Title:
Current Sheet Mass Leakage in a Pulsed Plasma Accelerator
Author:
John W. Berkery
Document type:
PhD Thesis, 2005
Access:
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Abstract:
In a pulsed electromagnetic plasma accelerator a current sheet accelerates a propellant gas through the j x 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.
