skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Plasma formation in a double-gap vircator

Abstract

Time-resolved light emission imaging was used to observe the plasma formation within the cavity of the double-gap vircator powered by a sub-microsecond generator ({approx}500 kV, {approx}10 kA, {approx}500 ns). The vircator generated well reproducible S-band microwave pulses of {approx}200 MW peak power and up to 200 ns full duration. The plasma light emission was observed {approx}30 ns prior to the ending of the generated microwave pulses at the surface of the aluminum foil separating the vircator cavity gaps, in the gap where the virtual cathode is formed. Estimations showed that the energy deposition into the foil by the high-current electron beam is sufficient for the surface plasma formation. The plasma ions accelerated toward the virtual cathode neutralize its electron space charge. The latter was confirmed by the increase in the electron current transmitted through the vircator cavity. In addition, the time of the plasma appearance was determined by comparing the measured transmitted current with that following from the one-dimensional model of a stationary un-neutralized two-stream electron flow. This time agrees with the maximum of the microwave power observed in the experiments, thus showing that the plasma ions cause the termination of the microwave generation.

Authors:
; ;  [1]
  1. Department of Physics, Technion, 32000 Haifa (Israel)
Publication Date:
OSTI Identifier:
21537925
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 108; Journal Issue: 10; Other Information: DOI: 10.1063/1.3510475; (c) 2010 American Institute of Physics
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; ALUMINIUM; CAVITIES; ELECTRON BEAMS; ELECTRONS; EMISSION; FOILS; IONS; MICROWAVE RADIATION; PEAK LOAD; PLASMA; PULSES; SPACE CHARGE; STREAMS; SURFACES; TIME RESOLUTION; VISIBLE RADIATION; BEAMS; CHARGED PARTICLES; ELECTROMAGNETIC RADIATION; ELEMENTARY PARTICLES; ELEMENTS; FERMIONS; LEPTON BEAMS; LEPTONS; METALS; PARTICLE BEAMS; RADIATIONS; RESOLUTION; RIVERS; SURFACE WATERS; TIMING PROPERTIES

Citation Formats

Queller, T., Shlapakovski, A., and Krasik, Ya. E.. Plasma formation in a double-gap vircator. United States: N. p., 2010. Web. doi:10.1063/1.3510475.
Queller, T., Shlapakovski, A., & Krasik, Ya. E.. Plasma formation in a double-gap vircator. United States. doi:10.1063/1.3510475.
Queller, T., Shlapakovski, A., and Krasik, Ya. E.. 2010. "Plasma formation in a double-gap vircator". United States. doi:10.1063/1.3510475.
@article{osti_21537925,
title = {Plasma formation in a double-gap vircator},
author = {Queller, T. and Shlapakovski, A. and Krasik, Ya. E.},
abstractNote = {Time-resolved light emission imaging was used to observe the plasma formation within the cavity of the double-gap vircator powered by a sub-microsecond generator ({approx}500 kV, {approx}10 kA, {approx}500 ns). The vircator generated well reproducible S-band microwave pulses of {approx}200 MW peak power and up to 200 ns full duration. The plasma light emission was observed {approx}30 ns prior to the ending of the generated microwave pulses at the surface of the aluminum foil separating the vircator cavity gaps, in the gap where the virtual cathode is formed. Estimations showed that the energy deposition into the foil by the high-current electron beam is sufficient for the surface plasma formation. The plasma ions accelerated toward the virtual cathode neutralize its electron space charge. The latter was confirmed by the increase in the electron current transmitted through the vircator cavity. In addition, the time of the plasma appearance was determined by comparing the measured transmitted current with that following from the one-dimensional model of a stationary un-neutralized two-stream electron flow. This time agrees with the maximum of the microwave power observed in the experiments, thus showing that the plasma ions cause the termination of the microwave generation.},
doi = {10.1063/1.3510475},
journal = {Journal of Applied Physics},
number = 10,
volume = 108,
place = {United States},
year = 2010,
month =
}
  • Instabilities and double-layer formation are systematically investigated on a bounded collisionless system with electron beam penetrating through a plasma. The Buneman instability is observed at low beam currents. Above a critical current, there appears a sudden nonoscillatory potential drop due to the Pierce instability, which traps ions in the potential well. This collisionless ion trapping provides a new formation mechanism of the double layer, which is controlled by changing the speed of the potential drop.
  • The axial profiles of plasma parameters for low and moderate pressures, such as the plasma potential, electron temperature, and number density, have been evaluated in magnetized inductively coupled plasma. The experimental results revealed in both cases the existence of a genuine current-free double-layer structure, separating two plasma regions with different properties. Based on the experimental results, a physical scenario for the self-assembling of the double layer is proposed. Also, the axial profile of the electron number density downstream is analyzed, emphasizing the role of neutral metastable ionization, and a simple analytical model is developed to fit the experimental data. Themore » model allows the estimation of neutral metastable number density downstream and the recombination rate coefficient.« less
  • The formation of a double layer (DL) in a two-dimensional (2D) electronegative plasma with a source (heating) section connected to a larger downstream section is described. A 2D particle-in-cell (PIC) code is used to exhibit the DL, which appears near the transition between the source and downstream chambers, over a range of pressures and electronegativities. Diagnostics of the PIC code allow the calculation of various plasma parameters, not easily measured in experiments, to be compared with an analytic theory. The theory consists of a collisionless one-dimensional model of a DL connected to 2D source and downstream global models. The conditionsmore » of positive and negative ion balance upstream and downstream, and the downstream energy balance determine the DL potential, electron temperatures, and other plasma parameters. A rescaled oxygen reaction set is used both for the simulation and for the analytic comparison. The PIC simulations exhibit a Maxwellian electron distribution in the source region at temperature T{sub h}, and a bi-Maxwellian distribution downstream, with a low energy population at temperature T{sub c}<T{sub h} and with a hotter tail also having temperature T{sub h}. At the upstream DL edge, an accelerated electron component is observed. Using these results in the model, a DL is found in reasonable agreement with that obtained in the simulation.« less
  • The quasi one-dimensional expansion of a collisionless plasma with a hot-electron tail in a gentle convergent-divergent nozzle is studied. A parametric investigation of the plasma response is carried out in terms of the relative density and temperature of the hot-electron population. The formation of a steepened layer is shown to be due to the anomalous thermodynamic behavior of the plasma, which creates a local minimum of the Mach number. The change from a quasineutral to a non-neutral steepened layer occurs when this minimum goes below one and several sonic points appear. The non-neutral double layer does not introduce further changesmore » in the plasma response. All gain in plasma momentum and thrust is related to the supersonic expansion in the divergent nozzle, with zero contribution of the double layer. A comparative analysis of thrust efficiency of plasmas with and without hot electrons does not find any gain in the presence of hot electrons; instead, a small penalty in the expansion efficiency seems to exist. The study is limited to Maxwellian electron populations and finite nozzles.« less
  • We experimentally study the dynamics of the plasma induced by the double-laser-pulse irradiation of solid target in water, and find that an appropriate choice of the pulse energies and pulse interval results in the production of an unprecedentedly mild (low-density) plasma, the emission spectra of which are very narrow even without the time-gated detection. The optimum pulse interval and pulse energies are 15-30 {mu}s and about {approx}1 mJ, respectively, where the latter values are much smaller than those typically employed for this kind of study. In order to clarify the mechanism for the formation of mild plasma we examine themore » role of the first and second laser pulses, and find that the first pulse produces the cavitation bubble without emission (and hence plasma), and the second pulse induces the mild plasma in the cavitation bubble. These findings may present a new phase of underwater laser-induced breakdown spectroscopy.« less