Neutral Particle Collisions Research Articles

Electron component of a plasma in a homogeneous electric field.

We investigate spatially homogeneous stationary solutions of the Boltzmann equation describing the electron component of a gas plasma in a homogeneous electric field. We consider both elastic and weakly inelastic collisions of electrons with the neutral-particle plasma component as well as the Coulomb interaction between charged particles, described by Landau's collision integral. The Boltzmann equation for the electron swarm is linearized around a Maxwellian with some unknown temperature. For some important cases the problem reduces to a single-parameter system of two integrodifferential equations. Its solution allows us to treat plasmas under a variety of conditions in a rather simple way. We solve this system with the help of the spline-collocation method, and find the first two corrections to the Maxwellian corresponding to symmetrical and flow components of the velocity distribution function. The equation for the electron temperature then comes from the energy balance condition and from the requirement that the solution of the kinetic equation be unique. The electron temperature and energy absorption are described by curves with hysteresis as functions of the electric-field intensity. The conductivity versus electron temperature has a single maximum. Its rising slope corresponds to Spitzer's well-known formula, when the electron temperature is not very high and electron--neutral-particle collisions are unimportant. All numerical results are obtained under the assumption that the cross section for electron--neutral-particle collisions is independent of electron energy but generalizations are possible, since the analytical solution of the problem is far advanced.

Electrostatic Probe Characteristics in a Magnetic Field

A wide variety of electrostatic probes are found to retain their exponential current-voltage characteristic in the presence of a strong magnetic field. Electrostatic (Langmuir) probes continue to be one of the most important and most widely used tools for making measurements in plasmas. Langmuir probes are routinely employed in order to measure plasma density, Ne, electron temperature, Te, and plasma potential, V,. A typical Langmuir probe consists of a bare metal electrode electrically biasable with respect to the machine ground as shown in Figure 1. The theory of current collection for biased electrodes of various different shapes (i.e., flat metal discs, cylindrical wires, or conducting spheres) is well understood in the limit of a magnetic field free plasma (Hoyaux, 1968; Laframboise and Parker, 1973; Schott, 1968). Langmuir probes are also routinely employed in strongly magnetized plasmas (Brown et al., 1971; Buchenauer and Molvik, 1988; Stangeby, 1982). Our understanding of current collection by biased electrodes in a magnetoplasma is in a much more primitive state. There has been a considerable amount of theoretical work aimed at describing the current flow across a magnetic field to a probe electrode in the presence of classical coulomb and neutral particle collisions (Niyogi and Cohen, 1973; Dote and Amemiya, 1964; Sanmartin, 1970; Sato, 1972; Laframboise and Rubenstein, 1976). Unfortunately, it is widely agreed that turbulent rather than classical processes are likely to dominate charged particle motion across a magnetic field (Bohm et al., 1949a; Stangeby, 1982). As a consequence of this, the classical probe theory in a magnetic field is highly suspect (Stangeby, 1982).

On pumping the strong water maser sources

The collisional pumping of water masers is studied. It is found that the strong water masers can be pumped collisionally in regions where the charged particles are at a significantly higher temperature than that of the neutral particles. Such conditions exist in the precursors of MHD shocks and in massive starburst galaxies and active nuclei. The ionization fraction required for this pumping mechanism to work is roughly equal to the ratio of the typical neutral particle collision rate to that of the charged particles, and it is therefore in the range 10 to the -5th to 10 to the -4th. A physical explanation of why population inversion in H2O molecules occurs under the above conditions is given, and the total power emitted is computed. Also, the efficiency for masing is determined as a function of ionization fraction and temperature difference between the charged and neutral particles. 45 references.

Confinement of a potassium plasma in a spindle cusp magnetic field

The confinement properties of a low-β (average) potassium plasma produced by contact ionization in a spindle cusp magnetic field were investigated. In this configuration, ne≊108–1010 cm−3, Te≊Ti≊0.2 eV, and the ions are weakly magnetized. Electron and ion densities, space potentials, and plasma flow velocities were measured in the ring and point cusps. The leak width of the escaping plasma was measured over a wide range of magnetic field strengths and neutral pressures. The dependence of the leak width on neutral pressure and magnetic field strength is accounted for by a simple model in which the plasma streams out of the cusps along the magnetic field lines while diffusing across the magnetic field due to the combined effects of neutral-particle collisions and Bohm diffusion. Measurements of broadband plasma noise suggest the presence of ion acoustic wave turbulence.

Confinement properties of a low-beta discharge in a spindle cusp magnetic field

An experimental study of the confinement properties of a low-(average) beta discharge plasma in a spindle cusp magnetic field is described. Electron and ion densities, space potentials, and plasma flow velocities were measured in the ring and point cusps. The leak width of the escaping plasma was measured over a large range of magnetic field strengths and neutral pressures. Leak widths varying between the hybrid gyrodiameter and the ion gyrodiameter were observed. The dependence of the leak width on neutral pressure and magnetic field is accounted for by a simple model in which plasma diffuses across the magnetic field as a result of neutral particle collisions while streaming out of the cusps along the magnetic field lines. At the lowest neutral pressures investigated, the scaling of the leak width with magnetic field strength suggests that noncollisional mechanisms (e.g., Bohm diffusion) determine the leak width.

Main Features of the Electron Kinetics in Collision Dominated Steady‐State rf Plasmas

AbstractThis paper reports about systematic investigations of the velocity distribution function and resulting macroscopic quantities of the electrons in a weakly ionized and spatially homogeneous rf plasma in the molecular gas H2 and about a comparison with an analogous study of the rf plasma in the inert gas Ne for a wide field frequency range. These results are obtained by a strict treatment of the non‐stationary electron Boltzmann equation considering the action of the rf field as well as that of elastic and inelastic electron ‐ neutral gas particle collision processes. Furthermore, such investigations could be extended to very high field frequencies for the rf plasma in neon. This provides the basis for a general characterization of the complex temporal behaviour of the isotropic and anisotropic part of the velocity distribution as well as of the several macroscopic quantities in the range from low to very high field frequencies. The ration of the field frequency to two dissipation frequencies which describe the lumped dissipation of energy and impulse due to elastic and all inelastic collision processes and which are only determined by the atomic data of these processes governs the temporal behaviour of the electron kinetics in the rf plasma in any gas.

Preparation of long-lived excited ions for lifetime measurement by selective electron capture

Lifetimes of long-lived excited singly or multiply charged ions can be measured by producing the latter via selective electron capture in collisions of slow multiply charged ions and neutral particles, and determining the decay characteristic of radiation emitted along the flight path of excited ions. As an example, ’’metastable’’ Ne+(3s4P) ions were produced by utilizing the impact of Ne+2 on Xe at an impact energy of 51.5 keV. For the shortest lived state, which probably corresponds to j = 3/2, a lifetime of 100±20 ns was obtained.

6-CENTIMETER Formaldehyde Absorption and Emission in the Orion Nebula

Formaldehyde absorption and emission lines in Orion IR nebula, indicating pumping suppression by neutral particle collisions

Admittance of an infinite cylindrical antenna immersed in a lossy, compressible plasma

An expression is derived for the driving point admittance of an infinite, perfectly conducting cylindrical antenna excited by a finite uniform gap and immersed in a lossy, compressible, isotropic plasma. This expression is based on the twin assumptions that the gap width is much smaller than the wavelength of the plasma (electroacoustic) wave and that the radius of the antenna is much smaller than the wavelength of the electromagnetic wave; it is similar in form to the corresponding expression for an infinite antenna in free space, and it is obtained in much the same manner. Conductance and susceptance curves computed from the admittance function are in good agreement with those obtained numerically by other authors for f ≥ 0.7 fP. The behavior of the admittance function at frequencies in the neighborhood of the plasma frequency depends mainly on the electron – neutral particle collision frequency, not the plasma temperature. In general, the effect of the temperature is quite small, although a temperature-related effect can produce large admittance values in very low-loss plasmas at frequencies well below the plasma frequency (f ~ 0.03 fP for T = 1500 °K). This investigation supports the view that the propagation constant of the current along a cylindrical antenna in a compressible plasma is nearly equal to the plane-wave propagation constant in an incompressible plasma.

Radiation from Linear Resonant Antenna in Weakly Ionized Plasma†

ABSTRACT In un earlier work of the author. called part I of the series of studies relating to the radiation from antenna immersed in weakly ionized plasma, a linear resonant centre-fed antenna was used as source. In the present work. called part II of the series, a thin, linear, non-resonant end-fed antenna is investigated under similar conditions. A theoretical study of its radiation resistance for the electromagnetic and electro-acoustic waves is made, taking the propagation constant as complex for both these modes. Also the effect of electron neutral particle collision is taken into consideration wherever effective. It is shown that the effect of plasma is more profound when the source frequency is lower than the plasma frequency. But at frequencies higher than the plasma frequency the effect is found to be not of much consequence. General expressions of radiation resistance for both the electromagnetic and the electro-acoustic waves have been obtained, and from these the behaviour of half-wave linear ...

Critical Frequencies in the Stagnation Region of a Shock Layer

To FACILITATE the estimating of stagnation-re gion plasma and electron, neutral-particle collision frequencies, a plot of alti­ tude versus velocity- with equilibrium plasma and collision fre­ quencies indicated parametrically was developed. Since the gas in the stagnation region is relatively dense and only weakly ionized, a so-called critical frequency is introduced to account for the influence of these elastic collisions. Alteration of the plasma frequency, when collisions are considered, is indicated on the above-mentioned plot. The term as used here, refers to an electrically neutral, ionized gas consisting of equal populations of electrons and ions, and neutral particles also. The assumption of elec­ trical neutrality implies that the effects of space-charge forces, namely internally generated electric fields, are not important at distances comparable to the shortest length of interest. In the case of electromagnet ic radiation, this length is the wavelength of the radiation considered. It is well known that a character­ istic frequencj^, called the plasma frequency, co2„ is associated with each plasma, namely,