24E10
Numerical Analysis of Potential Structure around Solar Power Sail IKAROS
in Solar Wind Plasma
太陽風環境におけるソーラー電力セイルIKAROS周辺の電位構造解析	 
Takanobu Muranaka, Iku Shinohara and Ryoji Takaki
村中崇信, 篠原育，高木亮治
Japan Aerospace Exploratin Agency
3-1-1, Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan
宇宙航空研究開発機構 〒252-5210	  相模原市中央区由野台3-1-1
We have numerically studied on the electrostatic potential structure around the solar power sail “IKAROS”
in solar wind plasma. Solar power sail is expected to a candidate of the next-generation spacecraft forinterplanetary flight. The potential structure is of importance for the payload design such as the location ofonboard scientific instruments. A unique electrostatic potential structure is recognized as a result ofinteractions between ambient plasma and a large-scale membrane of the spacecraft. A wake potential in thedownstream region and the reduction of the sheath by photoelectrons in the upstream region of thespacecraft are shown by the numerical simulation.

1. Introduction
Solar sail is one of a next-generation spacecraftthat is regarded as a candidate for interplanetary
flight. Solar sail consists of a large membranethat is typically made of strong metal-coatedpolymer material. The conductive surface of themembrane reflects the solar radiation, which
results in converting the solar radiation pressure
into the spacecraft thrust. If its electric power is
supplied by onboard solar arrays, the spacecraft iscalled as “solar power sail”. Today, JAXA had
launched the IKAROS spacecraft [1], the first
demonstration spacecraft of a solar power sail, inMay 2010 into the Venus transfer orbit. The scaleof the membrane of the IKAROS spacecraft is 14x 14 m 2 to obtain sufficient thrust from the solar
radiation pressure, (e.g. 10 -6 Pa at 1.0 AU).
During interplanetary flight, a solar sail is
exposed to the solar wind plasma andphotoelectrons that are emitted from the sunlit
surface of the spacecraft. The Debye length of thesolar wind plasma is comparable to the scale ofthe solar sail, and the plasma has the flow that issupersonic for ions (mostly for protons) but
subsonic for electrons. Hence, the sail is expected
to have the significant potential structure arounditself, and its quantitative estimation is ofimportance for the payload design such as thelocations of scientific instruments. We performed
a numerical experiment for the interactions
between charged particles and a solar sailassuming the IKAROS spacecraft by using athree-dimensional
electrostatic
full
Particle-In-Cell (PIC) [2] code we had beendeveloped [3]. The analysis was focused on theestimation of the potential structure around thesail including space charge effects of the charged
particles around the sail.
2. Numerical Analysis of Interactions between
Solar Sail and Solar Wind Plasma
We performed the numerical analysis for theinteractions between charged particles and theIKAROS spacecraft at 1.0 AU. In the simulation,
the motion of individual ions and electrons isexplicitly solved, and the electrostatic field iscomputed self-consistently. Table I shows thecomputation parameters and Fig. 1 shows thenumerical domain of the simulation. The IKAROS
spacecraft is assumed an aluminium conductor plate
whose area is of 14x14 m2 and thickness is of 0.5 m.
The thickness of the model is comparable with theDebye length of the photoelectrons but much
Table I. Computation parameters
Plasma density [m-3]Plasma temperature [eV]
Plasma drift velocity [m/s]
Photoelectron (PE) current flux
[mA/m2]PE1 temperature [eV]PE2 temperature [eV]
Flux ratio, PE1:PE2
mass ratio (mi/me)
Numerical domain [grid]
Object size [grid]
Grid size, dx, dy, dz [m]Time width, dt [10-7 s]
wpe*dt

6x106
10470
40
1.5
5.0
9:1
1836
256*128*128
1*28*280.5
1.00.0138

smaller than that of the ambient plasma. The
directions of solar flux and the flow of the solar
wind are normal to the surface of the sail in thissimulation. As simplicity, we neglect ambient
magnetic field. The emitting photoelectrons are
considered to have double Maxwellian distribution
as a better representation of the distribution in
tenuous plasma environment [4,5]Fig.1. Numerical domain of the simulation. The
rectangular object located at the center indicates theconductor sail model.

3. Results
Figure 2 (a), (b) and (c) shows the numericalresults of the spatial distribution of the ambient ions,photoelectrons, and the electric potential at 2500
time steps, respectively. The saturation value of thefloating potential of the sail is determined to be+8.3 V by the computation. A large ion wakestructure is formed in the downstream region of thesail as shown in Fig. 2 (a). A wake potential whose
minimum value is -3.0 V is formed due to the ion
wake shown in Fig. 2 (c). The emitted
photoelectrons from the upstream surface diffuse
around the sail even to the downstream surface
although the negative wake potential obstructs thediffusion shown in Fig. 2 (b). The dense layer of thephotoelectrons whose magnitude is up to 3.8x108
m-3 in 1.5 m thick from the emitting surface isrecognized in the figure. We also recognized
reduction of the electron sheath in upstream regionof the sail due to the photoelectron emission byanother comparative simulation with or without thephotoelectron emission.
Acknowledgments
We are grateful to the member of IKAROS
demonstration team for the useful discussion on thisresearch. The computations for this work were
performed using JAXA Supercomputer System(JSS).
References[1] O. Mori et al.: Proc. 28th International Symp. SpaceTechnol. and Sci., Okinawa, 2011, 2011-o-4-01v.
[2] C. K. Birdsall, and A. B. Langdon: Plasma Physics
via Computer Simulation, (McGraw-Hill, New York,1985).
[3] T. Muranaka, H. O. Ueda, H. Usui, and I. Shinohara:
Trans. JSASS, Space Tech. Japan, 7, ists26 (2009),
Pr_2_19.
[4] T. Nakagawa, T. Ishii, K. Tsuruda, H. Hayakawa,and T. Mukai: Earth, Planets and Space, 52 (2000)
283.
[5] T. Muranaka, S. Hosoda, H. Usui, and I. Shinohara:
Proc. 61st International Astronautical Congress,Prague, 2010, IAC-10-D5.3.6.

Fig.2. Spatial distributions of the charged particles andthe electric potential around the IKAROS spacecraft at2500 time steps. Contours show a) the ambient ions, b)
the photoelectrons and c) electric potential in XY-planeat Z=64 from the top in this figure.