Development of magnetized plasma jets using radial foils on COBRA

Summary form only given. This research focuses on the initial ablation phase of a thin (20μm) Al foil driven on the 1MA-in-100ns COBRA through a 5mm diameter cathode in a radial configuration. In these experiments, ablated surface plasma (ASP) on the top side of the foil and a strongly-collimated axial plasma jet can be observed developing midway through current-rise. Our goal is to establish the relationship between the ASP and the jet. These jets are of interest for their potential relevance to astrophysical phenomena. An independently-pulsed 200μF capacitor bank with a Helmholtz-coil pair allows for the imposition of a slow (150μs) and strong (~1T) axial magnetic field on the experiment. Without this applied field, time-gated axial pinhole images show significant azimuthal asymmetry in the extreme-ultraviolet (XUV) emission of ASP before and during the formation of the jet. A phase transition is clearly visible at a point of sharp change in emission intensity which closely corresponds to the point predicted by a 1D Ohmic heating model. With the axial field, the ASP develops considerable azimuthal motion, which reduces observed asymmetries and translates into a spiral motion in the jet. The additional azimuthal motion slows the development of the jet as observed in side-on XUV emission imaging. The conservation of canonical angular momentum is closely related to the observed jet behavior. Initial contributions come only from the applied field. Later on, the ASP gains mechanical angular momentum which is transferred to the jet as it is formed by ASP material collecting near the axis. Laser-backlit shadowgraphy and interferometry confirm that the jet-hollowing is correlated with the application of the axial magnetic field.