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Non-axisymmetric Flows and Transport in the Edge of MST

Posted on:2012-01-16Degree:Ph.DType:Dissertation
University:The University of Wisconsin - MadisonCandidate:Miller, Matthew CharlesFull Text:PDF
GTID:1452390008992710Subject:Physics
Abstract/Summary:
Magnetic reconnection occurs in plasmas all throughout the universe and is responsible for spectacular and perplexing phenomena. In the Madison Symmetric Torus (MST) reversed field pinch (RFP), reconnection occurs as quasi-periodic bursts of tearing instabilities (saw-teeth), which give rise to a number of processes that affect the RFP's global behavior and confinement. This work examines the structure of turbulent plasma flow in the edge region and its role in affecting momentum and particle transport through the use of several insertable probes and novel ensemble techniques.;Very few measurements exist of tearing mode flow structures. The flow structure has now been measured for m = 0 modes and is in good agreement with theoretical expectations for nonlinear resistive MHD calculated for the RFP using DEBS and NIMROD. The flows are predicted and measured to be different than the classical Sweet-Parker picture with symmetric inward flows.;The flow fluctuations have a profound effect on momentum transport, which is trans- ported rapidly at the crash. This work advances the understanding of this process by measuring the Reynolds stress associated with turbulent flow. Combined with measurements of the Maxwell stress, a new picture for magnetic self-organization in the RFP via two-fluid physics has emerged. The Reynolds and Maxwell stresses are measured to be an order of magnitude larger than the rate of change in inertia but oppositely directed such that they almost cancel. Two-fluid effects are significant because of the relationship be- tween the Maxwell stress and the Hall dynamo, a term only existing in two-fluid theories. This relationship inextricably couples the momentum dynamics with the current dynamics. Indeed, the parallel momentum profile exhibits a relaxation at the crash akin to the relaxation seen in the parallel current density profile.;Tearing modes also drive particle transport. Fluctuation-induced particle flux is resolved through a crash by measuring it directly as ⟨ neur⟩. The flux increases dramatically during a crash and is non-axisymmetric. Between crashes, the transport from tearing is small, which agrees with previous measurements that identified electrostatic transport as dominant at that time.
Keywords/Search Tags:Transport, Flow, Tearing
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