Pure electron plasma equilibrium and transport jumps in the Columbia Non-neutral Torus

by Hahn, Michael, Ph.D., COLUMBIA UNIVERSITY, 2009, 140 pages; 3343506

Abstract:

The Columbia Non-neutral Torus (CNT) is a simple stellarator currently being used to study electron plasmas confined on magnetic surfaces. This thesis describes experimental and numerical studies of the equilibrium and transport jumps in CNT.

Recently a conducting boundary conforming to the last closed flux surface of CNT was installed. This boundary condition has caused some significant changes to the equilibrium. Measurements of CNT equilibrium with the conducting boundary were compared to measurements without the boundary. The conducting produces denser plasmas for a given emitter bias, forces the plasma potential to zero at the boundary, and causes plasmas to have electron temperatures that are more sensitive to parameters.

One property of the CNT equilibrium is a large axial potential and density variation along the magnetic axis. It is caused by the toroidally varying cross sectional geometry of the magnetic surfaces. Experimental measurements without the boundary have now verified the theoretically predicted magnitude of the variation.

Plasmas are typically created by a single biased heated filament placed on the magnetic axis, but plasmas can be created by other configurations of emitters. Experiments were done to study the effect of emitting off of the magnetic axis. The equilibria of such plasmas are consistent with the surfaces inside the location of the emitter being in a global (cross-surface) thermal equilibrium. The consistency has been verified by comparing experimental data to numerical reconstructions.

Transport jumps were observed in the cathode characteristic in CNT. These jumps occur at particular emission currents whenever any parameter that increases transport is varied. The jumps also do not depend on the ion content of the plasma, which makes the explanation for these jumps physically different from similar jumps in neutral plasmas. Experimental studies have shown that the jumps are probably caused by a cathode instability. The instability causes a shift from an equilibrium state which is unstable to the cathode instability to a state with a self consistent stable equilibrium between the cathode and the bulk plasma. These states can also be accessed through processes other than the cathode instability.

AdviserThomas Pedersen
SchoolCOLUMBIA UNIVERSITY
Source TypeDissertation
SubjectsPlasma physics
Publication Number3343506

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