Cylindrical Discharge Plasma Research Articles

A characterization of plasma properties of a heterogeneous magnetized low pressure discharge column

An approach is presented for characterizing heterogeneous magnetized plasma discharge tubes through the scattering of electromagnetic plane waves. Here, we formulate the analytical problem of electromagnetic scattering from a gyrotropic plasma column. The scattering accounts for the heterogeneous composition of the cylindrical discharge plasma and facilitates determining its propensity for gyrotropic scattering, particularly when electron collisional damping may be prevalent. The analytical results are validated using computational simulations. Scattered fields from the magnetized plasma are measured experimentally, and, by comparing the analytical and experimental results, the unknown parameters of the discharge, i.e., characteristic plasma and electron collisional damping frequencies, are determined. The technique is relatively straight-forward to use and removes the need for commercial computational electromagnetic simulations when experimental data on scattering characteristics of such cylindrical discharge plasmas are available.

Measurements of parallel electron velocity distributions using whistler wave absorption

We describe a diagnostic to measure the parallel electron velocity distribution in a magnetized plasma that is overdense (ω(pe) > ω(ce)). This technique utilizes resonant absorption of whistler waves by electrons with velocities parallel to a background magnetic field. The whistler waves were launched and received by a pair of dipole antennas immersed in a cylindrical discharge plasma at two positions along an axial background magnetic field. The whistler wave frequency was swept from somewhat below and up to the electron cyclotron frequency ω(ce). As the frequency was swept, the wave was resonantly absorbed by the part of the electron phase space density which was Doppler shifted into resonance according to the relation ω - k([parallel])v([parallel]) = ω(ce). The measured absorption is directly related to the reduced parallel electron distribution function integrated along the wave trajectory. The background theory and initial results from this diagnostic are presented here. Though this diagnostic is best suited to detect tail populations of the parallel electron distribution function, these first results show that this diagnostic is also rather successful in measuring the bulk plasma density and temperature both during the plasma discharge and into the afterglow.

Sheaths, particle fluxes, floating potentials and electromotive force in a cylindrical discharge plasma containing an internal coaxial wall

A two-fluid theory of a positive column in a cylindrical geometry having an internal coaxial wall is given. The space charge density throughout the whole plasma and inertia and diffusion controlled conditions are taken into account. An eigenvalue problem with a free boundary is solved. The radial distributions of the number densities and of the drift velocities of the ions and electrons and the profile of the electric potential are calculated. The boundary of the regions of particle collection for the inner and outer walls is exactly determined. The results show that the particle fluxes and the number densities of ions and electrons and, in particular, the floating potential at the inner wall are different from the values at the outer wall. The differences increase with an increasing ratio of the radii of the outer and inner walls. Under these conditions a so called impressed voltage or electromotive force occurs within the plasma which compensates the electric voltage between the inner and the outer wall since no radial electric current flows.

Extension of Schottky's Theory to a Cylindrical Discharge Plasma Containing a Coaxial Rod

AbstractThe well known diffusion theory of the positive discharge column is extended considering a plasma which burns between two coaxial cylindrical walls. Recent experimental investigations carried out with the goal to enlarge the fluxes of ultraviolet radiation in fluorescent lamp tubes motivate such an extension. Indeed, an enhancement of the efficacy could be found installating a glass rod along the tube axis which acts as an additional recombination surface. In order to understand this effect also a description of the radial plasma behaviour, especially that of the electron density, and of the necessary charged particle production by ionization is of interest. The paper reports about such an extension, including a representation of a suitable numerical solution approach and a discussion of the results.

Collisional drift instability in hollow and coaxial plasmas

The drift instability, which is known to localize in regions of maximum density gradient, is observed in a cylindrical discharge plasma which may be made hollow or coaxial in shape. In the hollow plasma, two distinct radial density gradients occur, forming the outer and inner edges of the plasma. These plasma edges are the sites of two distinct modes of the instability which may differ in mode number but not frequency. In the coaxial plasma, three plasma edges occur, and three distinct modes are observed. The plasma is structured and the waves are excited by positive d.c. voltages applied to grids in the path of the plasma. In predictions of radial wave shape, a two fluid model which allows for arbitrary radial density profiles provides excellent agreement with experiment. Real frequency predictions of the model are Doppler shifted by E*B column rotations which are treated as uniform in the regions of mode propagation. In most cases, good agreement with observed frequency is obtained.