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Transcript
4.2
Single Stage Thruster
The single stage thruster is modeled using the natural boundary condition on domain
edges, a charge injection boundary condition on the emitter, and an outflow boundary condition on the collector. The potential of the collector is held at zero while
the potential applied to the emitter,
#o,
is varied. The charge injection boundary
condition is either a homogeneous current flux based on experimental measurements
from Masuyama & Barrett [29, 30] or the charge injection model with parameters
Pref = 10-
Cm- 3 and Eref = 100 kV/cm. The large Eref facilitates convergence given
the relatively coarse nature of the mesh. A denser mesh with 11428 elements allows for convergence of a high sensitivity charge injection boundary condition with
Pref = 10-5
Cm- 3 and Eref = 1 kV/cm but requires the use of a more powerful com-
puter.
Contour plots of the solution for the homogeneous charge injection boundary
condition are shown in figure 4-17. Corresponding plots for the charge injection model
are shown in figure 4-18. The primary difference in the results is the magnitude of the
charge density. The homogeneous case results in charge density on the order of 10-3
whereas the charge injection model results in charge density a full order of magnitude
lower.
The smoother charge injection characteristic does not drive enough of an
increase in current for a given voltage. The charge density solution in Figure 4-16 is
calculated with a higher sensitivity charge injection model on the coarse mesh. Even
at a lower applied voltage, the solution exhibits faceting and numerical instability.
Increased mesh density is required to use the less stable model.
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