2-DIMENSIONAL MAGNETIC-FIELD EVOLUTION MEASUREMENTS AND PLASMA-FLOW SPEED ESTIMATES FROM THE COAXIAL THRUSTER EXPERIMENT

Local, time-dependent magnetic field measurements have been made in th e Los Alamos coaxial thruster experiment (CTX) [C. W. Barnes et al., P hys. Fluids B 2, 1871 (1990); J. C. Fernandez et al, Nucl. Fusion 28, 1555 (1988)] using a 24 coil magnetic probe array (eight spatial posit ions, three axis probes). The CTX is a magnetized, coaxial plasma gun presently being used to investigate the viability of high pulsed power plasma thrusters for advanced electric propulsion. Previous efforts o n this device have indicated that high pulsed power plasma guns are at tractive candidates for advanced propulsion that employ ideal magnetoh ydrodynamic (MHD) plasma stream flow through self-formed magnetic nozz les. Indirect evidence of magnetic nozzle formation was obtained from plasma gun performance and measurements of directed axial velocities u p to upsilon(z) approximately 10(7) cm/s. The purpose of this work is to make direct measurement of the time evolving magnetic field topolog y. The intent is to both identify that applied magnetic field distorti on by the highly conductive plasma is occurring, and to provide insigh t into the details of discharge evolution. Data from a magnetic fluctu ation probe array have been used to investigate the details of applied magnetic field deformation through the reconstruction of time-depende nt flux profiles. Experimentally observed magnetic field line distorti on has been compared to that predicted by a simple one-dimensional (1- D) model of the discharge channel. Such a comparison is utilized to es timate the axial plasma velocity in the thruster. Velocities determine d in this manner are in approximate agreement with the predicted self- field magnetosonic speed and those measured by a time-of-flight spectr ometer.