The performance of a conventional ion engine emitting thermoelectrons degrades immediately when placed in the earth atmosphere and exposed to oxygen and humidity. A microwave discharge-type ion engine, however, has no such defect. In fact, although the engine was installed on HAYABUSA and had been stored for two years by the launch date, it functioned fully in space. Recently, we brought the ion engine to the U.S. and demonstrated its operation in public in an attempt to expand its application. We exported the engine in early March without special care in packaging such as environmental protection, seals, etc. After receiving the engine at the end of March in the U.S., we installed it onto the vacuum tank and successfully performed ion-engine acceleration within one day. Thus, the microwave discharge-type ion engine has both robustness and easy operability.
Development and expanded applications of electric rockets
Over the past 20 years, the organization has changed - NASDA and ISAS merged and were reorganized as Japan Aerospace Exploration Agency (JAXA). Meanwhile, the Aerospace Basic Act was enacted and allowed us to implement a wide range of space activities. Taking this opportunity, we plan to develop various electric-rocket technologies and expand their applications, for example, incorporating ion engines into geosynchronous and earth-orbiting satellites inside and outside Japan, not just deep-space missions. To realize this, we need to explore and introduce more innovative approaches and ideas. Some examples are listed below.
The most time-consuming process in ion-engine development is durability qualification. In the past, the durability-demonstration test corresponding to actual operation time was made on the ground. It takes up to two years to conduct a single test. As a new technique, we plan to adopt a life-span qualification that will combine a several 1,000-hour durability test and numerical analysis, rather than simply perform several 10,000-hour durability tests. We are preparing a 3D numerical-analysis tool "JAXA Ion-Engine Development Initiative (JIEDI)" for ion-acceleration grids.
Fig. 1 shows ion trajectories. Ions are accelerated to emit from left to right through screen grid holes, accelerator and decelerator (approx. 2mm in hole diameter and approx. 0.5mm in grid interval). Ions produced in the ion source enter into the screen grid hole (yellow shows ion trajectory). The ions first converge by electrostatic acceleration in the strong electric field between screen and accelerator, then diffuse a little in the decelerating electric field between accelerator and decelerator, and then are ejected. The ions infrequently collide with neutral particles existing in the area to cause elastic scattering (shown in green) or charge exchanges (shown in red). Due to these events, the ions deviate from the original trajectory, or are pulled back and hit the sides of the accelerator/decelerator grid holes, eventually causing spattering loss. The gradation of blue to red on the grid surfaces indicates the attrition rate taking into account the particle attachment. The figures are hole profiles after 20,000 hours. This calculation now becomes possible. If you compare this figure with Fig. 3 in ISAS News No. 230 (May 2000 , Japanese only), you can understand the preciseness and advancement of the current calculation.