Characterization of the resonant electromagnetic mode in helicon discharges

This dissertation is motivated by a collaboration between the University of Texas at Austin and NASA on the VASIMR project. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an effort to create a plasma-based thruster. The proposed thruster uses a helicon plasma discharge as its plasma source. An ICRH heating mechanism is then employed to impart energy into the plasma. A magnetic nozzle exhausts the plasma converting the ion gyromotion into superalfvenic longitudinal motion, thus creating thrust.;The physics behind the helicon plasma source is an area of active experimental and theoretical research and we are presenting experimental results which cast the problem of helicon plasma in a new light. Our experimental results point to the existence of a resonant electromagnetic mode in the helicon plasma. The data suggests that this mode is universal in the helicon discharge and can be excited over a wide range of frequencies. The resonant frequency is slightly lower than the driving frequency in steady-state. The radial component of the perturbed magnetic field of the resonance mode is peaked at a radius r* where the radial density gradient of the plasma has a maximum. The energy damping rate of the resonant mode is relatively low and cannot deposit the mode energy into the plasma in a single pass. The experimental data also shows that the resonant electromagnetic wave is responsible for depositing most of the available rf energy into the plasma. This supports the central conclusion of a new theory created by B. Breizman and A. Arefiev which predicts that the plasma density gradient forms a potential well which traps the electromagnetic energy. In broader terms, the trapping of the wave in a cavity formed by the density gradients of the plasma both radially and axially provides an explanation to the central dilemma of helicon plasmas: the seeming contradiction between the measured low damping rate of the wave and the measured short damping length. The potential well which traps the wave energy effectively provides an unlimited length scale to the mode allowing it to bounce within the boundaries of the cavity while continuously depositing energy into the plasma. The non-Maxwellian electron population detected in the plasma is linked to the high ionization efficiency of the discharge and can be explained with the operation of the resonant helicon mode.;An experimental method used as an active frequency probe into the plasma impedance is developed and presented.