Thomson-Scattering Diagnostics of Plasmas Produced in Miniature Microwave Discharge Ion Engine

Abstract

T HE adoption of small satellites, with their flexibility, short development time, and low cost, has been a breakthrough in space applications [1,2]. Until recently, however, size restrictions have limited the capacity of the available propulsion systems. Hence, the demand for millinewton-class miniature propulsion systems is expected to grow in the future with miniature microwave discharge ion engines as ideal candidates for use on these satellites [3–5]. Conventional ion engines produce high thrust efficiency (60–70%) with specific impulses above 3000 s [6–8]. A 30 W miniature ion engine has been developed for deorbiting 100-kg-class satellites [5]. To miniaturize the engine while maintaining its superiority in performance, a microwave discharge ion source was used. The results demonstrate good performance of the 16-mm-diam version of the source, including a thrust efficiency of 0.51. This is competitive with the miniature ion engine developed at NASA, for which the thrust efficiency is 0.56 [9], and has been the best performance at its size [9]. The thrust density of our engine was 5:6 N=m, which is several times higher than that of conventional ion engines (1:1–1:6 N=m) [1–3]. This higher thrust density is a significant advantage for small satellites, since it means the volume of the engine can be dramatically reduced. Understanding the reason for this superiority is essential in developing this engine. It has not yet, however, been thoroughly investigated, since it is difficult to measure the plasma properties without disturbing the plasma as a result of intrusive diagnostic methods, such as an electrostatic probe. The aim of this study is to measure plasma properties by laser Thomson scattering (LTS) to understand the physics inside this engine. LTS is nonintrusive, in that no physical object need be placed in the plasma [10–12]. Application of thismethod to the plasma produced in theminiature microwave ion engine faces the following difficulties. First, the ne is estimated to be less than 10 m . This results in a weak Thomsonscattering signal. Second, the effect of stray laser light becomes very strong. To overcome these difficulties, we used the photon-counting methodwith a doublemonochromator. These effortsmade it possible to detect LTS signals.