DIRECT MAGNETIC-FIELD MEASUREMENT OF FLOW DYNAMICS IN MAGNETIZED COAXIAL ACCELERATOR CHANNELS

A miniature magnetic probe array, consisting of ten spatially separate d coils, has been used to obtain profile information on the time-varyi ng magnetic field within the 2.54 cm wide flow channel of the Coaxial Plasma Source experiment (CPS-1) [R. M. Mayo ct al., Plasma Sources Sc i. Technol. 4, 47 (1995)] at the North Carolina State University. Two- dimensional (2-D) current profiles within the annular flow channel, wh ich were constructed from the time-varying magnetic field data, reveal several complex features reflecting the influence of gun inductance, the Hall effect, and the applied magnetic field. When an external, ele ctrode linking magnetic field is applied, the evolution of the 2-D cur rent profile shows evidence of an ionizing shock front identified by a narrow current sheet propagating through thp channel during the first few microseconds of the discharge. The thickness of this current shee t is on the same order as both the collisional mean-free path and the ion electromagnetic skin depth. In this applied field case, the plasma is prevented from advancing ahead of the current sheet by the applied magnetic field, which turns the ions and electrons without collisions . In the absence of an applied field, plasma is able to advance ahead of the current sheet, where it may initiate ionization downstream befo re the advance of the ionization front. (C) 1997 American Institute of Physics.