Design of a compact coaxial magnetized plasma gun for magnetic bubble expansion experiments

The design of a compact coaxial magnetized plasma gun and its associated hardware systems are discussed in detail. The plasma gun is used for experimental studies of magnetic bubble expansion into a lower pressure background plasma as a laboratory model for extragalactic radio lobe expansion into the interstellar medium. The gun is powered by a 120 ¿F, 10 kV ignitron-switched capacitor bank. High-pressure gas is puffed into an annular gap between inner and outer coaxial electrodes. The applied high voltage ionizes the gas and creates a radial current sheet. The ~100 kA discharge current generates toroidal flux and poloidal flux is provided by an external bias magnetic field. Axial J × B force then ejects plasma out of the gun. If the J × B force exceeds the magnetic tension of the poloidal flux by a sufficient amount then a detached magnetized plasma bubble will be formed. Using a multi-tip Langmuir probe array, a high speed camera and a B-dot probe array, the evolution of this plasma ¿bubble¿ as it interacts with pre-existing low pressure background plasma, is studied. In particular, details of the plasma bubble formation system including the main gun cap-bank power system, gas valve control system, bias flux cap-bank power system, and experimental data are provided.