Development Statue of Atomic Oxygen Simulator for Air Breathing Ion Engine

Transcription

1 Development Statue of Atomic Oxygen Simulator for Air Breathing Ion Engine IEPC Presented at the 32nd International Electric Propulsion Conference, Wiesbaden Germany Yasuyoshi Hisamoto 1 Graduate University for Advanced Studies, Sagamihara, Kanagawa, , Japan Kazutaka Nishiyama 2 Japan Aerospace Exploration Agency / Institute of Space and Astronautical Science, Sagamihara, Kanagawa , Japan and Hitoshi Kuninaka 3 Japan Aerospace Exploration Agency / Institute of Space and Astronautical Science, Sagamihara, Kanagawa , Japan Abstract: A completely new concept, the Air Breathing Ion Engine (ABIE), has been proposed for spacecraft drag compensation at a super low earth orbit. In order to experiment on ground test, it is necessary to simulate the atmospheric environment of there. In this study, we are simulating the environment using surface neutralization. As the atomic oxygen source, a 6 cm ion source using Electron Cyclotron Resonance (ECR) discharge was operated in changing condition inside a vacuum chamber. The atomic oxygen flux was measured by the mass change using polyimide covering QCM. We achieved a flux density of 1.5 x cm -2 sec -1, which corresponded to the orbital conditions at an altitude of 220 km for the initial stage of the experiment of the ABIE. On the other hand, it was so low utilization efficiency that slow neutral particles accounted for a substantial fraction of the beam. ABIE = air breathing ion engine ECR = electron cyclotron resonance AO = atomic oxygen RPA = retarding potential analyzer QCM = quartz crystal microbalance m = mass change E = reaction efficiency = specific gravity B = effective exposure area of QCM t = time F = fluence density f = flux density Nomenclature 1 Graduate student, Department of Space and Astronautical Science, hisamoto@ep.isas.jaxa.jp. 2 Associate Professor, Space Transportation Division, nishiyama@ep.isas.jaxa.jp. 3 Professor, Space Transportation Division, kuninaka@.isas.jaxa.jp. 1

2 I. Introduction HERE is an increasing need for super low Earth orbiting satellites. These satellites enable progress of aeronomy, T accurate gravity/magnetic field mapping, and high-resolution earth surveillance. They orbit the earth at an altitude of lower than 250 km for few years, where the effect of atmospheric drag cannot be discounted. Atmospheric drag slows down the satellite, thus the satellites require some kind of propulsions and propellants. The satellite lifetime is few weeks by using chemical propulsion systems, while it is few years by using electric propulsion systems. Generally, the longer the mission time is, the heavier the propellant mass is proportionally. The Air Breathing Ion Engine 1-4 (ABIE) is a new type of electric propulsion system, which can be used to compensate the aerodynamic drag of a satellite orbiting at a super low earth orbit. The ABIE is composed of an air intake, a discharge chamber, grids, and a neutralizer. In the ABIE propulsion system, the low density atmosphere surrounding the satellite is taken in and used as the propellant for the Electron Cyclotron Resonance (ECR) ion engines to reduce the required propellant mass. Therefore ABIE is a promising propulsion system for aerodynamic drag free missions longer than two years. In electric propulsion, the pressure of a discharge camber is lower than the propellant tank pressure, and the propellant flows from the tank to the discharge chamber. In the ABIE propulsion system, the static pressure of the atmosphere which represents the tank pressure is lower than the discharge chamber pressure. One of the most important studies about ABIE is the design of air intake, to carry the incoming flow into the discharge chamber. Thus, its design is a major challenge to realize the ABIE concept. In the case of ground test, it is necessary to simulate the relation between the ABIE and the rarefied atmosphere on such a super low earth orbit. The atmosphere at the super low earth orbit is different from that at the sea level. The major components of the atmosphere at the super low earth orbit are atomic oxygen, molecular nitrogen, and molecular oxygen. Hence, the environment surrounding ABIE is subjected to hypothermal atomic oxygen flux with energies equivalent to relative velocities of about 8 km/s. It is equal to 5 electron volts (ev) for atomic oxygen. What we need to simulate is how to duplicate the atomic oxygen flux in parallel with that energy. In order to study Thermal Protection System (TPS) or space degradation material, atomic oxygen generators are inevitable equipment. There are an arc heated wind tunnel and a laser-detonation atomic oxygen source. In the former case, it is such a high background pressure that it makes no sense to demonstrate the ABIE. In the latter case, the experimental study on the ABIE has been reported by Tagawa. 5 As the result, they succeeded in demonstration of the concept at quasi-steady. Ion beam of 16 ma was extracted from the grid which was applied acceleration voltage of 200 V. However, a steady operation generator is required to test dielectric withstanding voltage of the air intake under the constant pressure flowing through the discharge chamber. Air Inflow Air Intake Reflector or Diffuser 0.04eV (Diffusive) +4eV (Mirror) Orbital Velocity 8km/s Satellite Core =5ev@Atomic Oxygen Large ECR Ion Engine Microwave Antenna & Grid Support Permanent Magnets Grids Plasma Exhaust Velocity Air leakage Collimator (Infinite Conductance to Inflow) Thermalized gas in the discharge chamber is hard to leak to upstream side through the collimator Figure 1. Schematic drawings of Air Breathing Ion Engine without the neutralizer. 2

3 II. Experimental Apparatus For the steady operation, how to reflect low energy ions from the metal surface has the advantage in the deceleration and the neutralization. 6,7 As a consequence, high flux of neutral beam is able to be achieved. The characteristics of ion reflection have been studied in Princeton University 8. A. Plasma Source A 6 cm plasma source using ECR discharge was used as the AO simulator in this experiment. We utilized the ECR plasma source, because the plasma potential of an AO ion is the same order of magnitude as the energy of motion of orbital AO. Figure 2 shows brock diagram of experimental setup. This has a discharge chamber of cylindrical shape with 64 mm. The discharge chamber of an plasma source was made out of aluminum for the chamber wall, soft iron for the yoke. Samarium-cobalt magnets were set up annular shape inside the discharge chamber. Microwave of 4.25 GHz in the range from 20 to 70 W is transmitted by a monopole antenna which is surrounded by the magnets. The mass flow rate of 0 ~ 3 sccm is the range of a mass flow controller. O2 gas Mass flow controller Sub chamber Oscillator Stub tuner Microwave amplifier Plasma source Figure 2. Brock diagram of the experimental setup. The pressure of vacuum chamber is 3 x 10-3 Pa at 0 sccm, and 5 x 10-2 Pa at 3sccm. B. Surface Neutralization The grid was made out of aluminum. It is not impressed accelerated voltage, so that the grid has a functional role to increase the pressure of discharge chamber. The thickness of the grid is 10 mm. In response to the result of past study 9, there was the characteristic radial distribution of the generated atomic oxygen due to the magnetic field. We considered that ions were bounced at the ring cusp of the outer magnet. It became clear that any other area did not contribute to the generation of atomic oxygen beam. A modified grid has no hole around the center area. Figure 3 shows the surface neutralization mechanism in this study. We chose to reflect the atomic oxygen ion at the surface of same magnets creating the electromagnetic coupling region in discharge chamber. Some atomic oxygen ions with thermal velocity collided spontaneously with the magnets and were immediately neutralized at the surface. At that time, those atomic oxygen ions were converted into neutrals losing approximately half of their incoming energy. It was reported that the energy of ions decrease about 0.3 to 0.5 times as much as a space potential by reflection from metallic surface. What was need was a method to produce atomic oxygen ions of ~10 ev. Figure 4 shows the picture of the atomic oxygen simulator. 3

4 Yoke Surface neutralization Sidewall Oxygen gas Magnet Microwave Plasma Neutral beam Antenna Magnetic line Grid Figure 3. Mechanism of surface neutralization. Figure 4. Picture of AO simulator III. Results and Discussion The beam from the simulator is chiefly composed of AO, AO ions, and molecular oxygen. We acquired the performance of the AO simulator changing the experimental condition. A. Ion Flux and Ion Energy Measurements The ion current, measured in the downstream of the simulator, indicates the leakage of AO ions from the grid. We used Retarding Potential Analyzer (RPA) to determine the ion flux and ion energy. To decrease the influence of oxidation, this corrector was coated by gold. The RPA was set up at a distance of 5 cm from the edge of the grid. The dimensions of the RPA are shown in table 1. Table 1. Dimension of the RPA. Outer diameter 70mm Opening area 1.33cm 2 Grid material SUS304, 16mesh/inch Grid gap 5.5mm Corrector material Copper plate coated by gold An experimental result of ion flux is shown Fig. 5. This ion flux was calculated on the assumption that ions were singly ionized ions. Ion flux were less than 3.5 x cm -2 sec -1. It were almost constant without relying on microwave power. Figure 6 shows ion energy distributions dependence on microwave power at 1 sccm. In Fig.6 it can be seen that ion energy distributions have had an energy peak near 5 ev. 4

5 3.5E E+12 Ion flux, cm -2 sec E E E E E sccm 1.0 sccm 0.5 sccm 0.0E Microwave Power, W Figure 5. Ion flux dependence on microwave power. Normalized ion energy distribution W 35.8W 25.5W 23.5W 20.7W 16.5W Ion enrgy, ev Figure 6. Ion energy distributions dependence on microwave power at 1 sccm. B. AO Flux Measurement The atomic oxygen flux was measured by the polyimide erosion of polyimide covering QCM 10). For the atomic oxygen with 5 ev, 3 x cm -3 of polyimide is eroded per incoming the atomic oxygen. The density of polyimide is 1.42 g/cm 3. From the eroded mass, we determined atomic oxygen fluence, using Eq. (1). In response to the results of past study, 9 plasma potential was constant at 13 to 18 V without relying on the input power. Reflecting particles has 0.3 ~0.5 as much energy as incoming particles by taking surface neutralization mechanism into consideration. As a result, we considered that the generated atomic oxygen had the energy of 5 to 9 ev. F m E p p B (1) Given the time and area exposed, the atomic oxygen flux was obtained from the fluence. 5

6 f F t (2) To eliminate the effect of a contamination from the atomic oxygen source and temperature dependence, Au QCM which was not covered with polyimide was set up at same position. It took a small amount of time to change frequency. The changing rate was about 1 Hz/sec, so that the measuring time was 5 minutes. 1.6E E E+15 AO flux, cm -2 sec -1 1E+15 8E+14 6E+14 4E+14 2E sccm 2.0sccm 2.5sccm 3.0sccm Microwave Power, W Figure 7. AO flux dependence on microwave power. Figure 7 shows the experimental result which the flux density of atomic oxygen dependence on microwave power and mass flow rate. The flux density increased with an increase of input microwave power. On the other hand, mass flow rate gave little effect on the increase of the flux density. The flux density of 1.5 x cm -2 sec -1 was achieved at the condition of 3.0 sccm and 61 W. This corresponds to the orbital conditions at an altitude of 220 km for the initial stage of the experiment of ABIE. This flux density is translated to 0.2 ma/cm 2 in electric current as singly ionized ions. The effective area, where the atomic oxygen generated, is 15 cm 2 in this simulator. Consequently, total flux reached more than 3 ma. This is the same order of magnitude as the ion beam current of a miniature ion thruster 1 for small spacecraft. Thus, the ABIE of 1 W class is able to operate in this scale. IV. Conclusion The composition of the beam is summarized in table 2. In this study, we performed the atomic oxygen source using surface neutralization method in order to simulate the AO environment of super-low earth orbit. As a result, it was shown that the atomic oxygen close to the orbital velocity was obtained without positively acceleration. However, AO energy distribution is not measured directly. This is a future work. Additionally, the flux density related principally to the input power. It indicates that we can control the atomic oxygen flux just by input power. The achieved flux was adequately available as the simulator for the ABIE ground test. We used the new grid which was modified along the distribution of the generated atomic oxygen. The highaspect ratio grid acted for preventing leakage of un-ionized gas. On the other hand, it is a problem that the utilization efficiency of oxygen gas is low. In order to decrease the effect on the ABIE, we need to filter them, which is future works. Table 2. The composition of the beam. Energy Flux, cm -2 sec -1 AO Future work ~ AO ion ~ 5 ev ~ Molecular oxygen (Thermal velocity) ~ 2 x 10 sccm 6

7 References 1 Nishiyama, K., Air Breathing Ion Engine Concept, Proceedings of 54th International Astronautical Congress, IAC-03-S4-02, Nishiyama, K., AIR BREATHING ION ENGINE, Proceedings of International Symposium on Space Technology and Science, ISTS 2004-o-3-05v, Nishiyama, K., Toyoda, Y., Hosoda, S., and Kuninaka, H., Research Plan of Air Breathing Ion Engine, Proceedings of Space Transportation symposium, STEP , Fujita, K., Air Intake Performance of Air Breathing Ion Engines, Journal of the Japanese Society for Aeronautical and Space Science, Vol. 52, No. 610, 2004, pp (japanese). 5 Tagawa, M., Nishiyama, K., Yokota, K., Yoshizawa, Y., Yamamoto, D., and Kuninaka, H., An Experimental Study on Air Breathing Ion Engine Concept Using Laser-Detonation Atomic Oxygen Beam Source as an LEO Space Environment Simulator, 2009-b-04, K.W.Hohman, L.B.Olson, and T,R.Brogan, Development of Atomic Oxygen Source for Space Simulation Applications, Proceedings of 46th AIAA Aerospace Science Meeting and Exhibit, AIAA , D.H.Lee, J.W.Bae, S.D.Park, and G.Y.Yeom, Development of a low angle forward reflected neutral oxygen beam for materials processing, Journal of Thin Solid Films, , (2001), pp J.W. Cuthberson, W.D.anger, R.W.Motley, Reflection of low energy plasma ions from metal surfaces, Journal of Nuclear Materials, , pp , Hisamoto, Y., Nishiyama, K., and Koizumi, H., An Upper Atmosphere Simulator Using ECR Discharge for Air Breathing Ion Engine, Proceedings of 61th International Astronautical Congress 2010, IAC-10.C4.4.14, J.J.Osborne, I.L.Harris, G.T.Roberts, and A.R.Chambers, Satellite and rocket-borne atomic oxygen sensor techniques, Journal of Review of Science Instruments, Vol. 72, No. 11, 2001, pp