Current Sheet Mass Leakage in a Pulsed Plasma Accelerator
In a pulsed electromagnetic plasma accelerator a current sheet accelerates a propellant gas through the j×B force density. 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 percentage of the available propellant mass that is contained in the sheet. The goal of this work was to use experiments and an analytical model to understand the nature of the physical processes behind the mass leakage phenomenon and to quantify its effects on the performance of the accelerator over a range of pressures and propellants. Photography, interferometry, magnetic field probing, spectroscopy and momentum plate measurements are employed to gain an understanding of the evolution of the sheet and the performance of the device. It was found that 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. Specific impulse and efficiency decrease with increasing propellant pressure for discharges using argon propellant, because of a decreasing sweeping efficiency. In neon, performance stays constant with pressure because the loss of mass from the current sheet is made up for by a commensurate increase in wake mass. In helium and hydrogen, performance 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. The model indicates that for 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.