Preliminary Reports on Spacecraft Charging Property Measurement of Degraded Space Material

Transcription

1 Preliminary Reports on Spacecraft Charging Property Measurement of Degraded Space Material Mengu Cho (1), Kazuhiro Toyoda (1), Minoru Iwata (1), Hirokazu Masui (1), Teppei Okumura (1), Md. Arifur R. Khan (1) and Noor Danish Ahrar Mundari (1). (1) Kyushu Institute of Technology, Kitakyushu , Japan, , ABSTRACT Thorough investigation of spacecraft charging at an early stage of satellite design is desirable to prevent the fatal anomaly in orbit. Charging properties, such as secondary electron yield, photo-electron emission yield, conductivity of spacecraft surface materials are the important parameters to predict charging in orbit via a computer simulation. The material property database determines the accuracy of the charging simulation. As every material exposed to space environment changes its properties, the properties after environment exposure is as import as the properties of the virgin state. The present paper introduces a project carried out at Kyushu Institute of Technology to study how and why the charging property of surface materials exposed to simulated space environment changes. Facilities to simulate material exposure to atomic oxygen, UV and thermal cycling along with a facility to measure the secondary electron yield, photo-electron yield and conductivity have been built. This paper provides an update of the ongoing project along with introduction of the research facility and preliminary results. 1. Introduction Spacecraft charging is a serious issue for safety of spacecraft operation. Electrostatic charging caused by charging of spacecraft insulator may lead to fatal anomaly of a satellite. Thorough investigation of spacecraft charging at an early stage of satellite design is desirable. Spacecraft charging analysis tool, such as MUSCAT (1), SPIS (2), NASCAP-2K (3) can give the satellite designers insight into the risk of charging in orbit. One of the most important parameters in the charging simulation is charging properties, such as secondary electron yield, photo-electron emission yield, conductivity of spacecraft surface materials. Currently campaigns to measure the charging properties of various materials are underway in several countries to improve the material database of each charging simulation code. One of the critical issues in the material property database is the fact that every material exposed to space changes its properties. Although this fact has been recognized for long time, preparing the material samples for the measurement is not an easy task and there was little effort to measure the charging property of degraded space materials. maximum differential voltage V (V) coverglass bulk conductivity (1/ m) Fig.1: Maximum differential voltage calculated by NASCAP/GEO for various values of coverglass bulk conductivity under the worst GEO plasma condition. (Ref.4) Figure 1 shows an example of how the charging property affects the charging simulation result. In Ref.4 charging of a GEO telecommunication satellite was calculated using NASCAP/GEO. In order to study the dependence of the results on the material properties, we changed the coverglass bulk conductivity as shown in Fig.1 and checked how the maximum difference voltage on satellite surface, which is usually at the outermost coverglass of the solar paddle, changed. The result

2 changed drastically as the bulk conductivity changed by three orders of magnitude from to (1/ m). Therefore, even if the charging simulation predicts very little charging condition at BOL (beginning of life) condition because of high conductivity, serious charging may occur at EOL (end of life) due to increased conductivity due to environmental exposure. Of course, the opposite situation may occur if the conductivity increases after the environmental exposure. Kyushu Institute of Technology launched a project to study how and why the charging property of surface materials exposed to simulated space environment changes. KIT has built facilities to simulate material exposure to atomic oxygen, UV and thermal cycling along with a facility to measure the secondary electron yield and conductivity. In the present paper, we will introduce each facility and preliminary results. Figure 2 Schematic picture of AO facility. 2. Atomic oxygen facility Figure 2 and 3 shows a schematic picture and photograph of AO facility. It is based on so-called PSI method (5). A pulsed CO2 laser, maximum of 5J, is shot toward the nozzle. At the same time, the pulse valve opens introducing oxygen molecule gas into the nozzle. The oxygen molecules are dissociated and accelerated via nozzle expansion in the same principle of pulse laser propulsion. Production of atomic oxygen was confirmed by measuring optical emission spectrum of the gas flow where a strong emission at 777nm was observed. The quadrupole mass spectrum analyzer that are separated by 1.9m from the nozzle detects the incoming AO pulse. From the time difference between the laser shot and the arrival time of oxygen atoms, we can derive the velocity. Figure 4 shows the velocity distribution observed at the mass-spectrometer. The velocity is peaked at 8km/s simulating the AO flow in LEO. To irradiate the sample selectively by the 8km/s oxygen atoms, we will install a chopper system inside the chamber. Currently, we are characterizing the AO fluence from the mass loss of Polyimide placed on top of QCM inside the chamber. Along with the witness sample, we will expose samples to the AO flow and start making the degraded sample. Figure 3 Photograph of AO facility. Figure 4. Velocity distribution of AO pulse 3. UV exposure facility Figure 5 shows a photograph of UV exposure facility.

3 The vacuum chamber is a cylinder of 40cm diameter. It can achieve a background pressure as low as 5x10-4 Pa. The chamber is equipped with a water-cooled Deutrium lamp that can emit UV from 115 to 400nm. The UV intensity distribution at the sample holder is shown in Fig.6. The intensity is given in the equivalent sun intensity. The maximum intensity is 79 times and the average over the irradiation area is 40 times the sun intensity. To avoid contamination on the MgF2 window in front of the lamp, a cryogenic plate surrounds the sample area. Figure 7 shows a photograph of the thermal cycling facility. It is made of two vacuum chambers connected by a gate valve with total length of 1,000mm approximately. A turbo-molecular pump can achieve the background pressure as low as 5x10-5 Pa. Each chamber has an internal diameter of 300mm. In the photograph, the left side is the high temperature chamber. It can heat a sample inside to a temperature as high as 500 o C using IR lamps. The right side is the low temperature chamber. It can cool a sample inside to a temperature as low as -40 o C using a cold plate or -150 o C using a shroud filled by liquid nitrogen. There is a rail mechanism between the two chambers that can move the sample back and forth. In this way, we can give a heat shock to the sample by suddenly changing the ambient temperature. Figure 5 Photograph of UV exposure facility Figure 7: Photograph of thermal cycling capability 4. Secondary electron coefficient measurement Figure 6 Distribution of UV intensity at the sample location Figure 8: Photograph of secondary electron measurement system 4. Thermal cycling facility

4 Figure 9: Schematic picture of sample holder to measure the secondary and back-scatter electrons. = (0V ) + I sample + I stage (0V ) The denominator represents the total incident current. Some of the terms are negative and some are positive. By taking the sum of all the terms, we can deduce the amount of electrons entering into the hole. When we bias the grid to a more negative, typically -50V, than the sample surface potential, only the back-scattered electrons can reach the grid or the collector. The secondary electrons, whose energy is typically of the order of ev, are all reflected back to the sample surface by the grid. The back-scattered electron current yield is given by ( 50V ) = + I sample + I stage ( 50V ) The secondary electron emission yield is given by subtracting the back-scattered electrons from the total emission, as =. Figure 10 shows an example of measurement. We have measured of Ti. Our measurement agrees fairly well with the data taken from Ref.6. The electron beam is stable up to 300eV. We will have to bias the sample to a negative value to measure the secondary electron emission yield at lower energy. Figure 10: Example of secondary electron emission yield measure. Reference values (6) are also shown. Figure 8 shows a photograph of the secondary electron measurement facility. We have converted an Auger electron microscope to the secondary electron measurement device. The microscope already has most of the necessary functions to measure the secondary electrons, leaving only a handful of modification items. Figure 9 shows a schematic picture of sample holder we made to measure the total secondary electron emission yield. This is based on Ref.6. Primary electrons enter from the top to the bottom through holes in the center of the dome-shaped collector and grid. When the grid has the same potential as the sample surface, all the electrons emitted from the sample can reach either the grid or the collector. These electrons are the combination of secondary and back-scattered electrons. We define the total electron emission yield by, 5. Conductivity measurement Figure 11 shows a schematic picture of test set-up used to measure the conductivity. The measurement is based on so-called the charge storage method (7). A dielectric sample sandwiched by two cupper plates is placed inside a vacuum chamber. The top cupper plate has a square opening to expose the dielectric sample to the electron beam coming from the ceiling of the chamber. We irradiate the sample by the energetic electrons to charge the sample surface to a negative potential of the order of kv. Once it is charged, we shut-off the electron beam. Then, the Trek surface potential probe as shown in Fig.11 or the photograph in Fig.12 routinely measures the sample surface. The probe is mounted on a X-Y stage and scans the potential distribution over the sample surface. Because the Trek probe is placed inside the vacuum, it will make an ideal high-impedance voltage probe. To avoid the zero-shift, a reference plate made of

5 a grounded cupper plate is also placed inside the chamber. The charge stored on the sample surface leaks either to the bottom holder via the bulk conductivity or to the top holder via the surface conductivity. Figure 13 shows the decay of surface potential measured at different parts on the sample surface. We measure the potential at every 10mm. We carry out a numerical simulation solving a two-dimensional diffusion equation to find the best combination of the surface and bulk conductivities that match the measurement result most. For the case shown in Fig.13, the surface conductivity is negligible. Therefore, we can deduce the bulk conductivity from a simple exponential law. Figure 13: Example of surface potential decay due to leakage current of the dielectric. Figure 11: Schematic picture of experiment set-up to measure the conductivity Figure 12: Photograph of sample layout inside the chamber 6. Conclusion In order to improve the accuracy of spacecraft charging simulation, a new project has started to measure the charging properties of degraded material has started at KIT. Facilities to simulate material degradation due to atomic oxygen, UV and thermal cycling have been built and some are already operational. We have acquired capabilities to measure the secondary electron emission yield and the conductivities. Currently, we are modifying the Auger electron microscope to add the capability to measure the photoelectron emission yield, which complete the phase of facility construction. Soon, we will start a systematic campaign to measure the charging properties of the degraded material. Our purpose is not only to make an extensive database of material properties but also to understand the mechanism of how and why the charging properties change after the environmental exposure. That kind of basic research will lead to development of new spacecraft material with higher stability in the material properties over the long period of environmental exposure in orbit. Acknowledgement Authors would like to thank past and present KIT students M. Chiga, D. Kumagai, N. Tomozoe, K. Tsujikawa, A. Ueda, D. Irie, T. Kouno, M. Sakamoto, H.

6 Igawa for their help in development of research facilities. Authors would like to thank also K. Nitta of JAXA for her help in obtaining the test samples. Reference 1. Muranaka, T., Hosoda, S., Kim, J., Hatta, S., Ikeda, K., Hamanaga, T., Cho, M., Usui, H., Ueda, O. H., Koga, K., Goka, T.,: Development of Multi-Utility Spacecraft Charging Analysis Tool (MUSCAT), IEEE Transaction on Plasma Science, Vol.36, No.5, October, , Roussel, J-F., Rogier, F., Dufour, G., Mateo-Velez, J-C., Forest, J., Hilgers, A., Rodgers, D., Girard, L., Payan, D.,: SPIS Open-Source Code: Methods, Capabilities, Achievements, and Prospects, IEEE Transaction on Plasma Science, Vol.36, No.5, October, , M. J. Mandell, V.A. Davis, D. L. Cooke and A.T. Wheelock, "NASCAP-2k Spacecraft Charging Code Overview", 9th Spacecraft Charging Technology Conference, Tsukuba, Japan, April, M. Cho, S. Kawakita, M. Nakamura, M. Takahashi, T. Sato, Y. Nozaki, "Number of arcs estimated on solar array of a geostationary satellite", Journal of Spacecraft and Rockets, vol.42 no.4, pp , G. E. Caledonia, B. D., Green, H. R, Krech, "A high flux source of energetic oxygen atoms for material degradation studies", AIAA journal, Vol 25, NO.1, Jan C.D. Thomson, V. Zavyalov, J.R. Dennison, "Instrumentation for Studies of Electron Emission and Charging From Insulators", 8th Spacecraft Charging Technology Conference, Huntsville, USA, J.R. Dennison, P. Swaminathan, R. Jost, J. Brunson, N. W. Green, A. R. Frederickson, "Proposed Modifications to Engineering Design Guidelines Related to Resistivity Measurements and Spacecraft Charging", 9th Spacecraft Charging Technology Conference, Tsukuba, Japan, 2005