Volume 89, Issue 3, 01 February 2001
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
89(2001); http://dx.doi.org/10.1063/1.1337593View Description Hide Description
Zero-dimensional and two-dimensional plasma models and optical emission spectroscopy are used in tandem to investigate the power coupling efficiency for a pure hydrogen microwave plasma. The zero-dimensional model accounts for the vibrational kinetics of the chemistry of and H excited states, and the kinetics of ground-state species. The set of species conservation equations are then coupled to the electron Boltzmann equation (to account for the non-Maxwellian electron energy distribution function) and the total energyequation for solution. The two-dimensional model makes use of a simpler thermochemical description of the plasma. The chemistry is described with nine species and thirty chemical reactions. Three energy modes are considered to describe the plasma’s thermal nonequilibrium, and Maxwellian distribution functions for kinetic and vibrational modes are assumed. The non-Maxwellian nature of the electron energy distribution function is separately accounted for. Experimentally, the absolute line emission intensity is utilized to obtain number densities of up to five hydrogen excited states using the following transitions: Hα (6563 Å), Hβ (4861 Å), Hγ (4340 Å), Hδ (4102 Å), and Hε (3970 Å). The first three transitions were used for a 38 Torr, 1000 W hydrogen discharge, and all five transitions were used for a 121 Torr, 4000 W hydrogen discharge. The absolute continuum emission from the plasma was compared to numerical predictions. The comparison of the numerical and experimental data indicates that 90%–100% of the input power is deposited in the plasma and that both the line and continuum emission match within a factor of 3, with the exception of the high energyexcited states for the 4000 W plasma. A control volume heat transfer analysis validates the energy coupling.
Spatial characteristics of electron beams from symmetric and nonsymmetric crossed-field secondary emission sources89(2001); http://dx.doi.org/10.1063/1.1337594View Description Hide Description
The crossed-field secondary emission (CFSE) diode is a cold electron source based on a self-sustained secondary electron emission. The output electron beams are tubular and could be generated in a wide range of currents up to several hundred amperes. In this study, radial and azimuthal current density distributions of electron beams produced by symmetric and nonsymmetric CFSE diodes have been investigated. The electron beams are characterized by extremely high temporal stability. The wall thickness of the tubular beam with a current of A from the diode with a 5 mm anode–cathode gap was measured to be as small as 1.4±0.2 mm. In axisymmetric diodes, the azimuthal current distribution is uniform but this is only achieved by careful adjustment of the cathode–anode assembly. In nonaxisymmetric diodes, the distributions are strongly nonuniform and depend not only on the magnitude but also on the direction of the magnetic field. Results of the present research show that the CFSE electron sources are potent candidates for incorporation into medium and high power microwave devices.
89(2001); http://dx.doi.org/10.1063/1.1337597View Description Hide Description
The effect of driving frequency (13.56–50 MHz) on the electrical characteristics and the optical properties of hydrogen discharges has been studied, under constant power conditions. The determination of the discharge power and impedance was based on current and voltage wave form measurements, while at the same time spatially resolved emission profiles were recorded. As frequency is increased, the rf voltage required for maintaining a constant power level is reduced, while the dischargecurrent increases and the impedance decreases. Concurrently the overall emission intensity decreases and its spatial distribution becomes more uniform. Further analysis of these measurements through a theoretical model reveals that frequency influences the motion of charged species as well as the electron energy and the electric field, resulting in a modification of their spatial distribution. Moreover, the loss rate of charged species is reduced, leading to an increase of the plasma density and to a decrease of the electric field. Under these conditions, the total power spend for electron acceleration increases with frequency, but combined to the higher electron density, leads to a drop of the average energy gained per electron, a drop of the mean electron energy, and an enhancement of the low-energy electron-molecule collision processes against high energy ones.
89(2001); http://dx.doi.org/10.1063/1.1331071View Description Hide Description
Several results on the plasma-neutral gas structure generated in a dc cathodic arc are presented. The arc is operated at a current level of 100 A, with a coppercathode, and with oxygen gas at a pressure of 0.5 Pa. The employed diagnostics include spherical Langmuir probes and a calorimetric technique. The plasma potential, electron temperature, ion density, and ion kinetic energy are inferred at different axial positions in the discharge chamber. In order to explain the obtained experimental results, a simplified one-dimensional stationary model to describe the plasma-neutral gas structure is developed. A good agreement is found between the experimental data and the theoretical predictions when charge-exchange between metallic ions and neutral gas molecules and lateral diffusion loses of energetic particles are included in the theoretical treatment.