Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA24 / P36
3D DSMC Simulation of Neutral Particles in Miniature Ion Thruster
Yousefian Ali, Yamamoto Naoji
Department of Advanced Energy Engineering Sciences, Kyushu University, Fukuoka, Japan
Email: Yousefian.ali.089@s.kyushu-u.ac.jp
Abstract: Antenna configuration in an ion thruster determines the electron density distribution throughout the engine
which leads to unique ionization patterns. In order to maximize the ionization, high neutral density in these ionization
areas is essential. Moreover the neutral density distribution of ion thrusters is solely dependent on the configuration ofthe gas inlet. We have developed an original 3D DSMC (Direct Simulation Monte Carlo) simulation of neutral particles
in a miniature ion thrusters for one of our developed thrusters. The variation in neutral density distribution and velocity
distribution is analyzed to ensure the accuracy of the simulation.
Keywords: Electric Propulsion, Particle Simulation, Miniature Ion Thruster
1. INTRODUCTION
Our research involves the development of realistic
numerical simulation for Miniature microwave Ion
Thrusters. An ion engine produces high thrust efficiency,
exceeding 70%, with specific impulse of 3,000-8,000 s.
Therefore, miniature ion engines are candidates for use
as miniature propulsion system for small satellites [1].
Since the ion engine is so small that we cannot insert ameasurement
equipment
withoutperturbations.
Numerical simulations are effective tool to understandthe phenomena occurring in a miniature microwave ion
thruster [2]. We utilize the DSMC method for thissimulation as The DSMC method [3] works by moving
the simulation particles through a finite time step througha discretized computational domain. After each push,particles are paired up with other particles located in thesame computational mesh and collision probability iscomputed for these pairs. Those pairs that are colliding
then have their velocities modified in a way thatconserves energy and momentum. By analyzing the inlet
position and configuration we can determine the neutral
density distribution throughout the engine and examine
its effectiveness. For a comparative study we chose oneof the miniature thrusters Developed in Yamamoto
research group in Kyushu University (depicted in Figure 1)
as the sample engine and adjusted the simulation to it.
The antenna depicted in the Figure is a star shaped antenna,
since the shape of the antenna does not affect the neutral
density distribution, we picked an L shaped antenna forour simulations with a bottom gas inlet located right
below the L shape of the antenna in the shape of a 2 mm
square with a depth of 10 mm. The simulation properties
are indicated in table 1. As it can be seen in table 1, inorder to achieve a more accurate and realistic simulation
we have considered a very low super particle number
which will greatly influence the calculation time. To deal
with that we adapted a hybrid OpenMP-MPI
parallelization method for all the simulation cases. The
simulation runs for a real time of 40 ms which is enoughfor the system to reach a steady state in which the flux ofincoming particles form the inlet is similar to the flux ofparticles leaving the engine. This simulation considers
neutral Xenon particles at room temperature (298 K) fed
at the bottom of the inlet at the specified mass flow rate.
The simulation also includes elastic collisions between
particles and atom wall interactions which assumes aFig. 1. An ion thruster developed in Yamamoto lab inKyushu University adopted from “Numerical Analysisof Microwave Discharge Type Compact Ion Engine”
Masters Thesis. Kyushu University Fukuoka, Japan,(2010).
closed boundary except for the end of the engines orificewhere the particles leave the engine [5].
Table 1. Simulation Properties
Grid Size
Time Step
Super Particle Number
Mass Flow rate

0.5×10-3 m
5×10-8 s
0.5×109
0.49×10-1 mg/s2. RESULTS
In order to validate the results of the simulations, we havecalculated the average density of neutrals in thedischarge chamber and compared it with the theoreticaldensity expected using the vacuum conductance theory
as follows [6]. Therefore the calculated average density
for the neutral particles in our engine is 5.32×1020 m-3.
The average particle density of the simulation was within
the acceptable range of the theoretical value andtherefore the validity of the simulation is confirmed.
Furthermore the velocity distribution of particles within
the engine should be validated and it should follow a
maxwellian distribution with the most probable velocity
corresponding to [7]:𝐶"# = %2𝑅" 𝑇 = 194.25 𝑚/𝑠- 65 -Figures 2 and 3 indicate the velocity
distribution of particles within the engine
where the most probable velocity isvalidated. Figure 2 indicates the velocity
distribution for select random particles
within the engine. It is obvious that thesimulation
followsamaxwellian
distribution with the peak nearing the 200m/s which is close to the theoretical value.
In order to further evaluate the accuracy ofthis velocity distribution we separated theparticles in figure 2 into their respective
positions within the engine and as it isshown in figure 3, each section of the engine
Fig. 2. Velocity distribution of select particles throughout the engine
still follows the maxwellian distribution
with the most probable velocity peaking
near 200 m/s. The density distribution
results are shown in Figures 4 and 5. Figure
4 shows the vertical cross section of theengine drawn from the middle of the engines
X axis and Figure 5 indicates the horizontal
cross section at the antenna’s top L shape.
As mentioned earlier, the desired density
around the antenna relies on the antenna
shape. For an L shaped antenna we would
hope for a higher density of neutrals aroundthe L part of the antenna while for othersymmetric antenna shapes such as star
shape or disk shape, we prefer a more
Fig. 3. Velocity distribution of select particles throughout the engine
uniform distribution. Figure 5 indicates thewith respect to position
density distribution for the inlet located at
the bottom of the antenna. It can be seenthat there is a higher concentration of neutrals around the4. REFERENCES
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- 66 -Fig. 4. Velocity distribution of select particles throughout the engine

Fig. 5. Horizental cross section density distribution of neutrals for the bottom inlet- 67 -