26aD16P
ヘリコンプラズマにおける中性粒子挙動
Neutral particle behavior in helicon plasma桑原大介1), 篠原俊二郎1), 島田智広1), 谷田佑莉子1), 宮澤順一2)
D. Kuwahara1), S. Shinohara1), T. Shimada1), Y. Tanida1), and J. Miyazawa2)
1) 農工大工, 2) 核融合研
1) TUAT, 2) NIFS
Most practical electric thrusters have a lifetime
problem coming from a damage of electrodes
directly contacting with a plasma. A helicon plasma
thruster has been proposed as a long lifetime
thruster, because it works without any contact
electrodes. Therefore, it has a potential to become amain thruster for a deep space explorer, which
requires high-thrust and high-specific impulse.
Authors have been promoting the Helicon
Electrodeless Advanced Thruster (HEAT) project[1] that uses the helicon plasma for generating asource dense plasma, and electromagnetic forces foraccelerating the source plasma to gain the enoughthrust and specific impulse.
In thruster studies, a neutral particle, e.g., H2,
Ar, and Xe works as a fuel gas. In most cases, thesegases are supplied to a discharge tube by the use ofa simple nozzle, and a supplying pressure is low.
Therefore, the neutral particle fills a discharge tube
homogenous. However, there are two problems inthis configuration. First, there is a limitation of anelectron density increase, due to a neutral particle
depression in the center axis of the high-density
helicon plasma [2]. This limitation reduces thethrust performance directly. Second, thehigh-density plasma causes an erosion of discharge
tube wall. For the future MW class thruster, such asa main thruster of a human mars explorer, thisproblem becomes serious because the particle andheat fluxes of the plasma will increase drastically.
Here, in a field of magnetically confined fusion
plasma science, there is an advanced neutral particle
fueling method named a supersonic gas puffing(SSGP) [3]. This method uses a concentrated sharpgas beam by injecting a high pressure gas via theLaval nozzle, in order to achieve a deep penetration
length in the fusion plasma compared to theconventional gas puffing.
To solve above-mentioned problems, we haveproposed a supersonic gas puffing system for thehigh-density helicon plasma. By the use of theSSGP along the center axis of the helicon plasma,
the depression of neutral particles in the central

region will improve. In addition, a number ofneutral particles near the wall of the discharge tube
will be clearly decreased, causing an electron
density decrease, which suppresses the walldamage.
In this paper, we have carried out preliminary
studies of the helicon plasma generation using
SSGP. First, we have to estimate and compare
behaviors of the neutral particle using the SSGP
with those with the conventional gas puffing. We
have developed a Schultz type ionization gauge
which can measure a pressure even in the highpressure range (~100 Pa). Since the size of thisgauge is quite small, three dimensional distributionof the neutral particle density can be measured byscanning of its position. Figure 2 indicates a nozzle
which is developed in this study. The nozzle isinstalled in a vacuum chamber of Large Mirror
Device, an inner diameter of 445 mm and an axial
length of 1,700 mm. This chamber is evacuated by
two turbomolecular pumps, and the backgroundpressure of the chamber is ~5 × 10-4 Pa. The
injection of a neutral gas is controlled by anelectromagnetic valve, and an opening time is set
around 1 ~ 10 ms.
In the presentation, we will show
characteristics of the neutral particle density
distribution by the use of the SSGP. In addition,
preliminary results of characteristics of the helicon
plasma fueled by the SSGP will be presented.

Figure 1 SSGP Nozzle.
[1] S. Shinohara et al., IEEE Trans. Plasma Sci. 42(2014) 1245.
[2] A. Fruchtman, Plasma Sources Sci. Technol. 17(2008) 024016.
[3] A. Murakami et al., Plasma Fusion Res. 5(2010) S1032.