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 to open a novel window of gravitational wave astronomy. DECIGO will consist of three spacecrafts flying in a triangular formation with a side length of 1,000 km. The position of each satellite with respect to its two counterparts has to be controlled to ensure sufficient accuracy of the scientific measurements. Therefore, the propulsion system which can satisfy stringent requirements for drag-free control is indispensable. To accomplish this control by some propulsion systems, these thrust must be controlled precisely to counteract non-gravitational forces such as residual aerodynamic drag or solar radiation pressure. DECIGO Pathfinder (DPF) is the precursor mission to DECIGO designed to validate the core technologies. One of the enabling technologies in DPF mission is the precise micro-propulsion system necessary to achieve the unique propulsion requirements. The objective of this study is to develop a miniature microwave discharge ion thruster for this micro-propulsion system. In drag-free control, the thrust must be actively-controllable with fast response (＞ 10 Hz). It is different from a conventional ion thruster system, where the thrust is constant for a long time. In this experiment, the thrust dynamic range was between 7-100 %, and the thrust noise was less than 0.02 μN/Hz~(1/2) in the frequency range of 0.1-1.0 Hz. In addition, the thrust control with fast response was realized by the feedback control of the ion beam current.
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