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The Importance of Seed Ionization in CIV Space Experiments

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A remarkable aspect of Alfvens critical ionization velocity CIV is the ease with which one can observe its symptoms in laboratory experiments and the great difficulty in observing its presence in space experiments. The object of this paper is to compare and to contrast laboratory and space CIV experiments, focusing on one potentially important difference the creation of an initial ion beam. In the laboratory experiments, the magnetic field is at rest along with the neutral gas while the plasma moves across it. In space, on the other hand, both ambient plasma and magnetic field are at rest while the neutral gas moves relative to the two. Plasma instability cannot occur until the neutral beam has obtained an ionized component. In space, the neutral cloud expands rapidly and the ultimate extent of ionization depends strongly on the time it takes to develop an ion beam capable of producing super-thermal electrons. Photoionization, charge exchange, associative ionization and stripping are possible mechanisms for seeding of the neutral cloud. However, because these produce ions yet are not CIV related, their presence confounds the experiment. Thus the presence of CIV-like symptoms, such as electron heating and lower hybrid waves, may give false evidence as to the occurrence of CIV. In this work, we suggest that the CIV interaction may be delayed in space experiments and that this delay, coupled with the rapid expansion of the neutral cloud, may explain the observed low yields.

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  • Particle Accelerators
  • Plasma Physics and Magnetohydrodynamics

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