Table of contents

Under certain conditions an electron or ion is accelerated continuously when subject to a force greater than the dynamic friction; as a result it is decoupled from its thermal state of motion and is said to be `runaway'. Runaway particles lead to, and form part of, a class of particles which have been referred to elsewhere as `suprathermal'. The study of the way in which runaway particles are produced and accelerated is not only of importance in laboratory gas discharges, but also in a wide variety of applications in astrophysics. The subject of runaway electrons and ions is treated, first in a uniform ionized gas in which the particles have Maxwellian velocity distributions and, secondly, in a current conducting plasma. The conditions for decoupling and accelerating particles from their thermal state are discussed and, in particular, it is shown that in the case of a weak electric field ions are unlikely to achieve speeds very much in excess of the drift speed of the electrons, except in restricted regions where runaway electrons are continually forming and escaping. When the ratio of the thermal and magnetic energy densities, given approximately by β, is small, the runaway electron current grows rapidly at the expense of the conducting current. A number of ways are considered in which the runaway current may grow and become predominant. Finally, the generation of suprathermal ions by Fermi interactions is considered, from which it is evident that when β is the order of unity the ion gas is heated and when β is small relatively few ions are accelerated to high energies.

The construction of a coaxial plasma gun is described. At its output end the gun is provided with a radial magnetic field, which is trapped by the plasma.

The plasma which is shot out from the gun is studied by photographic and magnetic methods. It is demonstrated that the gun produces magnetized plasma rings with the same basic structure as the rings obtained in toroidal pinch experiments.

When the the plasma rings are formed, the magnetic field lines from the gun break, a result which is of interest from a theoretical point of view.

Electric current is sent through a hollow jet of mercury which is flowing over the surface of a glass tube. The jet is broken up by the action of the electrodynamic force. However, if at the same time a strong current is sent through a central conductor placed inside the glass tube, the jet can be stabilized. This occurs when the currents are parallel. Sausage instabilities have not been observed in the present experiment.

The essential features of the motion of a particle in an azimuthally varying magnetic field of `spiral ridge' form may be described by a pair of non-linear coupled equations of simple form. These reduce to the uncoupled linear radial and vertical betatron equations when the azimuthally varying component of field is zero. In this paper two pendulum analogues are described, which obey the same equations of motion as the particles. The first of these represents the radial motion only and consists of a torsional pendulum with a magnetic moment which interacts with a rotating magnetic field; the second, which represents both radial and vertical motion, consists of a compound pendulum supported by two sets of pivots at right angles, with a magnetic bob which interacts with a travelling magnetic field. The first of these analogues was used to make an extensive survey of the maximum stable amplitude of the motion as the parameters corresponding to ridge angle, azimuthal field modulation depth and radial field index were varied. No quantitative measurement was made with the second model, but the effect of ridges in stabilizing the vertical motion was demonstrated.

The injector is a d.c. accelerator delivering a pulsed proton beam of 15 mA at an energy of 515 keV. The installation is described and details are given of its most important features.

Various methods of producing simultaneous radiation damping of synchrotron and betatron oscillations in a strong focusing accelerator are examined and formulae for calculating the damping decrement are given. It is noted that the curvature of the orbit in the damping magnets must be different from that in the main sectors. It is shown that the total damping decrement is always the same and is independent of the type of damping system used.

The probability of spin re-orientation is estimated for polarized protons accelerated in a cyclotron in which the field falls off with increasing radius and varies azimuthally. Beam extraction is also considered. The depolarization probabilities are shown to be both small.

The propagation of electromagnetic waves in a narrow plasma waveguide is examined in a non-linear approximation. The dependence of the phase velocity on the amplitude is derived, and consideration is given to the problem of frequency multiplication. The amplitude of the second harmonic is calculated. Owing to non-linear effects a new possibility arises of varying the phase velocity of the wave by means of a variation of the amplitude and of ensuring radial and phase stability in accelerators and also of a certain modification of the methods of amplifying and generating micro-radio waves.