Miniature Microwave Discharge Ion Thruster Driven by 1 Watt Microwave Power

Abstract

T HE age of space exploration using self-propelled small spacecraft is approaching. In the past decade, research and development of small spacecraft have advanced extensively throughout the world. Numerous small spacecraft have been successfully launched and operated [1–8]. Small-spacecraft missions are increasingly used with widely varying applications. In recent years, planned small-spacecraft missions are increasingly in need of propulsive capability. Propulsion devices supply the spacecraft with attitude control, station-keeping, and orbit transfer functions. Furthermore, propulsion can support future spacecraft missions such as drag-free control from atmospheric or solar pressure, precise constellation flight for interferometer missions, and deorbiting maneuvers of end-of-life spacecraft into the atmosphere. The arrival of propulsion devices suitable for small spacecraft, namely, micropropulsion, is eagerly anticipated [9]. Ion thrusters [10] are promising propulsion devices intended for use not only with standard-sized spacecraft but also with small spacecraft. Their characteristics of high specific impulse (3000 s), high thrust efficiency (50%), usage of an inert propellant (xenon), and continuously controllable thrustmeet the requirements for smallspacecraft missions. Several studies of miniature ion thrusters have been reported for plasma generators of several different types, namely, direct-current electron discharge [11,12], radio frequency discharge [13], and microwave discharge [14–17]. Among these types, microwave discharge ion thrusters have the following unique characteristics suitable for miniaturization. First, the discharge chamber generating the plasma has a simple structure because of its lack of an electron cathode. In this type, electrons are heated using electron cyclotron resonance (ECR); neither a discharge cathode generating primary electrons nor corresponding power supplies are needed. Second, the thruster performance is degraded only slightly by scaling down. In general, discharge loss is affected by the surfaceto-volume ratio of plasma. In microwave discharge ion thrusters, however, the ECR heating zone is confined to the near-surface magnetic field, meaning that the surface-to-volume ratio is not sensitive to the system’s scale. Using these advantages, Takao et al. [14], Yamamoto et al. [15,17], and Nakayama et al. [16] developed miniature ion thrusters driven by microwave discharges and showed good performance. In spite of these benefits, however,miniature ion thrusters have not been used for small spacecraft yet. An important reason is the severe limitation of electrical power available on small spacecraft. In general, the electrical power generated in small spacecraft is about 1 W=kg. For example, the maximum usable power in a 10–50 kg small spacecraft was estimated at only 10–50 W. The miniature ion thrusters developed to date require a total power of at least 30W.Low power limitations have made it difficult to use those ion thrusters on small spacecraft. On the other hand, mass limitations are not so severe problem as that of the electrical power. Nakayama et al. [16] estimated the total dry weight as approximately 4 kg for their miniature microwave discharge ion thruster. Additionally, this weight can be reduced father by decreasing themicrowave power and discharge loss because the microwave power supply has a large fraction of its total weight attributable to the low energy conversion efficiency. Our objective is a miniature ion thruster with a total power consumption of less than 10 W and a total dry weight of less than 2.0 kg. One such miniature ion thruster can propel a 10–20 kg small spacecraft, although several thrusters could propel larger spacecraft of 20–100 kg. Moreover, devices that produce minute and controllable thrust are useful for spacecraft that require precise control of both attitude and position. The major challenge to achieving this goal is to decrease the total required electrical power while maintaining high thruster performance. That is to say that power for generating plasma should be limited to only 1–2Wand the discharge loss should be suppressed to less than 500 W=A. To date, no ion thruster has been reported to operate at such lowpower applied to the plasma and at such a low discharge loss. As a first step toward this goal, we have developed a miniature ion thruster driven by 1 W of microwave power with a 250 W=A ion production cost and 37% mass utilization efficiency. In this paper, we present the characteristics and performance of aminiaturemicrowave dischargebased ion thruster.