Spatial Distribution Of Plasma Density Research Articles

Stabilization effect of a strong HF electrical field on beam–plasma interaction in a relativistic plasma waveguide

The influence effect of a strong HF electrical field on the excitation of surface waves by an electron beam under the development of instability of low-density electron beam passing through plane relativistic plasma is investigated. Starting from the two fluid plasma model we separate the problem into two parts. The “temporal” (dynamical) part enables us to find the frequencies and growth rates of unstable waves. This part within the redefinition of natural (eigen) frequencies coincides with the system describing HF suppression of the Buneman instability in a uniform unbounded plasma. Natural frequencies of oscillations and spatial distribution of the amplitude of the self-consistent electrical field are obtained by solving a boundary-value problem (“spatial” part) considering a specific spatial distribution of plasma density. Plasma electrons are considered to have a relativistic velocity. It is shown that a HF electric field has no essential influence on dispersion characteristics of unstable surface waves excited in a relativistic plasma waveguide by a low-density electron beam. The region of instability only slightly narrowing and the growth rate decreases by a small parameter and this result has been reduced compared to nonrelativistic plasma. Also, it is found that the plasma electrons have not affected the solution of the space part of the problem.

Applicability of the hydrodynamic approximation to current-carrying plasma jets during their radial expansion

Supersonic spherically symmetric vacuum-arc plasma jets are considered using a two-liquid model. The jet starts from a radial distance of 3/spl times/10/sup -3/ m from the cathode surface with a radial directed electric current of 50-1000 A. Joule heating of the electron component and heat transfer to the ion component were calculated. The spatial distribution of plasma density, velocity, and electron and ion temperatures were obtained by numerically solving the equations of conservation of mass, energy, and momentum. The mean free path for the ion-ion collisions and the Mach number for the ion component of the plasma jet were also calculated as a function of the radial distance. The Knudsen number (Kn) for the ion component of plasma was calculated as a criterion of applicability of the hydrodynamical approximation. It was found that if Kn/spl Lt/1 at the starting radial distance, it remains much less than unity, in spite of the decrease in the plasma density during the radial plasma expansion.

Two-dimensional model of a capacitively coupled rf discharge and comparisons with experiments in the Gaseous Electronics Conference reference reactor.

We present results from a two-dimensional (2D) numerical fluid model of rf discharges in conditions close to recently published measurements of the spatial distribution of plasma density in the Gaseous Electronic Conference reference cell. The discharge is in pure argon at pressures in the 100 mtorr range, frequency 13.56 MHz, and rf voltage amplitudes on the order of 100 V. The model is based on solutions of the continuity, momentum (drift-diffusion), and energy equations for the electrons, continuity, and drift-diffusion equation for positive ions, coupled with the Poisson equation. The results of the model are qualitatively and quantitatively in good agreement with the experiments. The model predicts a maximum of plasma density off axis, as in the experiment. The ion current density on the electrode is also nonuniform, and increases radially in the conditions of the experiments. The effects of the rf voltage, pressure, and reactor geometry (electrode dimensions, gap length, guard rings, etc.) on the plasma properties and on the uniformity of the ion current on the powered electrode are also discussed. It is shown that the existence of a maximum of plasma density in the radial direction, in the conditions of the experiment, is due to the small value of the electrode spacing. The results show that the harmonic content of the discharge current is also geometry dependent. The comparisons show that 2D, three-moment fluid models can accurately describe the discharge and the effects of the chamber geometry on the plasma properties for pressure above the limit where collisionless electron heating does not play a significant role.

Fast-imaging method of plasma density spatial distribution by a microwave heterodyne interferometer

By using a microwave heterodyne interferometer assisted by microwave scattering techniques, computer tomographic images of spatial distributions of plasma density can be obtained. Two 9.5 GHz Gunn diodes, whose frequencies are offset by 35 MHz, are used in a heterodyne branch. In front of the detection horn antenna, multiple diodes are lined up, which scatter the microwave power with modulation frequency of 5 kHz, and the whole spatial information in one direction can be obtained by a pair of launcher-detector systems assisted by these diodes. This system is a simple and low-cost means to get images compared with popularly known multichannel interferometer techniques; still, precise, spatial resolution in the images has been observed.

Simulation of the solar wind interaction with non-magnetic celestial bodies

Laboratory experiments are described to simulate the solar wind flowing around nonmagnetic planets for three cases: non-conducting and ideally conducting planets, and a planet with a gaseous shell. A glass sphere was used as a model of a non-conducting planet (the Moon). Spatial distributions of plasma density and magnetic field strength that have been obtained agree with the data from measurements in space. However, the magnetic field does not increase before the rarefaction wave in the model experiment. A field increase was observed only for a conducting lunella: this argues in favour of the existence of a high conduction region on the Moon. A wax ball was used to model phenomena on the day-side of non-magnetic celestial bodies with a gaseoue shell (Venus, comets). Its surface easily evaporates in the plasma flow, and ionized evaporation products form an artificial ionosphere. The magnetic field frozen in the plasma flow is shown to be a determinative factor in the formation of a sharp ionospheric boundary. The supersonic plasma flow that interacts with the ionosphere gives rise to a stationary shock wave.

Carbon influx flow rate in an Ohmically heated plasma in the FM-1 spherator

The flow rate of neutral carbon atoms during the initial start-up phase of an Ohmically heated plasma in th e FM-1 spherator was determined from the spatially resolved measurements of the carbon line radiation and independent measurements of spatial distribution of plasma density and electron temperature. This was accomplished by comparing the measured carbon line intensity with a numerical solution of time-dependen t couple d differential rate equations for several carbon ionization states. The differential equations used in the present model correspond to a spatially homogeneous medium. However, th e effects of plasma inhomogeneity were taken into account by calculating the plasma emission at each location using the experimentally measured electron density and temperature (i.e. the measured radial distributions of density and temperature were folded into the numerical calculations). The empirical flow rate of carbon atoms into the discharge. Γflow(t),was found to be Γflow(t)≃ne(t)/τflow. where τflow was 30–100 ms depending upon discharge conditions

Spatial Distributions of Plasma Density in a High-Frequency Discharge with a Superimposed Static Magnetic Field

An experimental investigation has been carried out on the spatial distribution of charged-particle density in an inductively-coupled, cylindrical high-frequency plasma discharge with a superimposed perpendicular magnetic field. The studies were performed at and near conditions of electron-cyclotron-wave resonance and consideration was given to the influence of magnetic field and high-frequency power levels. It is shown that high-frequency power levels have relatively little influence on the volume distribution of charge, but that very unique distributions result under conditions of electron-cyclotron-wave resonance. Characteristic distributions are presented and discussed in the context of plasma anisotropy resulting from the superimposed magnetic field.