S2-8
Research on Completely Electrodeless Electric Thrusters
using High-Density Helicon Plasmas－the HEAT Project and Related Topics
高密度ヘリコンプラズマを用いた完全無電極電気推進研究－HEATプロジェクトを中心に
Shunjiro Shinohara
篠原俊二郎
Tokyo University of Agriculture and Technology
2-24-16, Naka-cho, Koganei, Tokyo 184-8588, Japan
東京農工大学 〒184-8588 東京都小金井市中町2-24-16
Although high specific impulse can be provided by electric propulsion systems, many of conventional
electric thrusters suffer from a problem of finite lifetime due to the electrode wastage. To solve thisproblem, we have executed the HEAT (Helicon Electrodeless Advanced Thruster) project to develop
completely electrodeless advanced-concept electric thrusters. The entire process, i.e., a high-density (up to1013 cm-3) plasma production by helicon waves and its electromagnetic acceleration by novel proposed
methods, can be achieved without using any eroding electrodes. Here, as a review in our project, someexperimental and theoretical approaches to realize this concept along with related topics are presented.

1. Introduction
Higher specific impulse Isp can be provided byelectric propulsion systems compared to chemical
ones. However, present-time electric plasma
thrusters suffer from a problem of finite lifetime
due to the erosion of various electrodes that are in
direct contact with plasmas. In order to solve thisproblem, several novel methods, e.g., VASIMR
concept [1], are proposed, mostly using helicon
plasma sources [2,3].
Here, helicon plasmas have been proven to bevery effective for use in fundamental research aswell as in a variety of additional applications. Thisis because that the helicon sources can produce
high-density (~ 1013 cm-3) plasmas with a broad
range of external operating parameters.
In this paper, we discuss the applicability of ourhelicon sources with new electrodeless acceleration
schemes as the Helicon Electrodeless Advanced
Thruster (HEAT) project [4,5].
2. High-Density Helicon Plasma
We have examined up to eight high-density
helicon plasma sources with various externalparameters. High-density plasmas were successfully
produced with a diameter ranging from 0.5 [5] to 74
cm [4] as well as with an axial length from 5.5 to486 cm [4], which enabled to predict the excellent
scalings of the plasma production efficiency [4]:total number of produced electron-ion pair divided
by electron density vs. input radio frequency power.
Furthermore, the selective excitation of theazimuthal mode number was also demonstrated[5,6].
This makes these sources promising for use in
the development of the next generationelectrodeless thrusters, as well as for fundamental
research and industrial applications.
3. Electrodeless Acceleration Schemes
We have proposed and examined many
acceleration methods [5] by external means, such as
the Rotating Magnetic Field (RMF), Rotating
Electric Field (REF) and Ponderomotive
Acceleration (PA) with Ion Cyclotron Resonance(ICR) acceleration schemes, along with sole, simple
high-density helicon acceleration. In these concepts,
as shown in Fig. 1, the acceleration electrodes (andproduction antennas) are indirect contact with theplasma, leading to a longer lifetime operation
because of the reduction in the strong plasma–wallinteractions. In these schemes, the axial componentof the magnetic field Bz is necessary.

Fig.1. Conceptual idea of electrodelesshelicon thruster.

In addition, the RMF and REF schemes require
the radial component of the magnetic field Br,
because the Br and the induction of the azimuthal
current density jθ in the two schemes create theaxial thrust Fz by the electromagnetic (EM) force, jθ
×Br.
Here, by the RMF method, the increase in theflow velocity by ~ 15 % was ascertained
experimentally. Although the increase in thisvelocity was also found, further studies arenecessary in the REF method, choosing operationalparameters: Numerical calculations of the REF andPA/ICR methods have been carried out to derive
optimal operational windows.
Simple helicon discharge using a target thruststand [7] showed that in the middle sized helicons,diameters of 5 and 10-15 cm (tapered dischargetube), Isp were 840 and 2,000 s, respectively, for Arplasmas. The ratio of the thrust to rf input power
was 8.5 and 16 mN/kW, respectively, for Ar and Xe
discharges. Here, the maximum thrust was 41 mN
for 3 kW input power for Xe plasmas. Small
diameter of 2 cm case using SHD [8] showed thatthe lighter ions have higher velocity: hydrogen caseshowed the ion velocity was up to ~ 30 km/s (Isp ~3,000 s) by Mach probe measurements.
4. Conclusions
We have reviewed our recent studies that wereperformed within the HEAT project, on
high-density (~ 1013 cm-3), helicon plasma
production and its acceleration schemes.
Here, the availability of the helicon plasmas,
which can accept various external parameters with agood production efficiency, to electrodelss thrusters
was discussed.
Next, the proposed acceleration schemes, such as
the RMF, REF and PA/ICR along with sole, simple
helicons, were discussed from the viewpoint of
their application in the development of advanced
electrodeless plasma thrusters.
To verify and improve these schemes, further
studies are required to extend our understanding on
the related physical phenomena and to specify themost appropriate parameter ranges
Acknowledgments
The author would like to acknowledge the great
contributions made by late Prof. K. Toki and late Dr.
K. P. Shamrai. Members of the Heat project are also
acknowledged.
This work was supported in part by theGrant-in-Aid for Scientific Research (S: 21226019)
from the Japan Society for the Promotion ofScience.

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