Vacuum Sheath Research Articles

Admittance of Dipolar RF Probes in a Simulated Ionosphere

This paper describes a laboratory investigation of two types of dipolar RF admittance probes which could be of use in the ionosphere and inner magnetosphere. Simple theoretical models are presented, using the quasi‐static approximation and vacuum sheath. Independent accurate measurements of both the electron density and temperature are made, using Langmuir probes and the dispersion properties of Bernstein waves, and the results are compared with the RF probe predictions. In general, good agreement is obtained, indicating both the usefulness of the probes as a way of measuring electron densities in space and the applicability of the model used. These results also give an estimate of the effective sheath thickness around the probe, which is smaller than previous measurements have suggested.

Prolate spheroidal antennas in isotropic plasma media

The problems of a gap-excited finite-sized prolate spheroidal antenna, with and without a confocal prolate spheroidal vacuum sheath, operated in a uniform cold and in a uniform warm lossy plasma medium, have been considered by applying boundary value problem techniques. The dependence of the input admittance, the current distribution, and the radiation pattern, upon the collision frequency, the electron temperature, the length of the antenna, the length-to-radius ratio for the antenna, the ratio of the plasma frequency to the operating frequency, and the thickness of a vacuum sheath have been investigated. Admittance curves are given from which the input admittance of a finite linear antenna can be estimated for a wide range of operating conditions and length-to-radius ratios. Conductance and susceptance maxima, dependent upon the vacuum sheath thickness, have been observed when using the cold plasma model for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\omega &lt; \omega_{p}</tex> . When using the warm plasma model in conjunction with the boundary condition that the normal component of the fluid velocity is zero at the antenna surface and at the vacuum sheath surface, the radiation resistance of the antenna is increased significantly for the parameters considered.

Infinite cylindrical antenna in a uniaxial compressible plasma

Numerical values are given for the admittance of an infinite cylindrical antenna in a uniaxial plasma, taking into account both plasma compressibility and a vacuum sheath. The susceptance is found to exhibit no significant sheath or compressibility dependence above the plasma frequency (ƒp) and only a slight dependence on these factors below ƒp. The conductance behavior shows no significant influence of compressibility or sheath in the range centered about ƒp, in which it is relatively constant in value except for a slight minimum at ƒp.

Characteristic Waves on an Infinite Cylindrical Antenna in a Plasma Medium

An analysis of the current on an infinite cylindrical dipole antenna immersed in a lossy plasma medium that may, in the general case, be both compressible and anisotropic, is discussed. The antenna current, for the isotropic, incompressible plasma with no sheath present is evanescent along the antenna axis for frequencies (f) less than the electron plasma frequency (fp). When f &gt; fp, the current is axially propagating with a wave number whose real component Kzr is given by , where KE0 is the free‐space electromagnetic wave number. Addition of a static magnetic field parallel to the antenna axis opens up a region of propagating antenna current below the electron cyclotron frequency (fh) with , so that the region of evanescent current is reduced to . A further decrease in the region of evanescent current behavior to approximately (fh + 9ft)/10 &lt; f &lt; ft is effected by plasma compressibility or a vacuum sheath. At the same time, the value of in the latter case may exceed KE0 by two or three orders of magnitude in the range (fh + ft)/2 ≲ f &lt; ft so that the electrical wavelength of a finite antenna in the plasma would be a more sensitive function of frequency than for the free‐space medium.

The admittance of an infinite cylindrical antenna in a lossy, compressible, anisotropic plasma

An analysis of the current on an infinite cylindrical dipole antenna which is excited across a circumferential gap of nonzero thickness and immersed in a lossy, compressible magnetoplasma with its axis parallel to the static magnetic field is described. Some numerical results are presented for the antenna admittance for the sheathless case, where the uniform magnetoplasma is in contact with the antenna surface. The admittance values are obtained from a numerical integration of the Fourier integral for the antenna current, and are given for plasma parameter values typical of the E region of the ionosphere.The admittance values obtained exhibit a maximum slightly above the electron cyclotron frequency, and in this regard are similar to the admittance when the magnetoplasma is incompressible but separated from the antenna by a free-space layer (the vacuum sheath). In addition, the admittance is found to have a slight minimum at the plasma frequency and to have a more pronounced minimum at the upper hybrid frequency where also the susceptance changes sign, these minima not being significantly affected by the plasma compressibility or vacuum sheath. These features of the calculated admittances are found to have a qualitative resemblance to experimental results obtained from antenna measurements in the ionosphere.

The admittance of the infinite cylindrical antenna immersed in a lossy, compressible plasma

Some numerical results obtained from an analysis of the admittance of an infinite cylindrical antenna excited at a circumferential gap of finite thickness and immersed in a lossy, compressible (warm) plasma are given. The linearized hydrodynamic equations are used for the electrons (ion motion is neglected). A free-space layer, or vacuum sheath, is used to approximate the ion sheath which forms about an object at floating potential in a nonzero temperature plasma. Values for the antenna admittance are obtained by a direct numerical integration of the Fourier integral for the current, and are presented as a function of frequency for plasma parameter values typical of the E region of the ionosphere. The admittance exhibits a maximum below the plasma frequency unless the electron temperature and sheath thickness are both zero; however, above the plasma frequency, the sheath and electron temperature have relatively little effect on the antenna admittance. The nonzero plasma temperature considerably enhances the antenna conductance below the plasma frequency compared with the zero-temperature case while at the same time reducing the dependence of the conductance on the electron collision frequency. A susceptance zero the location of which is not sensitive to the vacuum sheath thickness occurs near the plasma frequency.

THE ADMITTANCE OF AN INFINITE CYLINDRICAL ANTENNA IN A LOSSY INCOMPRESSIBLE, ANISOTROPIC PLASMA

A numerical investigation of the admittance of an infinite, circular cylindrical antenna excited at a circumferential gap of nonzero thickness, and immersed in a lossy incompressible magnetoplasma with the antenna parallel to the static magnetic field is described. A concentric free-space layer (the vacuum sheath) which separates the antenna from the external uniform plasma is included in the analysis to approximate the positive ion sheath which may form about a body at floating potential in a warm plasma. The numerical results for the antenna admittance show that: (1) in the absence of a sheath, a sharp admittance maximum is found at the electron cyclotron frequency, with the maximum more pronounced when the plasma frequency exceeds the cyclotron frequency than for the converse case; (2) the vacuum sheath shifts upward in frequency and reduces in amplitude the admittance maximum which occurs for the sheathless case at the cyclotron frequency; (3) a kink or minimum in the admittance is found at the plasma frequency.

SURFACE CURRENT EXCITATION ON AN INHOMOGENEOUSLY SHEATHED PLASMA-IMMERSED CYLINDER BY ELECTROMAGNETIC AND ELECTROKINETIC WAVES

A theoretical investigation is described which analyzes the effect of an inhomogeneous sheath on the surface currents and scattering cross section of an infinitely long, perfectly conducting, circular cylinder immersed in an isotropic collisionless, compressible plasma by plane electromagnetic (EM) and electrokinetic (EK) waves incident at an arbitrary angle with respect to the cylinder axis. The linearized fluid equations are used in the analysis, with the inhomogeneous sheath taken to extend a finite distance into the plasma from the cylinder surface, on the order of 10 electron Debye lengths, beyond which the plasma is uniform. The numerical results for the surface currents obtained from the inhomogeneous sheath analysis are compared with those obtained when the sheath is replaced by a free-space region, called the vacuum sheath. It is found that for EM wave incidence, the surface currents are practically independent of the sheath and the finite plasma temperature. For EK wave incidence, the sheath has an attenuating effect on the currents produced, compared with the sheathless case, with the attenuating effect of the vacuum sheath being greater than that of the inhomogeneous sheath. The scattering cross sections for EM wave incidence are, for the scattered fields with the same polarization as the incident wave, also independent of the sheath and finite plasma temperature. The scattered fields with polarization different from that of the incident wave have cross sections generally less than 10−2 of the scattered fields with the incident wave polarization.

Excitation of Surface Currents on a Plasma‐Immersed Cylinder by Electromagnetic and Electrokinetic Waves.: I. The Vacuum Sheath

The surface currents excited on an infinitely long. circular metal cylinder immersed in an isotropic compressible plasma by electromagnetic (EM) and electrokinetic (EK) plane waves are investigated. The formulation proceeds from velocity moments of the collisionless Boltzmann equation for the electrons. together with Maxwell's equations. and uses a linearized analysis. The sheath which forms about the cylinder is approximated by a free‐space layer between the cylinder and the uniform plasma, called the vacuum sheath. It is found from the numerical calculations that the vacuum sheath tends to screen the EK wave from the cylinder, and produces a large attenuation of the surface currents excited by the EK wave. The sheath and EK wave are found, on the other hand, to negligibly affect the currents excited by the EM wave.