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p[C/m31
X 101
U
12
0.05
10
0.04
0.03
8
0.02
6
0.01
0
4
-0.01
2
-0.02
-0.03
0
-0.04
-2
-0.05
-0.05
0
0.05
Figure 4-16: Charge density solution with charge injection boundary condition pref
10- Cm- 3 and Eref = 1 kV/m on the coarse mesh.
The post-processed thrust and current characteristics are compared to the experimental result from Masuyama & Barrett [29, 30] and ID theory in figure 4-19. The
homogeneous boundary condition case results in a maximum thrust error of 14%.
This is likely due to the natural boundary condition applied to the domain boundary.
The natural condition forces all emitted current to the collector; no charge escapes to
the surrounding. Figure 4-20 compares the collected and emitted current for all analysis models. Figure 4-19b also confirms that the emitted current for the homogeneous
boundary condition case exactly matches the imposed experimental results.
The low sensitivity charge injection case does not represent the experimental results very well. The maximum error in thrust is 75% and the maximum error in
current is 76%. The model results in high current below the corona ignition voltage
and low current at high voltages. The higher sensitivity charge injection model (noted
as case 2 in figure 4-19) correlates better with the experimental results but does not
converge on the coarse mesh at higher voltages. The maximum thrust error is 0.4 mN
with a current error of 1.8 x 10' niA. The experimentally measured current was zero
with a non-zero thrust; this indicates that the current level is below the measurement
resolution. The high resolution mesh allows the high sensitivity charge injection case
to converge properly at higher applied voltages. The thrust and current characteristics (see case 3 in figure 4-19) are within 6% of the homogeneous case and within 19%
65
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