DECIGO, a Deci-hertz Interferometer Gravitational Wave Observatory, is a space gravitational wave antenna.The purpose of DECIGO is to observe gravitational waves at the frequency band mainly between 0.1-1.0 Hz, and toopen a novel window of gravitational wave astronomy. DECIGO will consist of three spacecrafts flying in atriangular formation with a side length of 1,000 km. The position of each satellite with respect to its two counterpartshas to be controlled to ensure sufficient accuracy of the scientific measurements. Therefore, the propulsion systemwhich can satisfy stringent requirements for drag-free control is indispensable. To accomplish this control by somepropulsion systems, these thrust must be controlled precisely to counteract non-gravitational forces such as residualaerodynamic drag or solar radiation pressure.DECIGO Pathfinder (DPF) is the precursor mission to DECIGO designed to validate the core technologies. Oneof the enabling technologies in DPF mission is the precise micro-propulsion system necessary to achieve the uniquepropulsion requirements. The objective of this study is to develop a miniature microwave discharge ion thruster forthis micro-propulsion system. In drag-free control, the thrust must be actively-controllable with fast response (＞ 10Hz). It is different from a conventional ion thruster system, where the thrust is constant for a long time. In thisexperiment, the thrust dynamic range was between 7-100 %, and the thrust noise was less than 0.02 μN/Hz~(1/2) in thefrequency range of 0.1-1.0 Hz. In addition, the thrust control with fast response was realized by the feedback controlof the ion beam current.
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