船木 一幸

A Magnetic Sail is a deep space propulsion system which captures the momentum of the solar wind by a large artificial magnetic field produced around a spacecraft. To verify the momentum transfer process from the solar wind to the spacecraft, we simulated the interaction between the solar wind and the artificial magnetic field of the Magnetic Sail using the magnetohydrodynamic model. The result showed the same plasma flow and magnetic field structure as those of the Earth. The change of the solar wind momentum results in a pressure distribution along the magnetopause, which is the boundary between the solar wind plasma and the magnetosphere. The pressure on the magnetopause is then transferred to the spacecraft through the Lorentz force between the induced current along the magnetopause and the current along the coil of the spacecraft. The simulation successfully demonstrated that the change of the momentum of the solar wind is transferred to the spacecraft via the Lorentz force. The drag coefficient (thrust coefficient) of the Magnetic Sail was estimated to be 5.0, and it became clear that the Magnetic Sail has weathercock stability. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

If a dense plasma were exhausted near the center of a magneetic sail, the magnetic field could be expanded far away from the spacecraft, thus the energy of the solar wind can be captured by this huge magnetic field in spite of very low-density solar wind. Then the magnetic sail can propel a spacecraft by the solar wind in the inerplanetary space. Such a magnetoplasma sail was analytically studied, and large thrust to power ratio as much as 250mN/kW was explained. When applied to short-term deep space missions, the magnetoplasma sail has great advantage against other electric propulsion systems because of its ability to achieve larger thrust to power ratio. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

An ion beam optics for a 10-cm-diam 400-W-class microwave discharge ion thruster was fabricated and its applicability to a long-term space mission was demonstrated. The optics consists of three 1-mm-thick flat carbon-carbon composite panels with approximately 800 holes that were mechanically drilled and positioned with +/-0.02-mm accuracy. When mounted on an aluminum ring, spacing for the three grids was kept at 0.5 mm by three sets of spacers. The thruster produced an ion beam current of 140 mA with a microwave power of 32 W for plasma generation and a total acceleration voltage of 1.8 kV. Although the grid is sputtered by the impingement of slow ions produced in charge-exchange collisions between fast beam ions and neutral atoms leaking from the engine, the grid showed only slight damage even after an 18,000-h endurance test. Also, other qualification tests including a mechanical test under launch conditions as well as a thermal vacuum test simulating the spacecraft thermal environment were successfully completed. Hence, the grid system was qualified for spacecraft propulsion.