As one of the electromagnetic plasma acceleration systems, we have proposed a rotating magnetic field (RMF) acceleration scheme to overcome the present problem of direct plasma-electrode interactions, leading to a short lifetime with a poor plasma performance due to contamination. In this scheme, we generate a plasma by a helicon wave excited by a radio frequency (rf) antenna which has no direct-contact with a plasma. Then, the produced plasma is accelerated by the axial Lorentz force f z ¼ j h  B r (j h is an azimuthal current induced by RMF, and B r is an external radial magnetic field). Erosion of electrodes and contamination are not expected in this total system since RMF coils and an rf antenna do not have contact with the plasma directly. Here, we have measured the plasma parameters (electron density n e and axial ion velocity v i) to demonstrate this RMF acceleration scheme by the use of AC currents in two sets of opposing coils to generate a RMF. The maximum increasing rate Dv i /v i was $28% (maximum v i of $3 km/s), while the density increasing rate of Dn e /n e is $ 70% in the case of a RMF current frequency f RMF of 3 MHz, which showed a better plasma performance than that with f RMF ¼ 5 MHz. Moreover, thrust characteristics such as a specific impulse and a thrust efficiency were discussed, although a target plasma was not optimized.

COLLECTIONS This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED INStudy on electromagnetic plasma propulsion using rotating magnetic field acceleration scheme Physics of Plasmas 24, 043505 (2017) We have proposed Rotating Magnetic Field (RMF) acceleration method as one of electrodeless plasma accelerations. In our experimental scheme, plasma generated by an rf (radio frequency) antenna, is accelerated by RMF antennas, which consist of two-pair, opposed, facing coils, and these antennas are outside of a discharge tube. Therefore, there is no wear of electrodes, degrading the propulsion performance. Here, we will introduce our RMF acceleration system developed, including the experimental device, e.g., external antennas, a tapered quartz tube, a vacuum chamber, external magnets, and a pumping system. In addition, we can change RMF operation parameters (RMF applied current I RMF and RMF current phase difference φ, focusing on RMF current frequency f RMF ) by adjusting matching conditions of RMF, and investigate the dependencies on plasma parameters (electron density n e and ion velocity v i ); e.g., higher increases of n e and v i (∼360 % and 55 %, respectively) than previous experimental results were obtained by decreasing f RMF from 5 MHz to 0.7 MHz, whose RMF penetration condition was better according to Milroy's expression. Moreover, time-varying component of RMF has been measured directly to survey the penetration condition experimentally. © 2017 Author (s)

A two-dimensional scanning probe instrument has been developed to survey spatial plasma characteristics in our electrodeless plasma acceleration schemes. In particular, diagnostics of plasma parameters, e.g., plasma density, temperature, velocity, and excited magnetic field, are essential for elucidating physical phenomena since we have been concentrating on next generation plasma propulsion methods, e.g., Rotating Magnetic Field plasma acceleration method, by characterizing the plasma performance. Moreover, in order to estimate the thrust performance in our experimental scheme, we have also mounted a thrust stand, which has a target type, on this movable instrument, and scanned the axial profile of the thrust performance in the presence of the external magnetic field generated by using permanent magnets, so as to investigate the plasma captured in a stand area, considering the divergent field lines in the downstream region of a generation antenna. In this paper, we will introduce the novel measurement instrument and describe how to measure these parameters.

Helicon plasma sources using radio frequency (rf) waves are very useful, as they can produce high-density (∼10 13 cm −3 ) plasma easily with a broad range of external operating parameters. Various kinds of featured helicon sources in a wide range of geometrical scales have been developed and characterized to control plasmas as required. Furthermore, particle production efficiency has demonstrated excellent performance over a very wide range of plasma sizes. Highbeta (∼1) plasmas can also be easily achieved, showing the importance of the neutrals effect. As one of the many applications in areas ranging from fundamental to application fields, a space propulsion system with an advanced concept of an electrodeless condition (no direct contact between the plasma and electrodes/antennas) has been developed, owing to the expectation of a longer life operation. Among many proposals, two trials of electrodeless, additional acceleration methods are introduced, emphasizing the importance of some diagnostics. Here, we will review and describe the present status of our high-density rf plasma production with additional acceleration in an advanced propulsion field.

Time-varying, azimuthal electron current is obtained from measured two-dimensional profiles of excited magnetic fields, using the Rotating Magnetic Field (RMF) plasma acceleration method in an open magnetic field configuration. The RMF is applied orthogonally to cylindrical plasma, leading to the azimuthal current drive via the Hall effect. Here, dc azimuthal current, whose magnitude is equivalent to that of ac azimuthal current with twice the RMF frequency, is verified for the first time. In addition, an expected current reversal is found, with the RMF rotation direction changing by 180°.