| The phenomenon of current sheet canting in pulsed electromagnetic accelerators is the departure of the plasma sheet that carries the current from a plane that is perpendicular to the electrodes to one that is skewed, or tipped. Review of pulsed electromagnetic accelerator literature reveals that current sheet canting is a ubiquitous phenomenon—occurring in all of the standard accelerator geometries. Developing an understanding of current sheet canting is important because it can detract from the propellant sweeping capabilities of current sheets and, hence, negatively impact the overall efficiency of pulsed electromagnetic accelerators. In the present study, photographic, magnetic, and laser-interferometric diagnostics were implemented to measure the current sheet canting angle in an experimental pulsed electromagnetic accelerator. Eight different propellants (hydrogen, deuterium, helium, neon, argon, krypton, xenon, and methane) were tested in a rectangular-geometry accelerator, at pressure levels ranging from 75–400 mTorr. The photographic, magnetic, and interferometric diagnostics were used to infer the spatial configuration of the current sheet by measuring its optical emission, current density, and electron density, respectively. The three techniques showed quantitative agreement. Additionally, emission spectroscopy was used to measure the electron temperature in the current sheet plasma. The canting angle was found to increase with the atomic mass of the propellant and the current sheet was always found to tilt such that the anode current attachment leads the cathode attachment. Lighter atoms were observed to yield less canting (the measured angles ranged from approximately 10° for hydrogen to 70° for xenon). Hydrogen, deuterium, and methane were found to exhibit the peculiar, and possibly beneficial, property of having reduced current sheet canting at the highest pressure level. The experimental results also motivated further analysis of the data, and led to the conclusion that current sheet canting is a natural consequence of the manner in which current is conducted in pulsed electromagnetic accelerators. It is postulated that depletion of plasma near the anode, which results from axial density gradient induced diamagnetic drift, occurs during the early stages of the discharge, creating a density gradient normal to the anode, with characteristic length on the order of the ion skin depth. Rapid penetration of the magnetic field through this region ensues, due to Hall effect, leading to a canted current front ahead of the initial current conduction channel. In this model, once the current sheet reaches appreciable speeds, entrainment of stationary propellant replenishes plasma in the anode region, inhibiting further Hall-convective transport of the magnetic field; however, the previously established tilted current sheet remains at a fairly constant canting angle for the remainder of the discharge cycle, exerting a transverse J × B force which drives plasma toward the cathode and accumulates it there. This proposed sequence of events has been incorporated into a phenomenological model. The model is shown to give quantitative agreement with the experimentally measured canting angle mass dependence trends. |
