and equilibration are important, are considered.

The drift magnetosonic waves in a finite-pressure plasma are considered. Instabilities arising from the magnetic drift resonance of particles with waves are investigated for two cases: quasi-perpendicular and oblique propagations. It is shown that, in a wide range of both the density and temperature gradients, these waves can be excited even in a high-β (β≥1) plasma.