Fusion Res.
Res. SERIES,SERIES, Vol.
Vol. 6 (2004) 000-000
610–613J. Plasma Fusion

Observation of Plasma Hole in an ECR Ar Plasma
YOSHIMURA Shinji, OKAMOTO Atsushi1 and TANAKA Masayoshi Y.
National Institute for Fusion Science, Toki, 509-5292, Japan
1Nagoya University, Nagoya, 464-8602, Japan(Received: 9 December 2003 / Accepted: 11 March 2004)

Abstract
The first experimental observation of a plasma hole structure in an ECR argon plasma is reported. The plasmahole is a cylindrical density cavity, which is formed spontaneously in the center of the plasma. The steep density
gradient between the hole plasma and the ambient plasma is sustained by a thin interfacial layer, the width of which isa few ion Larmor radii. Supersonic rotation is found over the large cross-sectional area of Ar plasma hole. The axial
flow can also exceed ion sound speed. Spectral measurements revealed that the neutral density profile exhibits a hole
structure as well as that in ion density, the diameter of which is much shorter than the mean free path of neutral
particles.
Keywords:
plasma hole, structure formation, macroscopic structure, vortex, supersonic ion flow, rotating plasma,
ECR plasma, directional Langmuir probe

1. Introduction

2. Experimental setup

An electric field in a magnetized plasma inevitably
drives rotating motion of ions by E # B drift, giving rise to
the formation of macroscopic vortical structures, which has
been a topic of interest in plasma physics because of therelation to self-organization and to transport phenomena.
Various types of vortical structures have recently been
investigated in both nonneutral plasmas [1,2] and quasineutral laboratory plasmas [3-5]. One of the structures
observed in electron cyclotron resonance (ECR) plasmas
appears with sharp density depletion around the central axis,
that is, a cylindrical density cavity is spontaneously formed in
the plasma. Since a nonluminous cavity region, which seems
to be a hole in the plasma, is clearly seen in the perspective
image taken by a CCD camera, this structure is referred to as
plasma hole [4]. So far, the plasma hole has been observed
only in an ECR helium plasma (He plasma hole).
In this paper, the first experimental observation of aplasma hole structure in an ECR argon plasma (Ar plasmahole) is reported, especially pertaining to the density profileand the ion flow velocity field. The existence of supersonicion flow over the large cross-sectional area is a noticeablecharacteristic of Ar plasma hole in comparison with He
plasma hole. A two-dimensional density profile of neutral
particles associated with the Ar plasma hole, which has beenmeasured with a spectroscopic method, is shown to examine
the formation mechanism of ion density depletion.

The experiments have been performed in the HighDensity Plasma Experiment (Hyper-I) device at National
Institute for Fusion Science (NIFS) [6]. Hyper-I is a linear
device, which consists of ten magnetic coils and a cylindricalchamber with the inner diameter of 300 mm and axial lengthof 2000 mm. The plasma is produced by ECR heating with amicrowave of frequency 2.45 GHz. An argon gas has beenused at relatively low pressures p Ar # 0.1 mmTorr. The ion
flow velocity field, the density and the electron temperature
profiles have been measured with a directional Langmuirprobe (DLP) inserted from the radial ports. The details of theDLP and the method of flow-velocity measurement havebeen given in ref. [6].
A two-dimensional motor drive system with an opticalsystem is equipped at the end of the chamber to carry out
spectroscopic measurements. The focal point of the opticalsystem is set to infinity to collect the visible light emitted
parallel to the axis of the cylindrical plasma. We haveobserved the emission lines from the argon ions (488.0 nm)
and the neutral argon (425.9 nm). A CCD camera is also
installed to take the end view image of the plasma hole.

3. Observation of Ar plasma hole
A CCD image of the Ar plasma hole is shown in Fig. 1,evidently indicating a nonluminous hole region. The iondensity in the hole is about 1010 cm - 3 , on the other hand, thedensity in the bright region (ambient plasma) is nearly one
order of magnitude higher than that of the hole region. The©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author’s e-mail: yshinji@nifs.ac.jp
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Yoshimura S. et al., Observation of Plasma Hole in an ECR Ar Plasma

Fig. 1 CCD image of Ar plama hole.

Fig. 3 Azimuthal velocity profiles of Ar plasma hole (z = 730mm and z = 1175 mm).

Fig. 2 Density profiles of Ar plasma hole (z = 730 mm andz = 1175 mm).

hole structure has been spontaneously formed in the center ofthe plasma under the operation condition with the pressurep Ar + 8 # 10 - 5 Torr and an input power of 3 kW. Figure 2
shows the ion density profiles measured at two different axial
positions, z = 730 mm and z = 1175 mm, where z denotes
the distance from the microwave injection window. It can beclearly seen that the ion density distribution has a good axial
symmetry and builds up a steep transition layer to theambient plasma. This shock-like density gradient is sustainedby the thin interfacial layer between the hole plasma and theambient plasma, the width of which is about 20 mm, i.e., afew ion Larmor radii. It should be noted that the size of theplasma hole is almost constant (60 mm) at two different axial
positions, indicating two-dimensional nature of the plasmahole structure. At z = 1175 mm, the ion density reaches its
maximum at the transition layer ( x + 40 mm ), which isFig. 4 Axial velocity profiles of Ar plasma hole (z = 730 mm
and z = 1175 mm).

similar to that of He plasma hole.

4. Flow structure of Ar plasma hole
The profiles of azimuthal and axial ion-flow velocity
measured at two different axial positions are shown in Fig. 3
and 4, respectively. As seen in Fig. 3, the plasma rotates
azimuthally in the clockwise direction (direction of ion
diamagnetic drift) when viewing from the end of thechamber. It is also confirmed from the plasma potential
measurement that the rotation is driven by the E # B drift.
The hole plasma (core region) shows nearly rigid rotation at2611

Yoshimura S. et al., Observation of Plasma Hole in an ECR Ar Plasmaz = 730 mm, however, it weakly rotates at z = 1175 mm. It
should be noted that the ambient plasma rotates at asupersonic speed in both positions. The supersonic ion flowis also found in the axial direction at z = 1175 mm, as shownin Fig. 4, where the flow direction is from the microwave
injection point to the chamber end. Although the existence ofsupersonic ion flow over the large cross-sectional area is a
noticeable and important characteristic of the Ar plasma holein comparison with the He plasma hole, quantitative
treatment for the supersonic flow field requires more reliableinterpretation of the DLP data or other diagnostic methods
such as laser induced fluorescence (LIF) spectroscopy [7].

The two-dimensional neutral density profile of the Ar
plasma hole is shown in Fig. 6. The remarkable point is thatthe neutrals do form a hole structure, the size of which isapproximately the same size of the ion density depletion andis much shorter than their mean free path. Here, we consider
the plasma production profile in connection with the density
structure of Ar plasma hole. The number of particles ionizedin unit time and unit volume is given by

G vy Hi n e nn ? I ArI ,where the quantity G vy Hi , product of the ionization crosssection and electron thermal velocity averaged over velocity
distribution function, is a constant under constant electron
temperature. Hence, the profile of I ArI (r) is proportional tothe profile of ionization rate per unit volume. Figure 7 shows
the line intensity profile of ArI, indicating that thedistribution of the number of ionization events in unit timeand volume is also concave one. Thus we can conclude that
one of the causes of ion density depletion is the localizationof plasma production due to formation of neutral particle
hole. As a possible explanation for neutral hole formation, it
is suggested that rotating motion of the neutrals induced bycollisions with the rotating ions makes the apparent radialmean free path short, since the radial projection of arc length
corresponding to the ionization length is much shorter thanthat in a free streaming case.

5. Discussion
The plasma hole is characterized by the cylindricaldensity cavity, the size of which is about 60 mm in diameter.
The mean free path of neutral argon under our experimental
condition is more than 400 mm, which is much longer thanthe diameter of the plasma hole. In order to examine whether
a hole structure of which the scale is shorter than the neutralmean free path can be formed, the two-dimensionalspectroscopic measurement has been carried out. The
intensities of emitted spectral lines from neutrals and ions
2are, respectively, given by I ArI ? n i n n and I ArII ? n i , where
the quasi-neutrality condition and the constant electron
temperature are assumed. Figure 5 shows the electron
temperature profile of the Ar plasma hole at z = 730 mm.
Although the electron temperature slightly decreases withradial distance from 12 eV to 9 eV, neglecting the effect ofT e variation on the collision cross section is justified. Hencethe neutral density profile is obtained by the following
relation:n n (r) ?

I ArI (r).
I ArII (r)

(2)

(1)

Thanks to the two-dimensional nature of the plasma hole, theoptical measurement, which is a line-integrated quantity
along the axial direction, provides the radial distribution.
Fig. 6 Two-dimensional neutral density profile of Ar plasmahole.

Fig. 5 Electron temperature profile of Ar plasma hole (z = 730mm).

Fig. 7 Line intensity profile (ArI: 425.9 nm) of Ar plasma hole.

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Yoshimura S. et al., Observation of Plasma Hole in an ECR Ar Plasma

in-Aid for Young Scientists (B), 15740332, 2003.

6. Conclusion
A plasma hole structure in an ECR argon plasma wasfirst observed and examined experimentally. The Ar plasmahole is characterized by ion density depletion with the steep
transition layer and the existence of azimuthal and/or axial
supersonic ion flow. The two-dimensional neutral density
profile was measured spectroscopically, revealing that theneutrals also form a hole structure as well.

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Acknowledgements
This research was partially supported by the Ministry ofEducation, Culture, Sports, Science and Technology, Grant-4613