Theoretical and Numerical Investigation of Transport in the Columbia Non-neutral Torus

by Durand de Gevigney, Benoit, Ph.D., COLUMBIA UNIVERSITY, 2010, 184 pages; 3448331

Abstract:

The Columbia Non-neutral Torus (CNT) is a simple stellarator studying electron plasmas confined on magnetic surfaces. This thesis describes theoretical and numerical studies of equilibrium and transport for such plasmas in the framework of CNT.

A major feature of electron plasmas is the presence of a very large radial electric field. Trajectories of magnetically trapped electrons are largely improved by the induced precessional rotation. However this precessional rotation can resonate, at low magnetic fields, with the motion along the field lines and can lead to a deterioration of the orbits of passing electrons. Variations in potential on inner magnetic surfaces, due to geometric constraints, can also lead to open trajectories.

Debye shielding in a pure electron plasma is rather different from textbook Debye shielding. An analytic model is developed for the sheath in a pure electron plasma and compared with three-dimensional solutions obtained with a new solver for the equilibrium of pure electron plasmas on magnetic surfaces. In a pure electron plasma a negative potential perturbation is screened much less efficiently than a positive potential perturbation. This particularity can be used to improve the quality of the orbits but also to create open trajectories and inject positrons in the plasma.

Two sources of transport have been identified experimentally: collisions with neutrals and the presence of diagnostic rods. Transport from these sources is studied first theoretically then numerically with a new transport code and is compared to experimental results. Neutral-driven transport is proportional to the neutral pressure and is independent of the magnetic field strength B as long as the magnetic field is non-resonant. Rod-driven transport is due to the sheath around the rods and has two components: a convective part scaling as 1/B and a diffusive part scaling as 1/ B2.

Advisor
SchoolCOLUMBIA UNIVERSITY
Source TypeDissertation
SubjectsPlasma physics
Publication Number3448331

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