24P140-P
Experimental Studies on Ion Stream Line Detachment and Onset of Azimuthal
Rotation in a Diverging Magnetic Field
イオン流線のディタッチメントに伴う回転流の駆動と電磁場の運動量
Kenichiro Terasaka1, Shinji Yoshimura2, Mitsutoshi Aramaki3,
Hirotsugu Katanami1, and Masayoshi Y. Tanaka1
寺坂 健一郎1, 吉村 信次2，荒巻 光利3，片波 寛嗣1, 田中 雅慶1
1

Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
九州大学 〒816-8580 福岡県春日市春日公園6-1
2
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-63
Nagoya University, Furu-cho, Chikusa, Nagoya 464-8603, Japan
名古屋大学 〒464-8603 名古屋市千種布留町
The ion flow velocity in a diverging magnetic field has been measured with the calibrated directional
Langmuir probe in an electron cyclotron resonance plasma. The ions are unidirectionally accelerated by theambipolar electric field along with the magnetic field line in the weak diverging magnetic field region.
When the effect of field-inhomogeneity is not negligible in the ion motion, the ion stream line detachment
takes place and the plasma starts the azimuthal
rotation generated by the difference of motions
between the magnetized electrons and the detached ions.

1. Introduction
Recently, plasma detachment from the magnetic
field line is considered to be important in plasma
propulsion research works. The control of plasma
detachment is required for the development ofplasma thrusters applying the external magnetic
field, because the plasma return to the main bodyalong with the magnetic field line even though theplasma is exhausted from an end of the thrusters.
Then, many theoretical and numerical studies tooptimize the plasma ejection have been performed.
The ion stream line detachment takes place whenan adiabaticity of ions is not conserved along thestream line. Hooper suggested that the electric fieldis generated to avoid the charge-separation [1],since the location where the detachment takes place
differs between the electrons and the ions due to thedifference of each mass. In the detachment region,
the angular momentum of the magnetic field also
plays an important role in the structure formation ofplasma flows perpendicular and parallel to themagnetic field line. However, the generation of theelectric field and the effect of the angularmomentum of electromagnetic field on the plasma
flow structure formation in the detachment region
have not been understood, so far.
In our previous experiment with a linear electron
cyclotron resonance (ECR) plasma, we observedthat the ion stream line detachment takes placewhen the non-adiabaticity of ions becomes to unity[2]. To understand the effect of electrostatic andelectromagnetic fields on the plasma flow field in
the detachment region, we have experimentallystudied the structure formations of the ion flow
velocity field and the electric field in a diverging
magnetic field.
2. Experimental Setup
The experiments have been performed in highdensity plasma experiment-I device (HYPER-I [3])at the National Institute for Fusion Science. Figure1(a) shows a schematic of the HYPER-I device.
The HYPER-I device consists a cylindrical vacuum
vessel (0.3 m in inner diameter and 2.0 m in axialFig. 1 (a) A schematic of the HYPER-I device, and(b) the axial profile of the magnetic field.

length). We adopted a diverging magnetic field
configuration in this experiment as shown in Fig.
1(b). A steady-state argon plasma was produced by
ECR heating with a 2.45 GHz microwave. Theargon gas pressure and the input microwave power
were set to 0.1 mTorr and 6 kW, respectively. The
typical electron density and electron temperaturearem－3 andeV, respectively.
We measured the ion flow velocity withdirectional Langmuir probe (DLP [4]). To obtain
the absolute ion flow velocity, we experimentallycalibrated the DLP with a laser induced
fluorescence (LIF) Doppler spectroscopy, and thecalibration factor was evaluated asin
this plasma [5]. An axially movable DLP wasmounted on the 3-dimensional driving system, andwas inserted from an end port of the device.
3. Experimental Results
Figure 2(a) shows the axial profile of theelectrostatic potential normalized by the electrontemperature ( ⁄), where, , ,, andare the electrostatic potential, elementary charge,
the Boltzmann’s constant, and the electrontemperature, respectively. The normalized electrondensity () is also shown with a solid line. In theupstream region
m, the Boltzmann’srelation is well satisfied, and the ambipolar electricfield along the magnetic field line is generated. Inthe downstream region
m in which the ion
stream line detachment takes place, the potential
decrease is less than that in density. This indicatesthat the ion fluid does not move along the magnetic
field line as the electron fluid.
Figure 2(b) shows the axial ion flow velocity
measured with the axially movable DLP. A dashedline shows the one dimensional Bernoulli’s relationconsidering the ambipolar electric field. Theelectric field is evaluated from the electrostatic
potential data shown in Fig.2(a). The experimental
data show a good agreement with the theoreticalprediction, and it is clear that the ions are axiallyaccelerated by the ambipolar electric field.
We also show the axial profile of the azimuthal
ion flow velocity at a radial position of 50 mm inFig. 2(c). It is found that a rigid-body-likeazimuthal rotation is generated in the region
m. From the comparison with the radial
profiles of the flow velocity and the potential, thisrotation is explained by thedrift, which isgenerated by the radial electric field and theexternal magnetic field. This result implies that theions do not expand with the diverging magnetic
field line to conserve its angular momentum in thedetachment region.

Fig. 2: Axial profiles of (a) the normalized
electrostatic potential, (b) the axial ion flow velocity,and (c) the azimuthal ion flow velocity.

4. Conclusions
We have experimentally studied the ion flow
velocity field of an ECR plasma in a diverging
magnetic field. When inhomogeneity of themagnetic field is weak, the ions are accelerated bythe ambipolar electric field generated along themagnetic field line. In the lower magnetic fieldregion in which the ion stream line detachment
takes place, it is found that an azimuthal
rotation is generated. The radial electric field isgenerated due to the difference of motions between
the magnetized electrons and the detached ions.
These results indicate that the ions are likely to flow
straight to conserve the angular momentum in thedetachment region.
References[1] E.B. Hooper, J. Propul. Power 9 (1993) 757.
[2] K. Terasaka, S. Yoshimura, K. Ogiwara, M. Aramaki,and M.Y. Tanaka: Phys. Plasmas 17 (2010) 072106.
[3] M.Y. Tanaka, M. Bacal, M. Sasao, and T. Kuroda:
Rev. Sci. Instrum. 69 (1998) 980.
[4] K. Nagaoka, A. Okamoto, S. Yoshimura, and M.Y.
Tanaka: J. Phys. Soc. Jpn. 70 (2001) 131.
[5] K. Terasaka et al.: Jpn. J. Appl. Phys. 49 (2010)
036101.