Force-Free Equilibria and Moving Structures in a Magnetized Plasma Arcade

Magnetized plasma arcades and flux ropes appear in space and astrophysical contexts and have been the subject of recent laboratory, observational, and computational studies. We present results from a laboratory arcade experiment. The arcade-shaped plasma is formed between two parallel electrodes and is constrained by a magnetic coil surrounding the electrodes. ICCD cameras, photodiode cameras, magnetic probes, and plasma spectroscopy are used to characterize the plasma. A simple model for a force-free equilibrium in a thin magnetized arcade predicts the angle of the sheared arcade magnetic field lines as a function of plasma current and vacuum arcade magnetic field. The model is compared with images of tilted plasma loops within the arcade and we find that the simple model provides a good upper bound for the tilt angle observed. We also observe moving structures that form repetitively and propagate along the electrodes with a speed that is nearly independent of plasma current, magnetic field strength, ion mass (hydrogen vs. argon), and direction of propagation. The structures are present in visible emission and in magnetic field measurements, the latter of which identify these as propagating current filaments.

This work was supported in part by U.S.D.O.E. grant DE-FG02-08ER55002.