Microparticle Dynamics in Strongly Magnetized Low Temperature Plasmas
Lynch, Brian Jr
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
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Dusty plasma systems consist of charged microparticles (usually called dust grains) embedded in a background plasma. The dust component can significantly modify the background plasma by collecting electrons and ions, thus modifying the plasma density and the local electrostatic potential. Similarly, due to their net electric charge, the dust grains also respond to local perturbations in the background plasma. In this way, the behavior of the dust grains and the background plasma are closely coupled with each other. The vast majority of experimental dusty plasma studies have been performed under conditions where there is no magnetic field. However, in many plasma systems — particularly fusion and astrophysical plasmas, magnetic fields play a crucial role. Therefore, in recent years there has been an increased effort to study modifications to dusty plasma systems under the influence of large external magnetic fields. Modifications of the dust component may manifest themselves directly as guiding center drifts or indirectly by altering the background plasma and dust grain charging processes. In this dissertation, the dust grain g × B drift magnetization effect was directly observed using the Magnetized Dusty Plasma Experiment (MDPX) device. The MDPX device was rotated to a horizontal configuration so that the magnetic and the gravitational fields were oriented perpendicular to each other. Dust grain g × B drift motion in a radio-frequency (RF) plasma bulk several centimeters away from any space charge sheaths was then observed and used to calculate the dust grain charge. It was found that the calculated dust grain charge was much smaller than the estimates obtained by extrapolating from situations where the background plasma was unmagnetized. The large reduction in the dust grain charge is believed to be due to the onset of strong electron magnetization where the electron Larmor radii become comparable to the dust grain diameter. In this situation, the electron charging current to the grain becomes flux-tube limited whereas the ion charging current remains relatively unaffected. The measurements performed in this dissertation are a large step forward in understanding the nature of dust grain charging in strongly magnetized plasmas and lay a substantial experimental framework for future theoretical studies.