Abstract:When the satellite is on orbit, the surrounding plasma environment will interact with the spacecraft surface, accumulate charges on the spacecraft surface and cause surface charging effect, which could lead to electrostatic discharge and affect the running of the spacecraft. SMILE is a satellite operating in a solar synchronous and high inclination large elliptical orbit. The on-orbit motion will encounter ionospheric plasma, magnetospheric plasma and solar wind plasma, pass through the region of the outer radiation belt enriched by high-energy electrons. These environmental factors can cause the surface charging effect on satellite and affect on-orbit security of the satellite and the acquisition of scientific data. Utilizing the software simulation of spacecraft plasma interaction system, the charging effects of SMILE satellite surface in solar wind plasma, magnetic tail plasma and extremely harsh plasma environment have been simulated, and the charging potential distribution on its surface have been obtained. The results show that the surface charging potential varies in different environments, but all comfort with the design requirements. The analysis of surface current shows that the secondary electron emission has great influence on surface charging in various plasma environments. Under sun illumination, photoelectron emission dominates surface charging. By analyzing the charge current on the surface on the eclipse, the calculated results can supply the experimental curve of the secondary electron emission coefficient of indium tin oxide materials. Keywords:surface charging effect/ SMILE/ spacecraft plasma interaction system/ plasma
SMILE卫星的模型如图1所示. 模型包括: 载荷仓, 为边长为1 m的立方体; 推进器, 为位于载荷仓下的梯形台, 垂直高度为1.375 m, 底面边长为1.64 m; 六块太阳电池帆板, 载荷仓两侧各三块, 长为1.1 m, 宽为0.88 m, 每一块帆板分割成12条并行排列、相互间距为1 mm的条形面; 伸杆天线, 一端位于载荷仓-X面的圆柱体, 半径为0.06 m, 长为3 m; 安装在载荷仓与推进器表面的LIA、星敏、测控天线等, 具体如表1所列. 图 1 SMILE卫星模型图. 红绿蓝三个轴分别为x, y, z方向 Figure1. The model of SMILE. The red, green and blue axes are in the x, y, z direction, respectively.
航天器部件
电路节点
表面 材料
电路 设置/Ω
载荷仓(底面)
0
ITO
载荷仓
1—5
ITO
20000
伸杆天线
6
KAPT
20000
推进器
7—10
ITO
20000
太阳电池下表面
11, 12
CFRP
37500
太阳电池上表面
13, 14
ITO
20000
+X面测控天线顶端
15, 17
PCBZ
20000
+X面测控天线底端
16, 18
ITO
20000
星敏
19—21
ITO
20000
推进舱+X面探测器
22
AL
20000
推进舱-X面探测器
23
AL
20000
散热板对内面
24, 26, 28
ITO
20000
散热板对外面
25, 27, 29
PCBZ
20000
-X面测控天线
30, 31
PCBZ
20000
LIA安装面、测量面
32, 34, 36, 38
AL
20000
LIA对外面
33, 37
PCBZ
20000
LIA靠星体面
35, 39
ITO
20000
探测器镜头
40—42
ITO
20000
表1SMILE卫星模型电路节点、表面材料及电路设置 Table1.Design of nodes, surface materials and circuits of SMILE model.
SMILE卫星运行轨道包括太阳同步轨道(sun-synchronous orbit, SSO, 700 km, 98.2°)和高倾角大椭圆轨道(high elliptic orbit, HEO, 5000 km × 19 Re, 98.2°或67°). 其中, SSO可能遭遇的等离子体环境主要是电离层等离子体和极区沉降粒子, 由于SSO轨道不做科学任务观测且表面充电风险较低, 故不进行三维表面充电的仿真分析. HEO轨道可能遭遇的等离子体环境主要包括磁层等离子体和太阳风等离子体, 其中磁层等离子体环境可对SMILE卫星表面材料造成负高电位的充电风险. 根据欧空局的空间环境手册ECSS-E-ST-10-04 C, 对于负电风险分析一般采用GEO极端恶劣等离子体环境进行表面充电评估. 为评估LIA遭遇的正电位风险, 取最恶劣的磁尾瓣等离子体环境进行模拟. 所以共进行磁尾瓣等离子体、太阳风等离子体、GEO极端恶劣等离子体共三种等离子体环境下的模拟, 三种环境的参数如表2所列.
等离子体环境
离子密度
电子密度
离子温度
电子温度
cm–3
cm–3
eV
eV
磁尾瓣
0.1
0.1
540
180
太阳风
8.7
8.7
12
10
GEO极端恶劣
成分1
0.6
0.2
2000
4000
成分2
1.3
1.2
28000
27500
表2等离子体环境参数 Table2.Parameters of various plasma environment.
3.仿真结果阴影区时, 磁尾瓣等离子体、太阳风等离子体、极端恶劣等离子体环境下的航天器平均表面电位及表面电流随时间的变化分别如图2和图3所示. 因为航天器表面进行了等电位处理, 各节点间电位差很小, 所以仅选取一个节点展示表面电位随时间的变化. 同时为了对比阴影区和光照下的充电情况, 选取正对太阳光入射方向的节点4作为代表. 图 2 阴影区节点4的平均表面电位 (a) 磁尾瓣等离子体环境; (b) 太阳风等离子体环境; (c) GEO极端恶劣等离子体环境 Figure2. Average surface potential on node 4 on the eclipse: (a) The magnetic tail lobes plasma; (b) the solar wind plasma; (c) the GEO worst case plasma.
图 3 阴影区节点4的表面电流 (a) 磁尾瓣等离子体环境; (b) 太阳风等离子体环境; (c) GEO极端恶劣等离子体环境 Figure3. Surface current on node 4 on the eclipse; (a) The magnetic tail lobes plasma; (b) the solar wind plasma; (c) the GEO worst case plasma.
由图2至图4可以看出, 在阴影区, 仿真的最终时刻, 磁尾瓣等离子体环境下表面材料电位(由于伸杆天线电位对航天器整体影响较小, 因此不考虑其表面电位, 以下讨论同样如此)约+5.5 V左右; 太阳风等离子体环境下表面材料电位约–25.3 V左右; 极端恶劣等离子体环境下表面材料电位约–8582 V左右. 其中, 只有磁尾瓣等离子体环境下充电电位为正. 仿真的结果中, 磁尾瓣、太阳风和极端恶劣等离子体环境下航天器表面最大电位差分别为0.007, 0.005和1.022 V, 没有发生静电放电的风险. 图 4 光照下节点4的平均表面电位 (a) 磁尾瓣等离子体环境; (a) 太阳风等离子体环境; (c) GEO极端恶劣等离子体环境 Figure4. Average surface potential on node 4 under sun illumination: (a) The magnetic tail lobes plasma; (b) the solar wind plasma; (c) the GEO worst case plasma.