The Hayabusa2 spacecraft returned to the Earth in 2020 using an innovative propulsion system called a microwave ion thruster. This thruster uses microwave radiation to ionize atoms and generate thrust, along with a microwave discharge cathode to neutralize the resulting electric charge.

JAXA, the Japanese space agency that launched Hayabusa2, has used this thruster for many missions and is planning to continue using similar thruster designs in the future. However, the current iteration of thruster cannot power the large-payload, long-duration missions that JAXA is planning. To remedy that, Morishita et al. experimentally studied the plasma behavior, both inside and outside of the microwave discharge cathode, to improve its performance.

“In this study, we developed the microwave discharge cathode with an optical window to investigate the internal plasma by optical measurement,” said author Morishita Takato.

Studies previously measured the cathode plasma discharge from the exterior of the microwave cavity. However, attempts at measuring the cathode interior would interfere with the operation and change the results. The researchers developed a noninvasive way to study the interior cavity by using an optical window and employing laser-induced fluorescence spectroscopy.

Using this method, the team mapped the ion velocity and density distributions under multiple conditions of microwave power, gas flow rate, and electron current. They mapped a relationship between the ion velocity and current oscillation.

The authors believe their results will help improve the performance of the microwave discharge cathode, and by extension, the microwave ion thruster in future space missions.

“This is an important step to improve the performance and lifetime of the microwave ion thruster system,” said Morishita.

Source: “Plasma parameters measured inside and outside a microwave-discharge-based plasma cathode using laser-induced fluorescence spectroscopy,” by Takato Morishita, Ryudo Tsukizaki, Kazutaka Nishiyama, and Hitoshi Kuninaka, Journal of Applied Physics (2021). The article can be accessed at

This paper is part of the Physics of Electric Propulsion Collection, learn more here.