J. Plasma Fusion Res. SERIES, Vol.4 (2001) 379-382

Shear Flow Stabilization of Drift-Wave Fluctuations in an
EcR-Produced Linear Plasma
YOSHINUMA Mikirou", INUTAKE Masaaki, HATTORI Kunihiko, ANDo Akira.
KANEKO Toshiro, HATAKEYAMA Rikizo and SATO Noriyoshi
Tohoku University, Sendni 980-8579, Japan(Received: 5 December 2000 / Accepted:2t August 2001)

Abstract
It is observed that intensity of drift-mode fluctuations destabilized in the range of slightly negative
radial electric field are affected by control the ExB rotation frequency shear. When the shear is increased

with E, fixed, the drift-mode fluctuations increase once in a weaker shear region, attain its peak aroundthe shear of ion diamagnetic drift frequency, and then decrease in the stronger shear region. Thisbehavior suggests that the net ion drift shear is effective for stabilizing the drift-mode fluctuation.

Keywords:
drift mode, flute mode, shear flow, radial electric field, ECR-produced plasma

1. Introduction

measured experiments on this issue.

In confinement-improved modes observed in

The purpose of the present paper is to clarifyeffects of both radial electric field and its shear on lowfrequency fluctuations which are considered to be atokamaks [], stellarators [2], and a mirror machines [3],a sudden change of radial profile of plasma density andpotential is often observed and often correlated with
reduction of low-frequency density fluctuations [4].
Radial profile of plasma potential can be controlled
by applying bias voltages to a radially-segmented
endplate in open-ended Qz-U device [5]. It has beenreported that effect of strong ambipolar potentials whichmain cause of degradation of plasma confinement.

2. Experimental Setup
The experiment is performed in the Q1-Upgrade
device which has a cylindrical vacuum chamber with
about 4.5 m in length and0.2 m in diameter [7]. The
chamber is electrically grounded and the base pressure
is kept below 7 x 10-6 Torr. A plasma with electron

stabilize the drift mode are observed experimentally by
Mase et al. [6]. It is considered that there are effects ofradial electric field shear as well as effects of radial
electric field in the GAMMA10 results. Furthermore,
the dependences of the flute-mode and drift-mode

density n" = l0lo cm-3 and electron temperature Z" - 7
eV is produced by electron cyclotron resonance (ECR)
discharge with argon gas pressure of 5 x l0-5 Ton. A 6

fluctuation intensity on radial electric field are

GHz microwave with a power of 200 W is axiallycomplicated by the effects of radial electric field shear

injected through a horn-type antenna located at one end

in Q7-U experiment [7]. Therefore, to clarify the effectsof radial electric field formation on low-frequencyfluctuations, effects of radial electric field and its shear
should be separately investigated and it is necessary tociury out more systematically-controlled and precisely-ofthe device. Surface ofthe antenna aperture is covered*with a glass plate (0.2 m in diameter, 5 mm in width) toinsulate the plasma from the antenna which iselectrically grounded.
In the plasma-source region a shallow magnetic@2001 by The Japan Society of Plasma

Corresponding author's e-mail : mikirou@ ecei.tohoku.ac.jp379

Science and Nuclear Fusion ResearchYoshinuma M. e/ al., Shear Flow Stabilization of Drift-Wave Fluctuations in an ECR-Produced Linear Plasma

50

well is formed by depressing the magnetic field strength
locally and a plasma is produced in the vicinity of theZlo

two ECR surfaces (Bpcn = 2.14kG).In the experimental(Eregion the magnetic field is almost uniform (B = 2.3kG). The plasma produced in the source region diffuses'i;into the experimental region through a 0.1-m-diam metal

4oJeolimiter, the inner surface of which is electricallyinsulated. The limiter is set between the source andexperimental regions and determines the plasma

o20ooCL(i)10diameter(D=9cm).
Langmuir probes are inserted radially in theexperimental region to measure radial profiles ofelectron density, electron temperature, and space

0-8-64-20246
Radial Position (cm)potential. Fluctuations in the plasma are measured fromelectron saturation currents ,/", of the probes and50

analyzed by an FFT analyzet.

lmA metal endplate is set at the other end of theplasma to precisely control both radial electric field ando'5

its shear. The endplate consists of ten circular concentricsegments. Bias voltages can be applied independently toeach segment. Fluctuations are dominantly observed in
the region with steep gradient where just inside thelimiter edge [8]. The density gradient is very gentle in

h30c,
oo.

o20(,

i'(!
o.

I|/)10the core region, therefore it is expected that there is alarge shear of ion diamagnetic drift near the limiteredge. To precisely control of the potential profile with-!
i0{and without ExB drift frequency shear, the endplate isdivided into narrower segments in the region wherefluctuations are strongly observed.

44-202468
Radial Position (cm)Fig. 1 Radial profiles of plasma potential in cases where(a) the radial electric field is varied without shear
and (b) both the radial electric field and the shear
are varied.

3. Experimental Results
3.1 Radial Potential Profile Control
In the present experiment, we discuss rotation
frequency shear in a cylindrical system instead of thevelocity shear in a slab system. To estimate the local

expected that direction of ExB drift should be reversedin the shadowed region for the profile labeled by F inFig. 1(b). These profiles have finite shear of the ,ExB
drift frequency. The radial electric field and rotation
frequency shear are varied in the range of -400 V/m to300 V/m and 0 to 400 kHzlm, respectively, while there

value of E, and its shear, measured profiles of plasma
potential are approximated by 6th-order polynomials.
Figure I shows various profiles of plasma potential
formed by carefully controlling bias voltages applied tothe endplate segments. These potential profiles arelabeled A to H for convenience' sake as shown in Fig.

are no significant change of density gradient'l. Figure 1(a) shows potential profiles in the case whereradial electric field is mainly varied. Parabolic profilesof plasma potential are formed in the shadowed region

3.2 Observations of Fluctuations

Drift-mode and flute-mode fluctuations withfrequencies of a few tens kHz which are much lower
than the ion cyclotron frequency are observed in a radial
region where the density gradient is steep. We canclearly identify these fluctuations, from the fact that
dependence of the drift-mode frequencies on electron

where the fluctuations are strongly observed. A
parabolic profile of plasma potential causes rigid plasma
rotation by ExB drift. The profiles shown in Fig. 1(a)have no shear of ExB drift frequency. On the other
hand, potential profiles shown in Fig. l(b) are more
complicated in the shadowed region. It is easily

temperature is clearly different from that of the flute380

Yoshinuma M. er al., Shear Flow Stabilization of Drift-Wave Fluctuations in an ECR-Produced Linear plasmamode. The drift-mode frequencies changes almost
proportionally to the electron temperature. On the otherhand, the flute mode frequencies are independent ofthe

Doppler shifts due to the ^ExB drift. When only rhe
shear increases with the radial electric field being almost
fixed, there are no significant variation in the observed
frequencies by a Doppler shift as shown in Fig. 2(b). It
is note that the flute-mode fluctuations are strongly
excited only in the strong shear region. The flute-mode
is considered to be a Kelvin-Helmholtz type instability
which is destabilized by strong ExB flow shear, detailsof which will not be described in this paper.

electron temperature.

Figure 2(a) and (b) shows fluctuation spectra incase of dominant control of the electric filed and theshear, respectively. Frequencies and intensities of thefluctuations vary complicatedly, depending on either
radial electric field or its shear. When the radial electricfield is varied from negative to positive with the shear
being mostly fixed, frequencies of the drift mode andFigure 3 shows the observed frequency plotted as afunction of radial electric field. The observed

the flute mode are shifted to lower and higher,
respectively as shown Fig.2(a). Those are caused by

M$4. 'YT*-?
ooE^"',*n^u,.-..ffi(o.2Ytcn

ffiffi-,3v/.n-ogv/m

A#ffi ^.4. *0102030

drift mode has azimuthal mode number zr = 2. Namely,
the change of the m = 2 drift-mode frequencies
corresponds to that of m = 2 ExB drift frequencies. This
result well explains the observed frequencies of the.g-\--E

local value obtained by fitting the measured potential
profile to 6th order polynomial. According to the phasedifference measured by using two probes which are set
at the same radial but different azimuthal positions, theE-0.8v1?

ANA(==o.

frequencies are varied by a Doppler shift of Ex,B drift
rotation. The radial electric fields are estimated by using-1 .v/cn,Eg

drift-mode fluctuations. Therefore, the frequencies ofthe drift mode are Doppler shifted by locally estimated
ExB drift. The flute-mode frequencies are Doppler
shifted by ExB drift which is not a locally estimated buta globally estimated.
These behaviors are well correlated with radial
distribution of the fluctuations. Figure 4 shows the radial
distribution of fluctuation spectra displayed as a contour
map. The fluctuations which should be noticed are8eP9€E
uJ-g-z6v/cm

Frequency (kl|z)(b)

20

Dfift{nodp-=#o!itt:.

CL.-#ilE

ldfF,r/drl - 3.0 kHy'cn

2.3ktztcn

"d.

15E€

10oxN

3s

*ug
i.6kHz/c'roo

P0o6,-@r.rna--

O.Tkl,lzlcr,

o=- 'O
Crt.5

Ia*fu0102030

llE0,3kHz/crl-ro-15

Frequency (kHz)-20
Fig. 2 Fluctuation spectra with potential profile control.

The profiles are mainly controlled (a) radial

4€-2-10r231
Radial Etectric Field ErMcm)

electric field with the shear being mostly fixedll df', eldrl - 1.4 - 2.3 kHzl and (b) radial electricfield shear with the radial electric field being

Fig.3 Observed frequency of fluctuations plotted as afunction of radial electric field. E is estimatedfrom local data of potential profile.

almost fixed (E = 0.5 V/cm).
381

Yoshinuma M. et al., Shear Flow Stabilization of Drift-Wave Fluctuations in an ECR-Produced Linear Plasma

Idiamagnetic drift frequency, and then decrease in theDdft.mode Fhte<node7E(,strong shear region.

6c 5o6o

4. Conclusions

4Effects ofboth radial electric field and its shear on
low-frequency fluctuations have been investigated in anECR-produced plasma, where the radial potential profile
is precisely controlled by applying bias voltages to a l0-3o.

6Eo 2t10

segmented endplate.

Frequency (kHz)Both drift-mode and flute-mode fluctuations areobserved in a region where the density gradient is steep.

Fig. 4 Radial distribution of fluctuation spectra displayed
as a contour map.

The drift mode is destabilized in the region of weakly
negative electric field, where the direction of ExB drift
equal to that of electron diamagnetic drift. When theExB rotation frequency shear is increased with the E,E

being fixed, the drift-mode fluctuations increase once ina weaker shear region, attain its peak around the shearof ion diamagnetic drift frequency, and then decrease in
the stronger shear region. This behavior suggests thaton-tz

dl

the net ion drift shear is effective for stabilizing thedrift-mode fluctuation.

xLrJc-((DuJo'IU)

0.5

Acknowledgements
This work was supported in part by the LHD Joint
Planning Research program at National Institute for(Uo(t-4-3-2-101234

Fusion Science.

Radial Electric Field (V/cm)
Fis

References

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I2l V.Erckmann et al., Phys. Rev. Lett. 70, 2086

shear.

( I 993).

t3l O. Sakai et a/., Phys. Rev. Lett. 70, 4071(1993).
t4l K. Ida et al., Plasma Phys. Control. Fusion 40,indicated by shadow. The drift mode has a broad band
spectra and is locally distributed near the plasma edge.
The flute mode has coherent spectra and is globallydistributed over a wide range of radial position.

1429 (1998).

t5l A. Tsushima et al., Phys. Rev. Lett. 56, 1815
fl986).
t6l A. Mase e/ al., Proc. Int. Conf. Open Plasma

Figure 5 shows intensities of drift-mode
fluctuations contour-plotted as functions of radial
electric field and its shear. The drift mode destabilized

Confinement System for Fusion, Novosibirsk,Russia, p.51, World Scientific, Singapore (1994).

t7l M. Yoshinuma et al., J. Plasma Fusion Res.

in the region of weakly negative electric field, where thedirection of ExB drift equals to that of electron
diamagnetic drift. Furthermore, when the ExB rotation
frequency shear is increased with E. fixed, the driftmode fluctuations increase once in a weaker shear

SERTES 1, 251 (1998).

t8l M. Yoshinuma et al., Transactions. of Fusion
Technology, 39/lT, l9l (2001).

region, attain its peak around the shear of ion

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