Low Phase Velocity Research Articles

The hydromagnetic stability of the magnetospheric boundary

The hydromagnetic Kelvin-Helmholtz stability problem is studied for an infinite plane interface between compressible infinitely conducting fluids. The critical value of the relative streaming velocity for stability is studied by use of the equations for marginal stability without making the simplifying physical assumptions used by previous authors. In application to the magnetosphere boundary we find we can make some predictions without too precise a knowledge of all the parameters involved. At middle and low latitudes the first growing modes propagate across the Earth's field with a very low phase velocity and wave fronts closely aligned to meridian planes. The modes tend to exhibit circular polarisation in a plane almost perpendicular to the Earth's field. This behaviour should also occur at high latitudes when the magnetosheath field is closely aligned to he Earth's field.

A tentative explanation of the anomalous attenuation of electromagnetic surface waves on plasma columns

An explanation of the high attenuation of electromagnetic surface waves on plasma columns which occurs in the region of low phase velocity is based on a perturbation expression for the attenuation coefficient and cold plasma theory.

Superphase velocity effects caused by a single electron in a magnetoplasma

The effects that take place when an electron traverses a magnetoplasma with a velocity v are discussed at length starting from the equation of conservation of energy and momentum of both the photon and electron in their relativistic form. The electromagnetic properties of a homogeneous, lossless and cold magnetoplasma, including ions, are reviewed briefly and reconciled with the equations of conservation. The resulting equations are specified for the cases of perpendicular and parallel magnetic field, electron velocity and wavevector of the photon involved. It is found that a single electron injected from the outside can excite right- and left-hand polarized, ordinary and extraordinary waves, helicons, whistlers, Alfvén waves and waves which depend on the difference of mobile carrier densities. The low-frequency, low-phase velocity phonons turn out to be dependent on the relation ℏ ω mv 2 itp

Expansion Solutions and Wave Propagation in Partly Ionized Gases

Kinetic theory solutions of the Boltzmann equations for a partly ionized gas are obtained for the study of wave propagation in a three-fluid plasma. The following simplifying assumptions are made: The interactions between charged particles are divided into long-range Coulomb forces which effectively act as an external electric field, and a short-range Coulomb force which is cut off at the Debye length; the interactions between other pairs of particles are assumed to obey an inverse fifth-power law. Further, the gas is assumed to be sufficiently close to equilibrium for the usual expansions of the velocity distribution functions around the equilibrium distribution functions to be used. Under these assumptions, the general sets of coefficient equations are obtained, and a 13 and 20 coefficient approximation (which include the effects of self-collisions or viscosity and thermal conductivity) are given explicitly. For a five-coefficient approximation, corresponding to employment of the conservation equations, a dispersion relation is obtained and solved for the phase velocities and attenuation of longitudinal waves as functions of wave and collision frequencies. It is found that the solutions which correspond to waves of high phase velocity are exactly those obtained when the adiabatic law is assumed to hold. Solutions corresponding to low phase velocity waves, however, are found to be more highly attenuated than their counterparts in the adiabatic theory.

Nondegenerate electron beam parametric amplifiers

Cyclotron wave amplifiers with a pump-to-signal frequency ratio of 5:1 (rather than the usual 2:1) have been built and have given a single-channel noise figure of 2 db. This paper reports on the difficulties which had to be overcome and the restrictions which had to be observed before achieving what, at the start, had looked deceptively simple. The paper begins with a discussion of the relative merits of assigning signal, idler and pump waves in various ways to beam and circuit. In the preferred arrangement the circuit carries only the pump wave; signal and idler are cyclotron waves on the beam. The validity of Tien's equations for the propagation constants and of the Manley-Rowe relations for the power-frequency ratio is demonstrated for the case of cyclotron waves. It is also shown that in a nondegenerate tube, idler noise must be extracted, no matter how high the idler frequency may be. Early experiments by C. B. Crumly, using interdigital couplers for signal and idler, gave very poor noise figures. The twisted coupler, a new structure with high coupling efficiency, was introduced but failed to reduce the high noise. It was then found that a signal carried by a cyclotron wave of low phase velocity was attenuated as it traveled along the beam, and that the noise extraction process worked poorly. This was traced to the spread of axial electron velocities, a source of trouble mentioned by C. P. Lea-Wilson and later by E. I. Gordon. P.G. Everett's measurements of this effect are described. It is then shown how the consequences of the effect may be minimized. Nondegenerate tubes are described in which the velocity spread effect is rendered almost harmless. The twisted coupler structure is particularly well adapted to experiments with synchronous waves; a brief report on such experiments is included.