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.
To investigate characteristics and estimate propulsion performance of a high-density radio frequency plasma with a very small diameter, we have developed the Small Helicon Device (SHD) and measured an electron density n e , an ion velocity v i and an emission intensity of Ar II under various conditions such as a discharge diameter (down to 3 mm), a mass flow rate and a gas species. Using quartz discharge tubes with inner diameter (i.d.) of 3, 10, 20 mm with an Ar gas, n e of (2.5-6) × 10 12 (estimated from optical measurement), ∼2.5 ×10 12 and ∼ 2.0 × 10 12 cm −3 , respectively, at −40 mm downstream from an excitation antenna was obtained. Using the 20 mm i.d. tube with a H 2 gas, v i ∼ 40 km/s was achieved in the presence of the magnetic field gradient.
In order to miniaturize a high-density, inductively coupled magnetized plasma or helicon plasma to be applied to, e.g., an industrial application and an electric propulsion field, small helicon device has been developed. The specifications of this device along with the experimental results are described. We have succeeded in generating high-density (~10(19) m(-3)) plasmas using quartz tubes with very small diameters of 10 and 20 mm, with a radio frequency power ~1200 and 700 W, respectively, in the presence of the magnetic field less than 1 kG.
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.
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