S. J. Buchsbaum

Resonance in a Plasma with Two Ion Species

When a high‐density plasma column in an axial magnetic field possesses two (or more) in species of different charge‐to‐mass ratios, there exists a plasma resonance condition which involves only the ion cyclotron frequencies. At reasonance, the two ion clouds oscillate transversely to the static magnetic field and 180 deg out of phase with each other, while the electrons remain relatively motionless. The ratio of the ion oscillatory energy to that of the electrons is of the order of the ratio of the ion‐to‐electron masses. Collisions between the two ion clouds randomize the large ordered velocities of the ions with great efficiency. Thus, by exciting this resonance, considerable ion heating may be realized. The effect of varying the relative concentration of the two ions is discussed.

Interaction between Cold Plasmas and Guided Electromagnetic Waves

The interaction between cylindrically symmetric anisotropic plasma column and bounded electro‐magnetic waves is analyzed theoretically. The properties of a cylindrical cavity coaxial with a cold plasma column and a coaxial with a static magnetic field are determined. The shift in the resonant frequency of the cavity‐plasma system is calculated in the high‐electron‐density limit and compared with the numerical solution presented earlier.

Alfvén Waves in Solid‐State Plasmas

Alfven waves can be studied experimentally in certain solid‐state plasmas with greater facility than in gaseous plasmas. This results from the fact that in these solid‐state plasmas all the charge carriers have masses equal to or less than the free electron mass. Alfven waves can thus be studied at microwave frequencies. Previously published experimental results on microwave absorption in bismuth are reinterpreted in terms of the properties of Alfven waves.

Effect of Collisions on the Landau Damping of Plasma Oscillations

The effect of collisions on the Landau damping of a one‐dimensional longitudinal plasma oscillation in the absence of a magnetic field is analyzed. It is found that in a steady state, collisions (no matter how few in number) affect the velocity distribution of the trapped electrons and thus play a major role in determining the Landau damping. When the damping is small (Im k « Re k), it is reduced from its collisionless value by a factor νc2/(νc2 + Ω2) where νc is the electron collision frequency for momentum transfer and Ω2 = eEk/m is the frequency of oscillation of a trapped electron in the approximately parabolic potential trough of the wave.

LIMITS OF VALIDITY OF BORN APPROXIMATION IN MICROWAVE SCATTERING FROM TURBULENT PLASMA.

Microwave scattering from a turbulent weakly ionized plasma column is studied. The scattering cross section is proportional to the mean‐square plasma density fluctuation, 〈n2〉, as predicted in the Born approximation for 〈n2〉½ up to 5% of the critical density.

High‐Frequency Behavior of the Surface‐Wave Helicon Instability

The surface‐wave instability in helicon wave propagation analyzed by Baraff and Buchsbaum is reinvestigated here for the purpose of extending the analysis to values of ωτ greater than unity. We find that although collisions are essential for the mechanism of instability, the formulas and analysis of Baraff and Buchsbaum, derived for ωτ«1, are unchanged until ωτ exceeds some large value ω0τ. This value ω0τ is typically much greater than unity. At higher frequencies the surface wave penetrates from one boundary to the next and some of the Baraff‐Buchsbaum approximations start to lose validity.

Diffusion in a Microwave Plasma in the Presence of Turbulent Flow

Microwave breakdown and diffusion in a weakly ionized microwave plasma was studied in the presence of turbulent flow of the neutral gas. Breakdown was produced by pulsed microwaves in helium, hydrogen, nitrogen, and argon in a tube placed along the E field of an S‐band waveguide. The microwave power at breakdown (or at extinction) is a measure of the diffusion rate of the charged particles to the walls of the tube. This power was measured as a function of flow rate of the neutral gas. The minimum microwave breakdown power exhibits a sharp upward break when turbulent flow sets in.

The Dielectric Coefficient of a Plasma

ABSTRACT An experiment is described which indicates that the Lorentz polarization term should not be present in the expression for the dielectric coefficient of a plasma.

Experimental study of a plasma column in a microwave cavity

Experiments designed to study the production of a steady-state plasma column by microwave cavity means are described. At low plasma densities electrons are heated by cyclotron resonance in crossed microwave electric and static magnetic fields. Phenomena associated with large energies which the electrons possess near cyclotron resonance are discussed. Large plasma densities are achieved by resonating the plasma column by suitably varying the static magnetic field, the microwave frequency and the input power. In this manner, densities of the order of 1012 cm-3 are obtained at a neutral gas pressure in the micron range.

Coupling between electromagnetic and electron waves in a plasma

The coupling between electromagnetic and electron plasma waves in a uniform plasma in the presence of a static magnetic field is studied. The transport equations are used to represent the plasma and Maxwells equations to represent the fields. These yield a dispersion relation which is discussed here only for ions of infinite mass. Eight topologically different phase velocity surfaces suffice to represent the system of combined electromagnetic and electron waves for all values of plasma density and magnetic field strength. The plasma waves have cutoffs (phase velocity infinite) wherever the electromagnetic wave has a resonance (phase velocity zero); the coupling between the two waves is strong there and their respective velocity surfaces join smoothly one onto the other. Elsewhere the waves are distinct.