Pure Electron Plasmas Confined on Magnetic Surfaces

Abstract

Summary form only given. Small Debye length, low temperature, pure electron plasmas have been successfully confined in the Columbia Non-neutral Torus (CNT). These plasmas are created by steady state emission of electrons from an electron emitter physically located on the axis of the magnetic surfaces. Equilibrium density, temperature, and potential profiles have been measured by emissive Langmuir probes and they agree well with numerical calculations. Toroidal density variations of a factor of four are also predicted numerically. Confinement time is measured by dividing the total number of electrons by the emission current, which is equal to the electron loss rate. Confinement is currently limited in CNT by the presence of insulating rods in the plasma. The rods charge up electrostatically and create an E x B convection of electrons out of the magnetic surfaces. Confinement has been studied as a function of magnetic field, emitter bias, and neutral pressure. Electron loss rate is found to be proportional to B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in the low neutral pressure regime, which is consistent with rod-driven transport being the dominant mechanism. At higher neutral pressures where the neutral-driven transport is dominant, the loss rate scales linearly with p <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> , as expected, and approximately as B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1.5</sup> . Ion driven instabilities are also observed at high neutral pressure. A retractable electron emitter has been installed in CNT that creates the plasma without having an insulating rod present in steadv state. Measurements of confinement time in this unperturbed plasma should help to clearly identify other transport mechanisms.