Experimental Investigations of Magnetohydrodynamic Plasma Jets

Abstract

This thesis primarily focuses on understanding the plasma behavior during the helicity injection stage of a pulsed spheromak experiment. Spheromak formation consists of a series of dynamic steps whereby highly localized plasma near the electrodes evolves toward a Taylor state equilibrium. The dynamical evolution stage has been modeled as a series of equilibrium states in the past. However, the experiments at the Caltech spheromak facility have revealed that unbalanced J x B forces drive non equilibrium Alfvenic flows during these preliminary stages. The Caltech spheromak experiment uses coplanar electrodes to produce a collimated plasma jet flowing away from the electrodes. The jet formation stage precedes the spheromak formation and serves as a mechanism for feeding particles, magnetic helicity, energy, and toroidal flux into the system. Detailed density and flow velocity measurements of hydrogen and deuterium plasma jets have revealed that the jets are extremely dense with β [subscript thermal] ~1. Furthermore, the flow velocity was found to be Alfvenic with respect to the the toroidal magnetic field produced by the axial current within the plasma. An existing magnetohydrodynamics (MHD) model has been generalized to successfully predict the effect of plasma current on the jet's density and flow velocity. The behavior of these laboratory jets is in stark contrast to the often considered model for astrophysical jets describing them as equilibrium configurations with hollow density profiles. Other contributions of this thesis include the following. 1. The thesis presents an analytical proof that resistive MHD equilibrium with closed flux tubes is not feasible. This implies that sustained spheromak experiments cannot maintain helicity while being in a strict equilibrium. 2. The thesis describes measurements to characterize the circuit parameters of the high voltage discharge circuit used in the Caltech spheromak experiment. 3. The thesis also describes the setup of novel He-Ne laser interferometers used to measure the density of plasma jets. The ease of alignment of these interferometers was greatly enhanced by having unequal path lengths of the scene and reference beams. 4. Finally, the thesis details the setup for a soft X-ray (SXR)/Vacuum ultra violet (VUV) imaging system. Some preliminary images of reconnecting flux tubes captured by the imaging setup are also presented.