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|Title:||Electron Currents in Field Reversed Mirror Dynamics--Theory and Hybrid Simulation|
|Author(s):||Stark, Robert Armand|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Fluid and Plasma|
|Abstract:||The role of electron currents in Field-Reversed Mirror (FRM) dynamics has been studied through both analytic and computational means. In particular, the effects of electron currents near a magnetic field null is closely examined. Since the electrons are not tied to field lines in this region, they are able to partially cancel the ion current there in response to an inductive electric field and ion drag. Past workers, using a fluid description, have suggested that this cancellation may be virtually complete, making impossible the steady state operation of the FRM as well as its start-up by neutral-beam injection. However, the fluid approximation is invalid near a field null. We have developed a detailed non-fluid model for the bulk dynamics of the electrons in this region. The region is treated as unmagnetized; electrons are accelerated by the inductive electric field and collisions with ions; damping is provided by viscosity due to adjacent "fluid" electrons and scattering on the ions. The resulting equation of motion when combined with Faraday's Law predicts that, due to viscosity, current cancellation is far from complete, although electron currents near the null may slow the attempt to reverse the field via neutral-beam injection by a factor of five or so.
To model the dynamics of the FRM as a whole we have developed a 1-D radial hybrid code which also incorporates the above electron null current model. This code, named FROST, models the plasma as azimuthally symmetric with no axial dependence. A multi-group method in energy and canonical angular momentum describes the large-orbit ions from the beam. Massless fluid equations describe electrons and low energy ions. Simulation of neutral beam start-up in a 2XIIB-like plasma is discussed. FROST predicts that electron currents will retard, but not prevent reversal of the magnetic field at the plasma center. These results are optimistic when compared to actual reversal experiments in 2XIIB, because finite axial length effects and micro-instabilities substantially deteriorated the ion confinement. Nevertheless, because of the importance of the electron current in a low field region in the FRM, FROST represents a valuable intermediate step toward a more complete description of FRM dynamics. (Abstract shortened with permission of author.)
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987.
|Date Available in IDEALS:||2015-05-13|