Hybrid-Direct Kinetic Simulation of Hall Thrusters
The discharge plasma of a Hall thruster is known to be in a non-equilibrium state. Experiments have shown that ions and neutral atoms in the discharge channel are in non-equilibrium and the velocity distribution functions (VDFs) of ions vary spatially and temporally. Additionally, the electrons experience complex processes such as secondary electron emissions (SEE), collisions, and turbulence effects. It is important to obtain the VDFs and energy distribution functions (EDFs) of both heavy species and electrons.
In order to achieve a good resolution of VDFs and EDFs, a direct kinetic simulation is used to model ions and neutral atoms whereas the electrons are modeled as fluid. Here, the numerical results obtained from the hybrid-DK simulation (HDK) are compared with a hybrid-PIC simulation that was developed by Boeuf and Garrigus 
We have developed a 1-D axial model of a Hall Thruster acceleration channel. The details of the code is described in our paper (Physics of Plasmas, vol. 19, 113508, 2012).
Table 1 is the comparison of thruster performace obtained from (1) a hybrid-DK simulation with DK simulation for neutral atoms, (2) a hybrid-DK simulation with a fluid neutral model, (3) a hybrid-PIC simulation, and (4) experiment.The numerical results are in good agreement with experiment. Although the same electron fluid was employed in the hybrid simulations, there is a slight difference in the results. In addition, using a kinetic model for neutral atoms plays an important role.
Table 1: Comparison of thruster performance
|Mean discharge current, Id||3.59 A||4.03 A||3.94 A||4.5 A|
|Thrust, T||88.7 mN||89.0 mN||90.8 mN||80 mN|
|Specific impulse, Isp||1810 s||1810 s||1850 s||1600 s|
|Electric efficiency, η||0.74||0.67||0.69||0.5|
Figure 1 shows the high-frequency plasma oscillations of the discharge plasma of a Hall thruster near the channel exit. The main difference of the two simulations is statistical noise. The hybrid-DK simulation does not suffer from the statistical noise due to the use of macroparticles. As a result, the transit-time oscillation (~100 kHz) is captured well.
Figure 1: High frequency oscillations. (a) Hybrid-PIC, (b) Hybrid-DK.
Funding for this work is provided by the U.S. Department of Energy, Office of Science, Fusion Energy Sciences Program and the Air Force Research Laboratory.
Hara, K., Boyd, I. D., and Kolobov, V. I.,"One-Dimensional Hybrid-Direct Kinetic Simulation of the Discharge Plasma in a Hall Thruster," Physics of Plasmas, Vol. 19, 113508, 2012.