J. Plasma Fusion Res. SERIES, Vol.4 (2001) 359-362

Observation of Plasma Hole in a Rotating Plasma
NAGAOKA Kenichi*, ISHIHARA Tatsuzo, OKAMOTO Atsushi,YOSHIMURA Shinjit and TANAKA Masayoshi Y.lNagoya University, Nagoya 464-8602, JapantNational InstituteFusion Science, Toki

for509-5292, Japan(Received: 5 December 2000 / Accepted: 18 August 2001)

Abstract
Plasma hole, a cylindrical density cavity, formed in a rotating plasma has been investigatedexperimentally. The plasma hole is characterized by large aspect ratio (length/radius > 30), steep
boundary layer between the hole and the ambient plasma (10 ion Larmor radius), and extremely highpositive potential (130 V). The flow velocity field associated with plasma hole structure has beenmeasured, and is found to have interesting features: (i) plasma rotates in azimuthal direction at amaximum velocity of order of ion sound speed, (ii) plasma flows radially inward across the magnetic
field line, (iii) there present an axial flow reversal between core and peripheral region. It is found that theflow pattern of the plasma hole is very similar to the that of well-developed typhoon with core.

Keywords:
plasma hole, structure formation, plasma flow, rotating plasma, vortex structure, directional Langmuir
probe

1. Introduction

three-dimensional velocity field associated with theplasma hole, and compare the flow pattern with that ofStudying self-organized structure formation is ofimportance to understand macroscopic behavior ofplasmas in laboratory and space, and many theoreticaland computational studies on this subject have been
carried out so far [1,2]. In laboratory, non-neutral
plasmas have been used in order to investigate twodimensional vortical dynamics, relaxation of twodimensional turbulence into vortex crystals, and so on[3-5]. Recently, various types of vortical structure havebeen observed in quasi-neutral plasmas [6,7]. Amongthem, spiral structure and multi-pole vortex were experimentally observed in an electron cyclotron resonancetyphoons.

2. Experimental Apparatus
The experiments have been performed in the HighDensity Plasma Experiment Device I (Hyper-I) at
National Institute for Fusion Science (NIFS) [9], whichis schematically shown in Fig. 1. The Hyper-I device

consists of a cylindrical vacuum chamber (30 cm indiameter and 200 cm in axial length) and 10 magnetic
coils arranged to produce a weakly divergent magnetic

field configuration. The microwave (2.45 GHz)

(ECR) plasma, showing that rotating magnetized plasma
is a fertile ground for studying self-organized structureIaunched from a high field end of the chamber through aquartz window along the field line excites an electron
cyclotron wave (ECW), which produces and sustains theformation [8].
In this paper, we present the experimental results
on a new type of vortical structure, a plasma hole,observed in a rotating ECR plasma. We also show the*C o rr e sp ondin g autho r' s e - mail : na g aoka @ nifs. ac

plasma by ECR heating. The ECR point (875 G) locatesnear the center of the chamber. At another end (low@2001 by The Japan Society of Plasma
Science and Nuclear Fusion Researchjp359

Nagaoka K. et al., Observation of Plasma Hole in a Rotating Plasma

Fig. 1 Schematic of Hyper-l device.

field side), three-dimensional prove drive system isinstalled to introduce an axial probe. Radially movableprobes are introduced from side ports.
The microwave input power and plasma duration
time are 4 - 15 kW and 1 min., respectively. The typicalFig. 2 CCD image of the plasma hole.

parameter of the HYPER-I plasma are electron

temperature T"- 30 eV, maximum density n" - I x 1013

0cm-3 for argon and plasma potential ye - 30 V,

I 0.00respectively.
The density and the electron temperature have beenmeasured by a Langmuir probe, and the plasma potential

20.00

30.0040.00

EEO

by an emissive probe. The plasma flow has been

60.00

measured with a directional Langmuir probe (DLP). The

70.00

-30

velocity V(0) flowing at an angle 0 with respect to thereference axis is determined by two ion saturation
currents, /,(0) and I"(0+ n), according to the following

80.00
90.00
100.0rrelationship [10];-90-90-60 -30 0x (mm)V(0)=1.1"(0+n)-1"(0)

C, K

50.00

I,(0+ n)+1.(0)30 6090Fig.3 Density contour on z= 1100 mm plane.

where I is the angle between the normal of ion
collecting surface of DLP and reference axis, C, ionsound speed, and K the coefficient of the order of unity,respectively. This relation between flow velocity andDLP currents was experimentally confirmed to be valid
for all flow directions with respect to the magnetic field
when weakly magnetized condition for ions (ion Lamor

axisymmetric structure (m = 0).
This plasma hole consists of three parts; central
core (hole) region, boundary layer, and ambient plasma
region. The density in the hole region is about 10 times
lower than the maximum density at the boundary layer,

radius > probe radius) are satisfied.

and the size is about 6 cm in diameter and is sharply
bounded by the thin layer with steep density gradient.
The width of the boundary layer is about 2 cm, whichcorresponds to 10 times the ion Larmor radius. Thedensity profile measured at z = 2OO mm (which is notshown in this paper), is similar to that at z = 1100 mm.

3. Experimental Results
A localized density cavity is spontaneously formedin the center of the plasma column under the operation
condition with a pressure of 0.6 mTorr (He) and an
input power of 8.5 kW. The CCD image of the plasma

The aspect ratio of the plasma hole is estimated ashole observed from a chamber end is shown in Fig.2.lt
is found that the dark region (refer to as "plasma hole")lies along the magnetic field near the center axis of thecylindrical plasma. The two-dimensional density profile
measured at z = 1100 mm is shown in Fig. 3, indicating-R(length/radius) > 30.

The plasma potential was measured by a radially
movable emissive probe at z = 1100 mm. As seen in
Fig. 4, the plasma potential exhibits a dome shape withan extremely high positive value (130 V) at the center,360

Nagaoka K. et al., Observation of Plasma Hole in a Rotating Plasma90

9sor(mm)o)

50x (mm)--.

Fig. 4 Plasma potential profile at z= 11OO mm. Solid line
indicates the plasma potential in the presence ofhole structure under operating condition 6 x 10-a$Torr (He) and 8.5 kW microwave input, dashedline the potential of the uniform plasma under.l5L

operating condition 2 x 10'3 Torr (He) and 9.6 kW
microwave input.

.71

450'5r (mm)
(c)

1.0

while that of ambient plasma is about 40 V. Themaximum potential gradient is about 40 V/cm at theboundary layer. The dashed line in Fig. 4 shows thetypical potential profile of an uniform plasma without

G5o! 4to.0

the hole structure.

-1.0

The velocity profiles of plasma flow measured by

DLP at z = 1100 mm are shown in Fig. 5. As seen in{t026Fig. 5(a), the plasma rotates azimuthally in the directionof E x B drift and the maximum flow velocity is of theorder of ion sound speed at the boundary layer wherer(mm)Fig. 5 Velocity profiles of plasma flow measure by DLP
at z = 1 100 mm with the coefficient K = 0.5. {a): thethe electric field also reaches the maximum. Figure 5(b)azimuthal flow velocity, (b): the radial flow

shows that the plasma flows radially inward with
subsonic velocity. It is interesting to note that theparallel flow along the magnetic field shows a flow
reversal as seen in Fig. 5(c); in the ambient region theplasma flows from the ECR region located at e - 800mm to the chamber end, while in the hole region theflow direction is from the chamber end to the ECRregion. The axial flow also has the maximum value atvelocity, and (c): the axial flow velocity.

60
40-1.8-t_a20

the boundary region.

-1.3
-1.0

Fo
E

4. Discussion-0.75-0.50) -zo

The azimuthal flow profile shows that the plasmahole is considered to be a monopole vortical structure.
The vorticity determined by velocity field at z = ll00
mm is concentrated upon inner region of the hole and ismuch small in other regions, which is shown in Fig. 6.
According to this vorticity distribution, it is found thatthe plasma hole has Rankine-type vortex structure.

;

-0.25
00.25-40

0.50-60 L€0.{0-20 O

20.t060x (mm)Fig.6 Vorticity contour on z= 1100 mm plane.
361

Nagaoka K. et al., Observation of Plasma Hole in a Rotating Plasma

differences of dynamics of fluid motion, such as, theWhen the magnetic field was inversed, the holestructure reappeared in the plasma without any changein size, however, the azimuthal flow was in the opposite
direction. This indicates that the azimuthal flow is dueto the E x B drift. The radial and axial flows do notchange their directions even if the magnetic field isreversed. This is the same property of vortices formed in
Coriolis fields. This implies that there present a large
convective-cell motion in r - z plane.
Plasma hole and typhoon, which is a typical vortexstructure in neutral fluids in nature, have some common

basic equations, the radial force balance and so on. Thisimplies that a common scenario is underlying in
generating plasma hole structure, and is very interestingand important from the viewpoint of vortex-structureformations and self-organization in plasmas.

5. Gonclusion
Plasma hole structure was observed in a rotatingplasma and experimentally examined. The plasma holeis characterized by large aspect ratio, steep boundary
layer and extremely high positive potential. Moreover,flow velocity associated with the plasma hole isproperties; (i) the azimuthal flow forms a single-vortex
structure, (ii) the radial flow is in inward direction, (iii)
a flow reversal is present in axial direction, and (iv) thecore structure (eye of typhoon) is formed at the center.
In order to compare the two flow structures in moredetail, the azimuthal velocity of the plasma hole ismeasured using DLP. The plasma rotates azimuthally inthe direction E x B drift at the maximum velocity oforder of ion sound speed, and flows radially inward. Aflow reversal is present in axial direction. This flow
pattern is very similar to that of typhoons, suggestingplotted as a function of the radius r in Fig. 7, and thefollowing relation is found;that the presence of common scenario in forming
vortical structure.

V qr(0< r (ro)1-\ _0.5 __1 1r 2 rs)(Acknowledgements
Authors would like to thank Profs. M. Kono at

IChuo University and J. Vranjes at Institute of Physics(Belgrade) for helpful discussions.

lwhere ro (- 30 mm) is the position at which theazimuthal velocity reaches maximum. In many typhooncases, it is well known that a - 1 in the core region anda - -O.5 in the outer region. Thus the plasma hole andtyphoons have very similar flow pattern in spite of theReferences

tll M.R. Brown, J. Plasma Phys. 57, 2O3 (1997).
[2] J.B. Taylor, Phys. Rev. Lett.33,ll39 (1974).
[3] T.M. O'Neil, Phys. Today 52,24 (1999).
l4l D.Z. Jin and D.H.E. Dubin, Phys. Rev. Lett. 84,1443 (200U.
t5l Y. Kiwamoto, K. Ito, A. Sanpei and A. Mohri,./a'taa(,oaaatata

1.J.r. a aa.1 aaiPhys. Rev. Lett.85, 3173 (2000).

tata ao't6l M.Y. Tanaka, T. Sakamoto, H. Imaizumi, K.
Taniguchi and Y. Kawa| Proceedings of the 1996
International Conference on Plasma Physics Yol.
2, p. 1650.
[?] T. Ikehata, H. Tanaka, N.Y. Sato and H. Mase,r-ot..l i$l:aa/aPhys. Rev. Lett.81, 1853 (1998).

/aa
0.0

tSl M. Kono and M.Y. Tanaka, Phys. Rev. Lett. 84,
4369 (2000).

0204060t9l M.Y. Tanaka, M. Bacal, M. Sasao and T. Kuroda,r (mm)

Rev. Sci. Instrum. 69, 980 (1998).
tl0l K. Nagaoka, A, Okamoto, S. Yoshimura and M.Y.
Tanaka, J. Phys. Soc. Jpn. 70, I3l (2001).

Fig. 7 Azimuthal velocity as a function of the distancefrom the center of rotation. Dashed line indicatesthe relation Ve* f with a= 1, solid curve with a=-1, broken curve with a= -O.5, respectively.

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