Study of the "critical Ionization Velocity" Effect by Particle-In Simulation

Details Study of the "critical Ionization Velocity" Effect by Particle-In Simulation Study of the "critical Ionization Velocity" Effect by Particle-In Simulation

Feb 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991phdt........65l&link_type=abstract

Thesis (PH.D.)--THE UNIVERSITY OF IOWA, 1991.Source: Dissertation Abstracts International, Volume: 52-07, Section: B, page: 367

Computer Science

Ionization

Scientific paper

The Alfven "critical ionization velocity" (CIV) mechanism for the anomalously rapid ionization of a neutral cloud moving across a magnetized plasma at a supercritical velocity was proposed originally to explain the formation of the early solar system. Since then, many theoretical and experimental studies of this mechanism have been carried out. The CIV mechanism has been demonstrated well in the laboratory but not in all space experiments. This implies that the CIV mechanism is more complicated than previous theory has suggested. Motivated by this, a series of computer simulations of a neutral gas cloud moving across a magnetized plasma have been performed with a 2{1over2 } dimensional electrostatic PIC code "PANIC". We observe electron heating (mainly along the magnetic field) by waves driven by a cross-field ion beam which have frequencies in the lower-hybrid frequency range and subsequent ionization of the neutrals by electron impact ionization as suggested by traditional theory which is based upon the modified two-stream instability (MTSI) or modified beam-plasma instability (MBPI). It is also found that the efficiency of CIV mechanism is reduced by the finite-size of the gas cloud in the following ways: the momentum coupling between the cloud and the ambient plasma reduces the relative drift between the ambient plasma and neutrals; the MTSI will be quenched when the size of the cloud is smaller than the wavelength of the major MTSI mode; some energetic electrons are lost from the cloud at a rate characterized by the acoustic velocity instead of the electron thermal velocity along the magnetic field and the neutral drift velocity in the direction of plasma drift relative to the neutrals. The transverse charge -separation field across the cloud was found to result in the modulation of the drift velocity which will reduce the parallel heating of electrons and increase the transverse acceleration of electrons. The simulation results are used to interpret various chemical release experiments in space.

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