J. Plasma Fusion Res. SERIES, Vol. 4 (2001) 363-36'lMultipole Vortices in an ECR Plasma
OKAMOTO Atsushi*, ISHIHARA Tatsuzo, NAGAOKA Kenichi,
YOSHIMURA Shinjit and TANAKA Masayoshi y.r
Nagoya University, Nagoya 464-8602, Japan

lNational Institutefor Fusion Science, Toki 509-5292, Japan(Received: 5 December 2000 / Accepted: 27 September 2001)

Abstract
Spontaneous formation of multipole vortices in an ECR plasma is experimentally examined toinvestigate self-organization and structure formation in a rotating plasma. These vortices arecharacterized by steep density profiles with the peak densities 4-5 times higher than that of backgroundplasmas. The vortex pattern does not move with time, while the plasma rotates in the direction of electron

diamagnetic drift. It is found that there exists axial flow reversal in the rim of vortices.

Keywords:
multipole vortices, self-organization, structure formation, E x B drift, shear flow, ECR plasma

1. Introduction
Vortex, a localized vorticity distribution, is acommon entity of self-organization in rotating fluids,

vortices have the same polarity in vorticity, and that theions flow in the direction of electron diamagnetic drift.
It is also found that there exists an axial flow reversal inand many works have been done in ordinary fluids andsuperfluids. Recently, vortex formation and interaction

the rim of vortices.

in a non-neutral plasma have been attracting muchattention [1,2], and discussed in relation to 2-2. Experimental Setup

dimensional Eulerian fluid. On the other hand, a few
experimental works have been done in neutral plasmas,
in which interplay between a variety of processes suchas collective effect, nonlinear effect, viscosity andDensity Plasma Experiment (HYPER-I) device ar

The experiments have been performed in the HighNational Institute for Fusion Science [5]. A schematic
view of the HYPER-I device is shown in Fie. 1. The

compressibility etc. is expected to be important. Thus, inthe present work it is of primary interest to understandthe inherent features of vortex generated in a neutral
plasma [3,4].
Here, we concentrate our attention to characterize
the multipole vortices observed in an ECR plasma. The

experimental results on the spatial structure, density
profile, and flow velocity field of the multipole vortices
are presented. In Sec. 2, the experimental setup isCoollng Water

described, and the results and discussions are presented

Fig. 1 Schematic of HYPER-I device

in Sec. 3, where the most simplest structure, a pairedvortices is described in detail. It is found that the two*02001 by The Japan Society of Plasma

Corresponding author' s e -mail : aokamoto @ n fs. ac.jp

Science and Nuclear Fusion Research363

Okamoto A. et al., Multipole Vortices in an ECR Plasma

3. Results and Discussions
3.1 Multipole Vortices Formation
Figure 2 shows the end view images of theobserved structures for different magnetic field

HYPER-I device consists of ten magnetic coils and acylindrical chamber with the inner diameter of 30cm
and axial length of 200 cm. The magnetic field lines are

weakly diverging along the chamber axis, and make aso-called magnetic beach configuration. Plasmas areintensities. When the operation pressure is higher than 2
x 10-2 Torr, localized structures are spontaneously
formed in the plasma. These structures have been foundproduced and sustained by electron cyclotron resonance(ECR) heating with a 2.45 GHz microwave injectedfrom an open end of the vacuum chamber. Under theto be rotating plasma columns, and are called as
multipole vortices hereafter. The pattern of structures isstationary during the whole period ofdischarge, and thenumber of poles (bright regions in the figure) changes

typical experimental condition, the ECR point is locatedat 111 cm from the microwave launching position. The
ratio of microwave frequency to electron cyclotronfrequency, alo)ee, at the injection position is 1.1. Thedensity and electron temperature are typically n < 10r3
cm-3 and Z" = 5 eV, respectively, with the operationpressure po = 3 x 10-2 Torr (Argon) and 5-10 kW
microwave input power.
Ion flow velocity is measured with movabledirectional Langmuir probes [6] from the radial ports. A
three-dimensional (3-D) motor drive is equipped at anopen end of the chamber to measure the density profile.
A CCD camera is also installed to obtain the end view

with the external magnetic field. When the coil current
is 1"o11 = 100 A (Fig' 2(a))' the lowest magnetic field
intensity in this experiment, three large vortices areformed in the center of the chamber surrounded bymany small vortices. When the coil current is increased(1""u = 103 A), the center vortex disappears, while two
vortices in the upper side and the bottom side grow as
seen in Fig. 2(b). Although the number of large vortices
increases from three to four, the small vortices merge inthe peripheral region so that the total number ofvortices
decreases. For higher magnetic field (Fig. 2(c) and (d))'

image of vortices.

Fig.2 End-view images of multipole vortices: (a) 4.r = 100 A, (b) /oir = 103 A, (c) /"",r = 106 A, (d) ["u = 110 A, (e) l.or= 120
A, and (f) /"oir = 130 A.

364

Okamoto A. et al., Multipole Vortices in an ECR plasma

further merging of vortices occurs and small vortices in
the peripheral region disappear. The simplest structure

100

1.00

0.90
0.85
0.80

observed in the present experiment consists of two large

vortices as shown in Fig. 2(e), which is referred to asvortex pair hereafter. The localized structures disappear
for the coil current above 1.o1 = 130 A (Fig. 2(f)), and

4.70
0.65
0.60

EE0uniform plasmas are produced. All these structures

0.50
0.45
0.40

presented in Fig. 2(a)-(e) have a common feature. i.e..

rotational symmetry with z-radian rotation around the0.30
u,z50.20-50

center axis.

A change in the external magnetic field

0.1 0

corresponds to a shift of ECR point along the chamber-'t00 -50 0axis. Increasing the magnetic field, the ECR point50

0.05100x [mm]moves away from the microwave injection window. The

experimental observation shows that the number ofvortices and the configuration are very sensitive to thelocation of ECR point. This means that the formation ofmultipole vortices and generation of global convection
are strongly affected by the location of plasma
production region. The density profile and ion flow
structure have been measured with a Langmuir probe
and a directional Langmuir probe, respectively, theresults of which are presented in the next subsection.

Fig. 3 Density contour of vortex pair. The density isnormalized by the maximum density.

3.2 Density Profile and Flow Field Structures

dL=o.=
gof Vortex PairIEIn this subsection, the characteristic features of thevortex pair are presented. Density contour ofthe vortex
pair measured with a Langmuir probe is shown in Fig.
3. In accordance with the two bright oval regions in theCCD image (Fig. 2(e)), there are two density humps.
The full width at half maximum of the density hump is

1.0(a)

0.80.6

E o.4oCoo o.20.0about 10 cm x 5 cm on the cross section perpendicular
to the magnetic field. The peak density is 6 x 1012 cm-3,1.0o.!
tr 0.85and is 4-5 times higher than that of the backgroundplasma with a flat profile over the whole cross section.

tiLTo examine the axial structure of the vortices, wehave measured the density profile at axially differentpositions. The results are shown in Fig. 4, in which Fig.
4(a) is for 2O cm from the microwave launchingposition, and Fig. 4(b) for 110 cm. The density profile inFig. a(b) is identical to that measured along thehorizontal cord passing through the center of vortices (y= 0) in Fig. 3. The density profiles in Fig. 4 have

G0.6

Q0.4

tr(b)

oo

0.2
0.0-100

commonly two maxima 4-5 times higher than the-50 0

50100x [mm]background density. The distance between the peaks isabout 9 cm for Fig. 4(a), and 10.5 cm for Fig. 4(b),which is attributable to rhe weak diverging magnetic
field. According to these results, it is concluded that thevortex pair is axially extends over 90 cm.

Fig. 4 Density profile at two different axial positions:(a) 20 cm, (b) 110 cm from the microwave injection window. The density profile is normalized bythe maximum densitv.
365

Okamoto A. et al., Multipole Vortices in an ECR Plasma

BO

The azimuthal ion velocity associated with thevortex pair is shown in Fig. 5. The velocity
measurements have been done using a directional
Langmuir probe [6], along the horizontal cord passing
through the center of vortices. The direction of theexternal magnetic field is perpendicular to this figure'and the direction of magnetic field vector is indicated in
the upper right of the figure. The hatched regions in Fig.
5 correspond to the full width at half maximum of theo.2atC)-. 0.0-9.2

density humps, the dashed lines indicate the positions of
the maximum density. The ion velocity is normalized by
the ion sound velocity Cs. As seen in Fig.5, theazimuthal ion flow velocity is proportional to the-0.4
-100

-50

050x [mm]Fig. 5 Azimuthal velocitY Profile

distance from the center of vortices, exhibiting a rigid-like rotation. The two vortices rotate in the same
direction (counterclockwise rotation). The electron flow
velocity, which cannot be accurately determined by the
directional Langmuir probe, is found to be in the same
direction of ion flow.
When the magnetic field direction is inverted, noremarkable change has been observed in the vortical
pattern except for a change in the azimuthal position(tilting of the horizontal cord with respect to chamberaxis). Figure 6 shows the azimuthal ion velocity for theinverted magnetic field case. The asymmetric profile inthe azimuthal ion velocity is attributable to the tilting ofvortex pattern. It is easily found that the rotation of ion
flow changes its direction in this case. This result
implies that the ion rotation is due to F x B drift with aforce F acting in the radial and inward direction. The

B84.2o(J- 0.0-0.2-0.4-100-50

050x lmml

Fig. 6 Azimuthal velocity profile with reversed magnetic

potential profile measurement revealed that the electricfield.

field in the vortex is radially outward (< 1 V/cm) andthat the rotation direction is opposite to that of E x Bdrift. It is worth nothing that E x B rotation velocity
with a electric fieldE- I V/cm is I x 105 cm/s = 0.3
C5. The experimental result shows that there exists aninward force, which dominates an electric field of theorder of 1 V/cm. The origin of inward force is not fullyo.2oo\

understood yet, and the experiments are now inNprogress.

The axial ion flow profile is shown in Fig. 7, whichis independent of the direction of the magnetic field. In
the core region of each vortex (hatched region), thedirection of axial flow is the same as that of 1t grad-Bforce, where p is the magnetic moment. On the otherhand, in the peripheral regions, the flow direction is0.0-0.2-0.4-100-50

050x lmml
Fig. 7 Axial velocity profile

opposite to that of the core regions, exhibiting theexistence of strong shear in axial velocity at the edge
region. The vortex pair is considered to be confinedwithin this shear layer.
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Okamoto A. et al., Multipole Vortices in an ECR Plasma

4. Summary
We have observed spontaneous formation ofReferences

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