KAZUTAKA NISHIYAMA

Space Transportation Symposium FY2018 (January 17-18, 2019. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2018 (January 17-18, 2019. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2018 (January 17-18, 2019. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

© 2018 IOP Publishing Ltd. We report the experimental and simulated azimuthal ion velocities of a gridded ion thruster, which generates a roll torque around the thrust axis. Laser-induced fluorescence spectroscopy was applied to two microwave ion thrusters with opposite magnetic polarities. A comparison of the measured results revealed a net misalignment of the grid optics and showed that the ions are continuously accelerated from inside the discharge chamber towards a direction downstream of the grid optics. To investigate the effect of the electromagnetic field, the authors conducted a two-dimensional particle-in-cell Monte Carlo collision (2D-PIC-MCC) numerical simulation. The numerical simulation agrees with the measurements and reveals that the ions are azimuthally accelerated by a gradient B drift, curvature drift, E × B drift and the Lorentz force. The reproduced roll torque is 3.1 2.3 μNm and arises due to the mechanical tolerance of the grid optics. The roll torque shows good agreement with the result observed in the space operation. Therefore, the roll torque can be predicted by using our experiment and simulation.

© 2018 IOP Publishing Ltd. The microwave power absorption efficiency of the μ10 ECR ion thruster, utilized in the Japanese asteroid explorers Hayabusa and Hayabusa2, is investigated in order to allow performance measurement and provide information for its improvement. A model detailing the local electron behavior in a real ECR plasma discharge, based the magnetic field characteristics, is presented. Three methods to evaluate the microwave power absorption efficiency are proposed: an estimation based on the chamber geometry and magnetic field characteristics, a measuremen based on performance parameters and a measurement performed with Langmuir probes. The equations used for each method are analytically derived. The local electron behavior model is confirmed with a Langmuir probe experiment. Measurement of the microwave power absorption efficiency is performed with the two independent methods proposed. Results from the two experiments show good agreement with each other and with the theory. Finally, a diffusion model explaining the different electron temperature distributions observed in the chamber is proposed. The model and experiments clarify the physics behind previously observed performance variations and give valuable hints for future chamber improvement.

© 2018 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. This paper is a study to reveal the mechanism of the roll torque in the gridded ion thrusters. Through the space operation of ion thrusters, a small torque is observed around the thrust axis. In order to reveal the mechanism of the roll torque, the authors performed the laser-induced fluorescence and 2DPIC simulation. As a result, the torque of the experiment and the simulation showed good agreement. From the results of experiments and simulation, it was found that the cause of roll torque is drift inside the ion source, the misalignment of the grid, and leakage magnetic field in the ion beam.

Copyright © 2018 by the International Astronautical Federation (IAF). All rights reserved. Japan's second asteroid explorer Hayabusa2 was successfully launched on Dec 3, 2014 for sample return from Asteroid 162173 Ryugu by 2020. Four xenon ion thrusters using electron cyclotron resonance discharge propelled the spacecraft for 547 hours during the first year in space. Hayabusa2 completed an Earth gravity assist on Dec 3, 2015 followed by 798 hours and 2,593 hours of ion thruster operations called the first- and second- term transfer delta-V. The spacecraft has completed the third-term transfer delta-v period between January 10, 2018 and June 6, 2018 in which the final 2,475-hour ion thruster operation was executed before the rendezvous with Ryugu. Cumulative operating hours for four ion thrusters are 6,450 h, 11 h, 5,193 h and 6,418 h, respectively. This paper summarizes the 6,515-hour powered flight by the ion engine system producing 1,015 m/s delta-v, thruster performance change trend, roll torques generated during different combinations of ion thrusters operated, and spacecraft surface erosion histories measured by two quartz crystal microbalances located in the vicinity of the thrusters. In parallel to the space flight operation, an engineering model of microwave discharge neutralizer has been under long-duration testing on the ground since 2012 and accumulated 51,000 hours of diode-mode operation as of September 17, 2018.

© 2018 IOP Publishing Ltd. This paper presents the first laboratory-based study to measure the azimuthal velocities of ions in the beam of a gridded ion thruster. Through the operation of gridded ion thrusters in space, it has been confirmed that these thrusters cause an unexpected roll torque about the ion beam axis. To reveal the physical mechanism that produces this torque, laser-induced fluorescence spectroscopy has been applied to a microwave ion thruster that was installed in Japanese asteroid probes. This technique can be used to measure the azimuthal velocity by estimating the Doppler shift of the Xe II 5p 4()6p 5/2 to Xe II 5p 4()6s 3/2 transition at 834.659 nm. The measurement was conducted without a neutralizer cathode to avoid the possibility of the cathode affecting the trajectory of the ion beam. The measured velocity functions are the sum of the spectra of the high velocity beam ions and those of charge exchange ions. By deconvolving these spectra, the azimuthal velocities were successfully measured and were found to range from -700 to 620 m s-1 with an accuracy of 25%. The measured azimuthal velocity profile was accurately reproduced by the simulated velocity profile obtained using a model, which includes the effects of the maximum possible misalignment of the accelerator grid with respect to the screen grid and the Lorentz force produced by the magnetic field leaked from the discharge chamber. A roll torque of 0.5 ±0.1 μN m about the thrust axis was calculated from the velocity profile, which is lower than that reported in flight data, but additional mechanisms are suggested to explain this discrepancy.

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Space Transportation Symposium FY2017 (January 18-19, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan