Volume 92, Issue 4, 15 August 2002
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
Thomson-scattering observation of non-Maxwellian electron energy distribution in inductively coupled and plasmas and the effect of vibrational excitation and nonlocal electron kinetics92(2002); http://dx.doi.org/10.1063/1.1493659View Description Hide Description
Laser Thomson-scattering diagnostics of electron energy distribution function(EEDF) was carried out for inductively coupled and plasmas at 25 mTorr. Care was taken to ascertain that the observed Thomson spectra are unaffected by other laser-induced effects, in particular by Raman scattering. Admixing of and with Ar resulted in marked deviation of EEDF from Maxwellian. The mechanism leading to non-Maxwellian EEDF was also studied by a Monte Carlo simulation. The results suggest that the major factors responsible are rapid electron energy loss due to vibrational excitation and the effect nonlocal electron kinetics associated with localized electron heating.
92(2002); http://dx.doi.org/10.1063/1.1492014View Description Hide Description
Hall thrusters are an attractive form of electric propulsion that are being developed and implemented to replace chemical systems for many in orbit propulsion tasks on communications satellites. One concern in the use of these devices is the possible damage their plumes may cause to the host spacecraft.Computermodels of Hall thruster plumes play an important role in integration of these devices onto spacecraft as the space environment is not easily reproduced in ground testing facilities. In this article, a hybrid particle-fluid model of a Hall thruster plume is applied to model the SPT-100 thrusters used on the Russian Express satellites. The emphasis of the article is on making assessment of the model through direct comparison with measurements of ion current density and ion energy distributions taken on board Express spacecraft. A model for simulating atom–ion collisions is described. The sensitivity of the plume simulation results to various aspects of the physical modeling is investigated. The plume model is able to predict many of the most important characteristics of the measured data.
92(2002); http://dx.doi.org/10.1063/1.1494128View Description Hide Description
Laser-accelerated beams of Mega-electron volt protons have been produced at the Los Alamos Trident laser facility and used for high-resolution point-projection protonradiography of Au grids. The effective proton source size affords an inherent resolution of 2–3 μm in the object plane. The proton beam is characterized by ion time-of-flightFaraday cup measurements and nuclear particle track detectors. Laser-driven protonradiography appears promising as a valuable research tool for probing plasmas or modest density objects.
92(2002); http://dx.doi.org/10.1063/1.1491023View Description Hide Description
A global model for electronegative plasma, in which the negative ion distribution is assumed to be a parabolic profile in the axial direction with a flat central region and a similar edge profile in the radial direction in the electronegative region, is applied to study the power and pressure dependences of plasma parameters in low-pressure discharges. The electron density increases approximately linearly with the power. The electron temperature also increases with the power due to the decrease in neutral number density with increase in power, resulting in the increase in plasma potential. The density of is a weak function of the power, while the densities of and which are strongly correlated to the densities of the respective radicals, depend on the power. On the other hand, the decrease in electron temperature with the pressure significantly results in a decrease in the degree of dissociation. The electron density also decreases gradually with the pressure except for the case of pressure lower than 5 mTorr. The densities of and decrease gradually with the pressure at pressures higher than 5 mTorr, while the density of increases gradually with the pressure. The electron energy probability function (EEPF) is measured with a Langmuir probe in an inductively coupled rf (13.56 MHz) discharge over a pressure range from 2 to 30 mTorr, while keeping the power injected into the plasma at about 70 W. The measured EEPFs are approximately Maxwellian at any pressure, although there is a slight deviation from a Maxwellian distribution at pressures higher than 10 mTorr. The results estimated from the measured EEPF are compared to the model and show reasonably good agreement.
92(2002); http://dx.doi.org/10.1063/1.1491022View Description Hide Description
Analysis of discharge mechanism in especially designed pseudosparks that facilitate pseudospark characteristic low-erosive conduction at very high currents is presented. The alternate formulation of criterion for transition to arc discharge phase developed here enables us to account for the emergence of arcs at each aperture after a threshold electric charge is transmitted in these devices. The use of a large number of apertures is found to have no influence on discharge condition that initiates arc formation at individual apertures.