| In a pulsed electromagnetic accelerator a current sheet accelerates a propellant gas through the j x B force. In the ideal case all of the gas is entrained and accelerated by the sheet. An observed departure from this ideality is current sheet mass leakage, a phenomenon through which a wake of plasma is left behind the sheet along the cathode. This leads to a decrease in sweeping efficiency, the percent of the available propellant mass that is contained in the sheet. The present work describes experiments and an analytical model designed to quantify and explain the effect of current sheet mass leakage on the performance of the accelerator. High-speed photography, interferometry and magnetic field probing are employed to gain an understanding of the evolution of the sheet and the performance of the device. After an initial bifurcation phase, the current sheet in this device enters a steady-state phase of propagation during which the mass, velocity and canting angle are approximately constant. It is found that non-dimensional impulse and efficiency decrease with increasing propellant pressure for discharges using argon propellant, because of a decreasing sweeping efficiency. The performance of neon discharges stays constant with pressure because the loss of mass from the current sheet is made up for by a commensurate increase in wake mass. The performance of helium and hydrogen discharges increases with pressure, because while the sweeping efficiency stays constant, the wake velocity increases. The trends in the behavior of the sweeping efficiency have been explored with an analytical model of the current sheet. It is proposed that in the lighter propellants, which have a higher ion Hall parameter, the ions in the sheet are subject to a directed motion towards the cathode, causing a high degree of leakage of plasma into the wake. The heavier propellants, with low ion Hall parameters, are subject only to a diffusive leakage of ions at the cathode. However, these sheets are found to be highly permeable to the ambient propellant. |
