No abstract

In order to establish a completely electrodeless electric thruster, we have been studying the proposed electromagnetic acceleration methods, and estimating plasma performance using various diagnostics. Plasma thrust is the most important feature of the thruster; therefore estimation of the plasma thrust is necessary. In this study, we have developed a pendulum-target-type plasma thrust stand. Our experiment uses a Large Mirror Device and a high-power radiofrequency source (7 MHz, ∼5 kW) to produce high-density helicon plasma. The thruster uses both permanent magnets and electromagnets for generating magnetic field with a large radial component to increase electromagnetic acceleration by the proposed method of including an azimuthal current. In this paper, details of the developed thrust stand and experimental results for thrust, thrust efficiency and specific impulse are presented.

In our proposed method of the completely electrodeless electric propulsion system, a high-density (∼ 10 13 cm −3) helicon plasma is accelerated by the Lorentz force, i.e., the product of the azimuthal current j θ and the radial component of magnetic field B r. In order to promote the plasma acceleration scheme, we used permanent magnets (PMs) designed to increase B r in comparison to the present electromagnets (EMs). As an initial try of the plasma acceleration by our system, electron density n e and ion velocity v i of generated plasma using PMs' magnetic field were measured, and we have obtained the maximum value of n e = 2.5 × 10 12 cm −3 and v i = 2.2 km/s. In addition, we have also introduced a combined, flexible operation of using PMs and EMs leading to better plasma performance.

Electric propulsion is an established high-efficiency method in deep space explorers. However, most of the applied methods feature electrodes in direct contact with the plasma, thus its lifetime is limited by the electrodes' erosion. We developed electrodeless electric propulsion systems in order to overcome this problem, and performed optical measurements to estimate the high-density helicon plasma performance of the systems. The electron and neutral particle density profiles were measured by a high-speed camera, and the velocity of the singly-charged Ar ions was determined by a high-resolution monochromator. Additionally, a preliminary experiment of a spectroscopic method using an intensity ratio based on a collisional radiative model with a CCD monochromator was performed. The plasma parameters were in good agreement with the results obtained by an electrostatic probe, and the non-invasive optical measurements presented here can constitute an effective tool for evaluating an electric propulsion system.

To investigate characteristics of helicon plasma with a very small diameter, we have developed the Small Helicon Device (SHD) and measured the electron density under various conditions over a wide range of radio frequencies. Using a tube with inner diameter of 20 mm, an electron density n e of ~ 10 13 cm-3 near the antenna region was obtained with an excitation frequency f of 7 MHz, rf power P rf of ~ 1000 W, and axial magnetic field in the magnet coil region B of 560 G. In the case of an inner diameter of 5 (10) mm under conditions of f = 12 (7) MHz, P rf ~ 1000 W, and B = 280 G, n e of ~ 10 11 cm-3 was succesfully achieved even away from the antenna region.