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Calculations of changes in magnetic moment of a particle passing near an X-type neutral line are discussed. The trapping of particles by these changes, in the low field region about the line, is considered, along with the effects of this trapping around magnetically guarded supports for internal ring machines. Previous calculations concerned particles confined to a symmetrical zone around the separatrices of the field or whose canonical momentum along the neutral line vanished. The present calculation is intended to remove this type of restriction.

Drift waves in a plasma with a density gradient are expected to behave like standing waves in the direction parallel to the density gradient, confined to a region around the point of maximum gradient and evanescent outside this region. Because of finite Larmor radius effects the higher radial harmonics are predicted to have lower frequencies. The authors show that in the quadrupole, confinement at the inner boundary is probably due to a layer of rapid velocity shear and with this assumption the observed radial eigenfunctions are in reasonable agreement with those predicted; the lower frequency drift wave observed in earlier work is identified as the first radial harmonic. There is a significant departure from the theory, however, in that the drift wave is observed to become a radially propagating wave in the outer regions of the plasma. The energy outflow associated with this propagation is evaluated and shown to be small compared with the input rate due to the drift wave instability mechanism.

It is shown that random reconnections of magnetic field lines in a diffuse pinch can lead to the appearance of reversed axial field near the discharge tube wall. It is suggested that the magnitude of the effect will depend on the degree to which current can be exchanged between adjacent field lines at a point of reconnection and on this basis a mathematical model is developed which describes magnetic field configurations of the diffuse pinch which are intermediate between two extremes: the force-free paramagnetic model with no field reversal in the limit where the current does not follow the field at all at a recombination point, and the Bessel function model in the limit where the current rigorously follows the field lines. An iterative procedure for the generation of intermediate configurations is described, and numerical results are presented and compared with experimental results from ZETA.

Spontaneous oscillations observed in the UMIST steady-state quadrupole are identified as drift waves with one full wave-length around a closed line of force, and with the potential perturbation anti-symmetric about the maximum field point. Existing theories do not properly describe this mode, for which trapped particles are important. The authors develop the theory of the mode and solve the resulting eigenvalue equation to obtain the dispersion curve; the result agrees with experimental evidence within experimental error. The excitation is identified as due to the 'dissipative trapped particle' mechanism, and the corresponding theoretical growth rate also agrees closely with experiment. In their conditions the excitation is due to electron-neutral rather than electron-ion collisions; as a result the temperature gradient is destabilising rather than stabilising, and they show that this also agrees with experiment.

For pt.I see ibid., vol.21, no.11, p.907 (1979). It has been suggested that an electric field applied along a neutral line in a plasma should drive an Ohmic current along the line even in a collisionless plasma. The authors have attempted to verify this by studying single particle motion in a quadrupole magnetic field with an applied longitudinal electric field. The results support the existence of the effect, though not conclusively. Expressions are derived for the total current flow along the neutral line and a possible application is discussed briefly.

It has been suggested that charged particle motion near a neutral line, where the adiabatic approximation is inapplicable, can be described approximately as a diffusion process in one dimension. This suggestion is confirmed by following particle trajectories both computationally in an idealised magnetic field and experimentally in the magnetic field of a quadrupole. Measurements made with a beam of electrons give estimates of the diffusion coefficient numerically close to those obtained computationally, and also reproduce the predicted variation with particle energy and magnetic field gradient.

An outstanding problem in the behaviour of the Zeta plasma is the origin of the fluctuations of density; it has been shown experimentally that they do not arise directly from the observed turbulent motions of the plasma, and there is no known instability which can produce them directly. It is proposed that they arise as a by-product of resistive instabilities through their action in cross-connecting lines of force belonging to magnetic surfaces at different radii. This idea is developed quantitatively and it is shown that it predicts fluctuation levels in reasonable agreement with experiment. This result is interpreted as giving support to the tangled discharge model of the Zeta plasma.

Plasma from a duoplasmatron is injected along the axis of a linear quadrupole. Peak number densities of about 5*109 cm-3 and temperatures of about 10 eV are achieved. The lifetime is about 100 mu s; losses occur mainly at the ends, transverse losses being negligible. This hot plasma is embedded in a cold plasma (0.5-1 eV) part of which drifts upstream towards the source. It is suggested that this arises from the action of a positive sheath at the far end of the device, maintained by an excess electron current from the source. Density and potential fluctuations are observed throughout the plasma but are large only in a zone bounded approximately by the critical surface and the surface of minimum field line length.

It is known that the adiabatic invariance of the magnetic moment of a charged particle breaks down when the particle passes near a field zero. The extreme case of this breakdown, arising when the particle orbit encloses the field zero, is investigated by computation of particle orbits. It is found that the magnetic moment changes by a factor with a maximum value of about 5.75; the exact value depends on the phase of the particle in its orbit, and the details of the dependence are explained on a simple model.

A theory is developed which leads to an expression for the partition of energy between mechanical and magnetic modes in a turbulent magnetohydrodynamic system in the presence of a strong external field. Modifications to this expression due to specific plasma effects are obtained and are not significant. Results are also obtained for the frequency spectra and diffusion coefficient. Measurements of the turbulent fluctuations in the Zeta device are described, and from these, values for the partition are obtained. It is found that there is only complete agreement with the theory if an anomalous resistivity is assumed at low operating pressures.

The Zeta discharge in the low density regime is characterized by strong turbulence, rapid loss of plasma, a resistance much larger than that predicted from the electron temperature, and a high ion temperature. A model for the energy flow in this regime is proposed, based on the convective transport of the plasma to the walls. This model leads to coupled equations for the ion and electron temperatures and for the turbulent energy. From these equations, and from their integrated forms which govern the energy balance of the discharge as a whole, predictions of the ion and electron temperatures are obtained which are in gross disagreement with observation. It is shown that agreement can be obtained by introducing an additional energy loss process, thermal conduction along the lines of force which are assumed to follow a random path to the wall. Two other possible energy loss mechanisms are considered and shown to be inadequate alone, although some contribution from either in conjunction with thermal conduction cannot be ruled out.