Performance improvement and plasma diagnostics of microwave ion thruster
Microwave discharge ion thrusters have been developed by our laboratory, and the 10-cm class thruster is mounted on Japanese asteroid explorer “Hayabusa” and “Hayabusa2”. Usually, the plasma is generated by the thermionic cathode, which is the life-limiter due to spattering. On the other hand, the microwave thruster utilizes the resonance of the microwave and electrons, which contributes to the long lifetime and simple system. For future Japanese space missions, such as the Phaethon flyby mission DESTINY+ and the Solar powered sail mission OKEANOS, we are improving the performance and investigating its related plasma physics by probing, non-intrusive measurement of laser spectroscopy, and numerical simulation.
Development of the Isp 7000 sec class microwave ion thruster
In order to respond to long-distance and long-term exploration missions such as Jupiter’s exploration, we develop higher specific impulse microwave ion thruster based on the flight model of “Hayabusa” and “Hayabusa 2” spacecraft. It is possible to reduce the amount of fuel to be loaded by increasing the specific impulse. Since the specific impulse of the ion thruster increases by increasing the voltage applied to the grid, a voltage of 7.5 kV which is five times higher than the flight model is applied. To realize higher TRL, improvement of a gas isolator and of the grid design at 7.5 kV are investigated. We research the above topics experimentally and numerically.
Performance improvement of microwave cathode
The microwave discharge cathode has been developed as a neutralizer of the microwave ion thrusters of Japanese asteroid explorer “Hayabusa” and “Hayabusa2”. The ion thruster consists of the ion source and cathode. The ion source generates thrust by accelerating ions whereas the microwave discharge cathode neutralizes this ion beam to prevent the spacecraft from being negatively charged. Because the microwave discharge cathode generates plasma without electrodes, it has tens of thousands hour lifetime and sub-10 W power consumption. We have achieved over 50,000 hours in ground test and 15,000 hours in space. The microwave cathode is still under developing for future space mission in terms of lifetime at higher electron currents.
Development and performance improvement of MPD thrusters
MPD thrusters are 100 kW-1 MW class electrical thrusters. For future space missions, such as interplanetary cargo ships, we develop and improve the thrusters based on following three points;
1. Quasi-steady state
In place of the general power supply (PFN), we developed a power supply suitable for the operation of an MPD thruster. It became possible to acquire the performance with sufficient reliability even in short pulse operation.
2. Applied magnetic field
Magnetic field is applied to realize efficient performance at lower power range.
3. Operation with various propellants
Not only argon gas but also xenon which has been proven in an ion thruster or hall thruster and hydrogen which is expected to have higher performance are tested.
Development of long lifetime electrothermal PPT for micro-satellite mission
A pulsed plasma thruster (PPT) is a type of electric thrusters and produces thrusts by exhausting sublimated gases from polytetrafluoroethylene (PTFE; Teflon®) as a solid and non-toxic propellant. Because of its simple structure and safe propellant, this thruster is expected to be installed in small satellites of 50 kg class. Our laboratory studies an electrothermal PPT, which accelerates aerodynamically. This type can generate larger thrusts than an electromagnetic PPT, which accelerates by Lorentz force. A problem of electrothermal PPTs is degradation of its thrust with continuous operation because the wall surface of the cavity made by PTFE is used as propellant. To solve the problem, we have proposed and developed a “Electrothermal PPT with propellant feeding system” which supplies additional PTFE to a ceramic cavity. Maintaining the volume of the cavity by supplying PTFE, we aim to maintain performance, permanent operation, and contribution to future small satellite missions.
Hybrid Electro-Chemical Thruster
Our laboratory is developing the world first hybrid electro-chemical thruster (HECT). In a HECT, both electric and chemical energy are deposited into the propellant, the former coming from combustion, the latter from the solar panels. Based on our thermodynamic performance model, such thruster would offer unrivalled performance for thrust-Isp combination, outclassing conventional rocket engines and electrothermal thrusters in the range of 450-800s Isp, thanks to the combined utilization of electric and chemical energy. Possible HECT configurations are mainly differentiated, in the actual development, by the choice of the electric power deposition method (resistive heating, DC-arc heating, MHD heating, etc.). Our performance and feasibility studies led us to seek the development of a resistojet-based HECT, in which a 3D printed high performance resistor is utilized. Heat transfer plays a major role in the HECT performance, with regenerative cooling and film cooling having different effects compared to the case of regular combustion engines. Our CFD investigation seeks to define how to minimize the heat loss to the walls, which in the HECT reduces the Isp even if regenerative cooling is utilized. Preparation of the first experiment is at an advanced stage and a proof-of-concept is expected to be performed in the coming months.
Investigation of space radiation environment realizing human Mars exploration
Beyond International Space Station and manned Moon exploration, many countries plan to explore and act on the Martian surface in the future. However, human crews will get harsh radiation exposure due to space radiation and have difficulty living on the Martian surface. To find safe places where human crews can live and explore for a long-term, we investigate Martian space radiation environment using numerical simulation and laboratory scale experiments. In the numerical simulation, we calculate numerous space radiation using massive parallelism of JAXA supercomputer system. In laboratory scale experiments, we construct small Martian space radiation environment using microwave ion thruster µ1. Results from these simulation and experiments clarified that Martian local residual magnetic field can change trajectories of space radiation and decrease radiation exposure of human crews significantly.