Ph.D Student | Nemirovsky Ron |
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Subject | Pinch Collapse of Plasma in a Capillary: The Dynamical Effect of Neutral Atoms |

Department | Department of Physics |

Supervisor | Professor Emeritus Amiram Ron |

This work focuses on the role of neutral atoms in the dynamics of a capillary discharge Z-pinch, in the regime for which soft-x-ray amplification was demonstrated. The popular one-fluid MHD model assumes that all the particles in the plasma are charged, and drift together. We, however, show that for the case discussed here, large portions of the plasma contain an appreciable amount of neutral atoms. Since these are not affected by the electro-magnetic forces, but only by the much weaker mechanical forces, they flow with much smaller velocities than the ions and the electrons. To roughly examine the dynamic influence of this effect, we use simple, intuitive considerations to extend the single-fluid MHD by introducing a separate fluid for the neutral atoms in addition to the standard electrons-ions fluid). Results of calculations using this preliminary extended model give new predictions or the dynamics of the pinch collapse, with some features in better resemblance with the measured data.

Convinced by the results of our intuitive considerations we derive the hydrodynamic equations of motion for a partially ionized plasma, when the ionized component, and the neutral components have different flow velocities and kinetic temperatures. Starting from the kinetic equations for a gas of ions and a gas of atoms we consider various processes of encounters between the two species: self collisions, interspecies collisions, ionization, recombination, and charge exchange. Our results are obtained by developing a general approach for the hydrodynamics of a gas in a binary mixture, in particular when the components drift with respect to each other. This is applied to a partially ionized plasma, when the neutral-species gas and the charged-species gas have separate velocities. We further suggest a generalized version of the relaxation time approximation and obtain the contributions of the inter-species encounters to the transport equations. Finally we use these results to approve the preliminary model used for the numerical evaluation.