24P125-P
Large diameter double-layer plasma source using permanent magnets
and its characteristics
永久磁石利用ダブルレイヤープラズマ源の大口径化とその特性
Tatsuya Suzuki1, Kazunori Takahashi1,2 and Tamiya Fujiwara1,2
鈴木達也1, 高橋和貴1,2，藤原民也1,2
1

Department of Electrical Engineering and Computer Science, Iwate University,
4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
岩手大学工学部 電気電子・情報システム工学科 〒020-8551 岩手県盛岡市上田4-3-5
2Soft-Path Engineering Research Center (SPERC), Faculty of Engineering, Iwate University,
4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
岩手大学ソフトパス工学総合研究センター 〒020-8551 岩手県盛岡市上田4-3-5

A diameter of a source tube of a permanent-magnets-expanded plasma source is enlarged from thepreviously reported 6.6 cm to 13 cm for a purpose of an increace in an electron-pressure-induced
thrust for electric propulsion. A supersonic ion beam of about 13 cm in diameter is detected near thesource exit simultaneously with a spontaneous formation of a potential drop. A plasma density, anelectron temperature, and an rf power efficiency are experimentally investigated at various gas
pressures and rf powers. The results are compared with particle balance and power balanceequations.

1. Introduction
Formation of current-free electric double layers(DLs) and subsequent acceleration of ions inmagnetically expanding radio-frequency (rf)plasmas have been observed in experiments,
theoretical models, and PIC simulations. A newconcept of electric propulsion device utilizing thisphenomenon has been suggested [1]. In this type ofplasmas, the accelerated ion beam is electricallyneutralized by the energetic electrons overcoming
the potential drop [2,3]. More recently, direct
measurements of the thrust from the magnetically
expanding DL plasma or simple rf plasma source
have been demonstrated [4-6].
According to the balance equations between avolume ionization and a surface loss of the charged
particles, and between a electric power and an
energy loss, the thrust resulting from an electronpressure increases with an increase in a source
diameter. The diameter of the previously reported
DL plasma source using the permanent magnets
was 6.6 cm [7]. Here a source diameter is enlarged
up to 13.3 cm and its characteristics are presented.
2. Experimental setup
A schematic diagram of the Large diameterPermanent Magnets expanding Plasma machine ofIwate University (L-PMPI) is shown in Fig. 1(a). A
plasma source, consisting of a 60 cm long and 13.3
cm inner diameter glass tube, is contiguously
connected to a 100 cm long and 76 cm diameterFig.1. (a) Schematic diagram of L-PMPI. (b) Axial
profile of the calculated axial component Bz of themagnetic field strength for the configuration (I)(dashed line) and (II) (solid line).

stainless steel vacuum chamber, which is evacuatedto a base pressure of about 104 Pa. The source isr (cm)4200.5246Normalized IEDF

160020406080

Vc (V)
Fig.2. Contour image of the radial profile of thenormalized IEDFs measured at 3 cm downstream ofthe source exit for the argon gas pressure of 40 mPa
and the rf power of 300 W.

terminated upstream by an axially movableinsulator plate at z = 28 cm, where z = 0 cm isdefined as the source exit. In the present experiment,
two types of magnetic field configurations provided
by the permanent magnets are used as a shown inFig. 1(b); all of the magnets has inward
magnetization in the radial direction. For theconfigurations (I) and (II), the field strength in thesource is about 80 and 140 Gauss, respectively. Anargon gas is introduced from the upstream side of
the source tube through a mass flow controller andthe operating gas pressure in the diffusion chamber
is maintained in the range 25-300 mPa, where anargon gas pressure is measured with an ionization
gauge connected to the side wall of the diffusion
chamber. A double-turn rf loop antenna winding thesource tube at z = 10 cm is powered from a 13.56MHz rf power generator through a matching circuit
and an argon plasma is produced by aninductively-coupled discharge.
An axially and radially movable retarding fieldenergy analyzer (RFEA) facing the source tube or aLangmuir probe (LP) is inserted from thedownstream side of the diffusion chamber. Ion
energy distribution function (IEDF) is measured by
the RFEA using the pulsed probe technique [8], andthe LP is used to estimate the local plasma density
and the electron temperature.
3. Results
Fig. 2 shows the contour image of the radial
profile of the IEDF, which is measured at z = 3 cm
and normalized by the maximum of the IEDFs, forthe argon gas pressure of 40 mPa, the rf power of300 W, and the configuration (I). Around collector
voltage of Vc ~ 30-45 V corresponding to the local
plasma potential p, the thermal ions exist within

the radius of |r| ≤ 6.5 cm. The accelerated group ofions is clearly detected within the radial range sameas the thermal ions around Vc ~ 50-65 V, whichcorresponds to the beam potential beam. The energyof the ion beam could be estimated as beam = e(beam p) ~ 20 eV, where e is the elementary charge.
The calculated ion beam velocity for the argon ion
and an ion sound speed for the measured electrontemperature of 7.5 eV are 10 km/s and 4.2 km/s,
respectively. Therefore, the detected ion beam issupersonic [9].
In the configuration (II), the electron temperatureand the plasma density are measured for various
argon gas pressures and rf powers, and arecompared with theoretical value derived from thebalance equations between the volume ionizationand the surface loss, and between the electric power
and the energy loss in a global model. For the rf
power of 300 W, the electron temperature decreases
from about 8 eV to about 5 eV with an increase in
the argon gas pressure from 25 mPa to 300 mPa.
And the electron temperatures are about 6-7 eV for
all of the rf power (50-800 W) and the gas pressureof 40 mPa. An rf power efficiency is about 80% for
the gas pressure of 40 mPa and the rf power of 300
W; it increases with increase in the argon gas
pressure and the rf power. The theoretical values ofthe electron temperature and the plasma density arein fair agreement with the measured ones.
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