J. Plasma Fusion Res. SERIES, Vol.4 (2001) 106-110

Electric Field Measurement and Limiter Experiment on LHD
NISHIMURA Kiyohiko*, IDA Katsumi, KAWAHATA Kazuo, YAMAZAKI Kozo, ASHIKAWA Naokor,
EMOTO Masahiko, FUNABA Hisamichi, GOTO Motoshi, IDEI Hiroshi, IKEDA Katsunori,
INAGAKI Shigeru, INOUE Noriyuki, ISOBE Mitsutaka, KANEKO Osamu, KHLOPENKOV Konstantin,
KOMORI Akio, KUBO Shin, KUMAZAWA Ryuhei, LIANG Yunfengr. MASUZAKI Suguru,
MINAMI Takashi, MIYAZAWA Junichi, MORISAKI Tomohiro, MORITA Shigeru,
MURAKAMI Sadayoshi, MUTO Sadatsugu, MUTOH Takashi, NAGAYAMA Yoshio,
NAKAMURA Yukio, NAKANISHI Hideya, NARIHARA Kazumichi, NARUSHIMA Yoshiro,NODA Nobuaki, NOTAKE Takashi2, OBUCHI Takashil, OHDACHI Satoshi, OHYABU Nobuyoshi,
OKA Yoshihide, OSAKABE Masaki, OZAKI Tetsuo, PETERSON Byron J., SAGARA Akio,SAITO Kenji2, SAKAKIBARA Satoru, SAKAMOTO Ryuichi, SASAO Hajimer, SASAO Mamiko,SATO Kuninori, SATO Motoyasu, SEKI Tetsuo, SHIMOZUMA Takashi, SHOJI Mamoru,SUZUKI Hajime, TAKAYAMA Arimichi, TAKEIRI Yasuhiko, TANAKA Kenji, TANAKA Naokir,
TOI Kazuo, TOKUZAWA Tokihiko, TORII Yuki2, TSUMORI Katsuyoshi, WATANABE Kiyomasa,
WATARI Tetsuo, YAMADA Hiroshi, YAMADA Ichihiro, YAMAGUCHI Satarou,YAMAMOTO Satoru2, YOKOYAMA Masayuki, YOSHIMURA Yasuo and MOTOJIMA Osamu
National Institute for Fusion Science, Oroshi-cho 322-6, Toki 509-5292, JapanlGraduate University
Advanced Studies, Hayama 240-0193, Japanfor2Dep. Of Energy Engn. and Science, Nagoya University, Nagoya 464-8603' Japan(Received: 5 December 2000 / Accepted: 2 October 2001)

AbstractTransition from the ion root to the electron root in neutral beam heated plasmas was observed in theLarge Helical Device for the first time. The measured values of the radial electric field were found toagree well with those estimated by neoclassical theory. The pedestal configuration, which is characterized
by a high edge temperature gradient, was observed in LHD plasma, resulting in the improvement of theenergy confinement. A radially movable limiter was inserted into the pedestal region. The pedestal
configuration was still observed at the edge region bounded by the limiter without changing its width andgradient, and no confinement degradation was observed.

Keywords:
radial electric field, electric field transition from the ion root to the electron root, radially movable
limiter, pedestal configuration

are 3.5-3.9 m and 0.6-0.65 m, respectively. The
maximum magnetic field strength is 2.89 T at themagnetic axis of Ru = 3.6 m. The available heatingpowers are 4.2 MW from two Neutral Beam Injections

1. lntroduction
The Large Helical Device (LHD) [1-4] is thelargest superconducting heliotron type device with I :2lm = l0 continuous helical coils and three pairs ofpoloidal coils. The major and minor radii of the plasma*C o r re sp o ndin g autho r' s e - mail : ni s imura @ LH D. nifs. ac j p106

@2001 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchNishimura K. et al., Electric Field Measurement and Limiter Exoeriment on LHD(NBIs), 1.4 MW from one Ion Cyclotron Range ofFrequency (ICRF) heating system, and 0.9 MW ofElectron Cyclotron Resonance (ECR) heating with six
gyrotrons. Achieved plasma parameters are as follows;(a) peak electron temperature of f"(0) = 4.4 keV at <ne>= 5.3 x 1018 m-3 and Pabs = 1.8 MW, (b) peak energy

can be attributed to a reduction of electron thermaltransport in the outer region of the plasma resulting in
sharp temperature gradient [12]. A radially movable
limiter was inserted to this pedestal region in order toinvestigate the pedestal formation [13]. Pedestal
formation was observed without changing the width at
the edge region bounded by the limiter maintaining its

confinement time of Te= 0.3 s, Z" (0) = 1.1 keV at <ne>= 6.5 x 10re m-3 and Puo, = 2.0 MW, (c) higher stored
energy of W, = 880 kJ, (d) the highest beta value
achieved in helical systems of <P> = 2.4 Vo ar 1.3 T.
The achieved energy confinement time in LHD iswidth. No serious degradation of energy confinement
was observed in the limiter experiment.

2. Electric Field Measurement

systematically higher than that predicted by theThe radial electric field (E.) is derived from thepoloidal and toroidal rotation velocity and pressure
gradient of the Neon impurity ions by means of radialforce balance. The poloidal and toroidal rotation

International Stellarator Scaling 95 (ISS95) [5] by up toa factor of 1.6 and is comparable with ELMy H-mode
confinement capability in tokamaks. This improvement

is attributed to configuration control and to theformation of a high edge temperature (pedestal) [6]. In

velocity and the pressure gradient of the Neon impurity
ions are measured with charge exchange spectroscopy

this regard, the radial electric field (4) is one of the key
parameters in helical systems for high temperature(CXRS) [4] at the mid plane in LHD (verticallyelongated cross section). The transition from theplasma confinement because neoclassical theory
suggests a reduction of a helical ripple loss and animprovement of an energy confinement by the radial

negative E, (ion root) to the positive E, (electron root)
was observed at electron densities betow 1.0 x l0rem-3.
Figure I shows the radial electric field profiles for threemagnetic axis positions; (a) R* = 3.6 m (inward shift),(b) R", = 3.75 m (standard), (c) R"* = 3.9 m (outwardshift). The electron densities in each measurement arealso shown in Fig. l. By shifting the magnetic axis

electric field. In general, the ambipolarity relationshiphas multiple solutions since the electron and the ion areusually in different regimes of collisionality. The
electric field caused mainly by the ion flow is called theion root (negative E,) and the electric field caused
mainly by the electron flow is called the electron root

inward, the neoclassical transport is expected todecrease [15]. The critical electron densities for thetransition from the electron root to the ion root are(positive E,) [7l.As the absolute value of the positive E,(electron root) is larger than that of the negative E (ionobtained from the density-scan experimental results; (a)0.3 x 101e m-3 (inward shift), (b) 0.7 x lOre m-3(standard), (c) 0.8 x l01e m-3 (outward shift). This is dueroot), more improvement of the energy confinement isexpected by a transition of the radial electric field fromthe ion root (negative Er) to the electron root (positive4). The electron temperature profiles are measured witha multi-channel YAG Thomson scattering system [8]and electron density profiles are derived from a multichannel FIR laser interferometer [9] measurement using
Abel inversion. The transition of the radial electric fieldfrom the ion root to the electron root has been observedto the differences in the ratio of edge ion temperature toelectron temperature [16]. With T") T, the transition
from the ion root to the electron root occurs at a higher
collisionality. The edge radial electric field increases
sharply up to 15 kV/m in the electron root as theelectron density decreases. On the other hand, the edge
radial electric field in the ion root increases gradually up

in neutral beam heated plasmas with ECR heating [10]in the Compact Helical System (CHS) tlll. Nonthermal electrons heated perpendicularly by the ECR

to -5 kV/m over a wide range of electron density of
1.0-3.0 x 10re m-3 [16]. Small radial electric field isobserved in the plasma core region. The critical electron
density for the transition in the case of inward shift iseasily escape through the loss-cone. In such case, thetransition was triggered by the enhancement of electron

lower than that in the case of outward shift. Thebehavior of the radial electric field and their absolutevalues in both the electron and ion roots are in goodloss. In LHD, the transition without the enhancement ofelectron loss, as described above, has been observed inneutral beam heated plasmas without ECR heating. The

agreement with predictions of neoclassical theory. The
critical electron density required for the transition also

general trends in the observations are in accordance withthe prediction of neoclassical theory.

agrees well with neoclassical predictions. The reductionof ion thermal diffusivity by the electric field associatedAn improvement in the overall energy confinement
to7Nishimura K. et al., Electric Field Measurement and Limiter Experiment on LHD(b)(c),electronrcot, f-10j-----

eleclron root{.55"

03-f10lr

illffitl
IT-ion rool

=0.9pFig. 1 Radial profiles of the radial electric field in case of (a) F", = 3.6 m (inward shift configuration), (b) B", = 3.75 m(standard configuration), and (c) 4, = 3.9 m (outward shift configuration). Dashed lines show neoclassicalpredictions.

into the plasma at the high field side (under the helicalcoil winding) as shown in Fig. 2. The limiter position iscontrolled remotely with accuracy of 0.5 mm.
Figure 3 compares the radial profiles of electronwith the transition from ion root to electron root wasobserved for the plasma with Ru^ = 3.75 m [16].

3. Limiter Experiment

A pedestal configuration, which has a hightemperature gradient at a plasma edge [6], is observed inLHD. Usuat plasma discharges in LHD have been2500

carried out with an open helical divertor configuration.

To compare pedestal formation in an open divertor
discharge with that in a limiter discharge and toinvestigate a dependence of confinement on plasma

2000
'1500

minor radius, a radial movable limiter was installed at
7.5L lower port of LHD. Figure 2 shows a schematic
figure of the limiter and magnetic flux surfaces with Ru*= 3.6 m. The limiter head, which was made of carbon(IG430U) with high thermal conductivity, was inserted

o'I'.,ooo

500

4.0

Iirlitr8 Ead

3.5

Crrhm

3.0

It0tlSod0tld'fEo2.5

c

1.5

2.0

1.0

O r/o liniterO uith'liniter(0.80)

im&i Fi"' .'."''-%
Eatfra.

O

iooa?

oioio-"--"-'o

0.5
0.0
-1.0

-0.5

0.0
p0.5

1.0

Fig. 3 Comparisons of electron temperature and density

profiles with and without limiter. The equivalent
limiter position is indicated by arrow.

Fig. 2 Schematic figure of movable limiter'108

Nishimura K. et aL, Electric Field Measurement and Limiter Experiment on LHD

density and temperature in the open divertor discharge

divertor discharges [14] and the pedestal was similar tothe internal thermal barrier in a Tokamak [6]. As thepedestal formation is observed in the limiter discharge,
good energy confinement is expected. Figure 5 shows
the normalized energy confinement time according to
the ISS95 scaling at various limiter positions. The ISS95
scaling is expressed as follows [5]:and limiter discharge. The arrows in this figure indicate
the equivalent limiter position. The electron temperature

profile was bounded well by the limiter but the plasma
density profile was not bounded well by the limiter. In
the case of the open divertor discharge, however, theplasma density exists in an ergodic region (p > 1.0),
which surrounds closed magnetic surface region. A clear
change of the electron temperature gradient can be seen

?fses - 0.079a3'2tR3'65P,$se 4r"tos| B?83+oa

at p - 0.85 in the open divertor discharge and p - 0.6 inthe limiter discharge. A high temperature gradient isalso maintained in the limiter discharges.
Dependences of the central electron temperature

where ao and rRo are plasma minor and major radii,
respectively, and P,o, total absorbed power, <n"> line
averaged electron density, B, magnetic field strength,and+2s rotational transform at radial position of 2/3ar.
An enhancement factor of 1.1+0.3 over the predictionsof the ISS95 scaling was observed at every limiter
position. In this experiment, plasma parameters other(Z"s), electron temperature at the pedestal knee position(Z"p"o), line averaged electron density (<n.>), pedestal

width (Apo"j, and the temperature gradient at thepedestal region (?"eped/App"J on various limiter positions

are shown in Fig. 4. The position of p = 1.2 in thisfigure means the farthest limiter position. Namely, thesedata are in the open divertor discharges. The electron
temperature at the pedestal knee and the pedestal width
remain constant with limiter position. As a result, a hightemperature gradient in the pedestal region is also
maintained in this experimental range.
It was shown that the formation of a pedestal
configuration led to a good energy confinement inthan ao do not change much and their dependences on ?E1.5
tiltl
Lil
^
^.-.-..1 .u n9..v-Avv:

A\r'oo^x^tA (-R ll

1,0HYOo;iPPiii0,iiio

1.5

I8

1.0':"'" s ? q . ..
..a.olo
1.-i-.t. r,8 0.5

0. 0

3.0 .i-0.7 0,8 0.9 1,0 11

2.0;.-3 IgFig. 5 Comparison of the energy confinement time withrhe lss95 scaling.

1.0 io

0.0
o.6

1.2

Limjter position (p)EF.^o.
o..o..
ITp"d,ti

4.0

0.0
6.0

0.5

5.0

0.4

4.0

E 0,3

3.0 _s

E0.2

6o.2

2'o r'

o.1

1.0:
Io0.0
o.7

0.8

0.91.0

1.1

0.0
1.20.

1

1.3

' Limiter posltbn (p)FFig. 4 Dependence of the central electron temperature

0. 0

I"o, the pedestal electron temperature [p"d,average electron density <n">, pedestal width
Lpo"o, and the temperature gradient at pedestal

0.45

0,50 0,55 0.60 0.65(ao) (m)T"o"o/Lpp.a on limiter position. The limiter positionis expressed in flux coordinate, and r= 1.2 means
the farthest position.

Fig. 6 Dependence of the energy confinement time on
the plasma minor radius.
109

Nishimura K. et al., Electrrc Field Measurement and Limiter Experiment on LHD

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4. SummaryThe transition from the ion root to the electron rootin neutral beam heated discharges was observed in theLarge Helical Device for the first time without ECR
heating. The measured values ofthe radial electric field
were found to be in good agreement with thoseestimated by neoclassical theory. The measured highelectric field associated with the transition from the ionroot to the electron root and the reduction of ion thermal
diffusivity supports the thesis that the helical ripple loss

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Acknowledgment
Authors would like to thank to the members of thedevice engineering group for their operational supports

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and ex-Director General A. Iiyoshi and present Director

General M. Fujiwara for their continuous

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110