Numerical Simulation of the Critical Ionization Velocity Mechanism.

Details Numerical Simulation of the Critical Ionization Velocity Mechanism. Numerical Simulation of the Critical Ionization Velocity Mechanism.

Jan 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........78b&link_type=abstract

Thesis (SC.D.)--MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 1992.Source: Dissertation Abstracts International, Volume: 53-06, Secti

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The 'critical ionization velocity' (CIV) of a neutral gas is related to a form of anomalous ionization first proposed by Alfven. Although the CIV phenomenon has been verified in laboratory experiments, space-based experiments have provided only inconclusive or negative results as to the existence of CIV in the space environment. If the existence of CIV can be confirmed in space plasmas, there may be wide applications of the theory to astrophysical models such as cometary coma formation and Io's plasma torus, as well as engineering implications regarding space shuttle glow, MPD thruster operation, and thruster firings from spacecraft. The purpose of this thesis is to investigate the mechanism of CIV, develop an estimate for the rate of CIV, and apply these rate estimates to explain the discrepancy between laboratory and space experiments. Many of the results are achieved through particle-in-cell (PIC) simulations. Unlike previous PIC simulations of CIV, the current work employs an implicit PIC code to allow the use of realistic mass ratios and collisional cross sections in the simulation. The inclusion of realistic mass ratios and collisional cross sections results in realistic estimates of the characteristic times required for CIV to develop. The results of the simulations and analyses indicate that CIV operates through the initiation of an ion beam through some form of seed ionization. This ion beam is unstable to the modified two-stream instability (M2SI). The M2SI efficiently transfers energy from the beam ions to electrons. The electrons heat to energies above the ionization energy of the neutral gas. Electron impact ionization of the neutral gas then reinforces the ion beam and leads to a positive feedback loop resulting in an anomalous form of ionization occurring on a time scale much faster than classical ionization processes. The results are used to recommend an improved experimental design for space experiments, as well as to investigate the possibility of CIV during an arcjet thruster firing. If CIV occurs during an arcjet thruster firing, the plasma density near the vehicle may be greatly enhanced. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.) (Abstract shortened with permission of school.).

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