Table of contents

In this paper the stability of plasma in a toroidal system is investigated, for drift type oscillations which have phase velocities in the range v_Tc » ω/k_|| » v_Ti. Making the assumption that the shear is not too small, our main attention is turned to finding out whether or not curvature of the lines of force causes any new instabilities to appear not already present in the slab-model approximation to the magnetic field. It is well known that, in this approximation, there is just one large-scale instability, namely the temperature drift instability. It is shown here that, when the temperature gradient differs from zero, particle drift due to the magnetic field gradient allows large-scale oscillations to be excited with a localization region having a width which exceeds that characteristic of the electrons:

x > x_θ = ρ_i/θ(m_e/m_i)1/2, ω < k_||v_Te,

where ρi is the ion Larmor radius and θ is the shear. To this instability we give the name `magnetic drift instability'. The criterion for it to be excited is somewhat easier to satisfy than that for temperature drift instability, although the localization width is smaller. One would therefore expect that its contribution to transport processes would not exceed that of the corresponding effects from the temperature drift instability. It is shown that both ion and electron magnetic drift instabilities exist. When joule heating is present and when the electron temperature considerably exceeds the ion temperature, only the electron instability is excited. The influence of corrugation of the field is also considered. It is shown to be unimportant in the presence of high shear. For low shear and in the presence of a temperature gradient corrugation does not stabilize the oscillations.

A simple plasma accelerator that injects plasma across a magnetic field is described. The accelerator consists of two disk-shaped electrodes 1 cm in diameter and placed 5 mm apart in the magnetic field of a long solenoid. The electrodes are oriented so that the current flow is perpendicular to the magnetic field with j × B directed away from the accelerator. A fast acting gas valve, built into the anode, squirts hydrogen into the inter-electrode gap as a lumped parameter pulse line is discharged across the electrodes; a plasma stream is produced which crosses the magnetic field as well as spreading along the field lines. Typical operating conditions are: voltage drop 150 V, current 60 A, pulse time 100 μsec, magnetic flux density 2 kG, ion energy 100 eV, total number of ions produced 6 × 10¹⁵. A simplified accelerator has been made by using electrodes of hydrogen loaded titanium; in this case the anode automatically dispenses hydrogen during the current pulse and the fast acting gas value is unnecessary.

The present work investigates the effect of an anisotropy of the velocity distribution function upon the spectrum of plasma oscillations. It has been shown that the excitation of harmonics of electron cyclotron frequency may occur not only for longitudinal oscillations, but also for transverse electromagnetic waves, easily radiated from the plasma, as well.

It is shown that the effective potential of a "test charge" in a Vlasov plasma falls off, in general, as the inverse third power of the distance of the observer from the test charge, at large distances. The law applies when either (1) the test charge is in motion relative to the plasma, or (2) the plasma is anisotropic. Only when the test charge is at rest in an isotropic plasma does the exponential Debye factor appear in the effective potential.

A high-current well-focused multimomentum source of molecular ions has been developed as part of a high-energy energy-spread injection system. The multimomentum injection system is designed to produce a trapped plasma with continuous distributions of momenta and energies in order to reduce velocity-space instabilities. The ion source beam was delivered at 16-20 keV. It was composed of nine or more different species of molecular and atomic ions of hydrogen, deuterium and other gases which were extracted and focused identically and simultaneously. Maximum beam divergence was 2° from the beam axis. Average beam intensity was 130 mA/cm² in a 2 cm dia. beam 40 cm from the source. Electrostatic focusing was accomplished by an array of 6 mm dia. accel-decel extraction and focusing apertures. The beam was 85% composed of molecular ions, of which either the diatomic or triatomic component could be maximized by an adjustment of the gas fed to a reflex arc. Operating with a mixture of hydrogen and deuterium gas, the relative intensities of the several mass-to-charge ratio beam components indicated a random distribution of protons and deuterons.

The present work investigates the small amplitude oscillations in bounded beam-plasma systems with a sharp interface between two plasma media present. The dynamic character of the electromagnetic wave field is taken into account which makes it possible to go beyond the quasistatic approximation usually employed. The space charge and current of the beam travelling along the external magnetic field are assumed to be compensated. The thermal motion of plasma and beam particles is neglected.

Oscillation frequency and growth rates have been found for whistlers and waves corresponding to the quasistatic approximation, in a cylindrical plasma waveguide with a low density beam moving near the axis.

It is demonstrated that waves excited by a beam of electrons in a cold collisionless magnetoactive plasma can be absorbed in the inhomogeneous region of the system. Absorption takes place in the neighbourhood of the resonance of the excited wave frequency with the plasma frequency ω₀, cyclotron frequency ω_o (when the inhomogeneity is in the direction of the external magnetic field) or hybrid frequency Ω = √(ω₀² + ω_o²) (when the inhomogeneity is transverse to the magnetic field). The particle acceleration that results, occurs in the direction of the magnetic field (plasma resonance) and in the transverse direction (cyclotron and hybrid resonances). As well as being absorbed, the waves are also reflected (in regions of hybrid resonance with a longitudinal inhomogeneity of the system and in regions of cyclotron and plasma resonance with a transverse inhomogeneity of the system).