23P043
Potential profile measurement with heavy ion beam probe
on the Large Helical Device
大型ヘリカル装置における重イオンビームプローブによる電位分布計測
Akihiro Shimizu, Takeshi Ido, Masaki Nishiura, Mitsuki Nakamura1, Ryohei Makino1,
Masayuki Yokoyama, Hiromi Takahashi, Yasuo Yoshimura, Takashi Shimozuma,
Hiroe Igami, Shin Kubo, Haruhisa Nakano, Katsumi Ida, Mikiro Yoshinuma, LHD Group
清水昭博, 井戸毅，西浦正樹，中村充希1，牧野良平1，横山雅之，高橋裕己，吉村泰夫，
下妻隆，伊神弘恵，久保伸，中野治久，居田克巳，吉沼幹朗，LHD実験グループ
1

The National Institute for Fusion Science, 322-6, Oroshi-cho, Toki, Gifu 509-5292, JapanDept. of Energy Science and Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan
核融合科学研究所 〒509-5292 土岐市下石町322－6
1
名古屋大学工学研究科エネ理工The radial profile of potential in the core region of the Large Helical Device (LHD) plasma was measured
with a Heavy Ion Beam Probe (HIBP). The radial profiles of Er were estimated from differentiation of theexperimental data of HIBP. In relatively low temperature (~0.8keV) and medium density plasmas (~0.5 1019 m-3), which is heated by neutral beam injection, the small negative radial electric field was observed.
In high electron temperature (~2 keV) and low density plasmas (~0.2  1019 m-3), which is heated by NBI
and electron cyclotron heating, the positive radial electric field was observed. These Er profiles werecompared with the calculation result from neoclassical theory, and the theoretical prediction almost
coincided with the experimental data of Er in the core region of LHD plasmas.

1. Introduction
The radial electric field Er is a very importantparameter to study plasma confinement inmagnetized toroidal plasmas. For example, inimproved confinement discharge in tokamaks, theshear flow suppresses the turbulence in plasmaand high performance of plasma is realized.
In helical configuration, of which magnetic
configuration is not axisymmetric, trapped
particles in helical ripples enhance diffusion loss
in low collision regime, so called in 1/ regime(diffusion coefficient is proportional to 1/).
This helical ripple induced loss is intrinsically
non-ambipolar, therefore Er is formed to satisfy
ambipolar condition. This Er reduces the helicalripple diffusion loss, so the formation of Er in
helical device is important.
In order to study physics of the formation of Er
in LHD, heavy ion beam probe (HIBP) wasinstalled and has been developed [1-3]. We
measured the profile of potential in wide plasma
parameter space such as high ion temperatureplasmas and high electron temperature plasmas.
2. Experimental setup
LHD is a helical device, magnetic configurationof which is characterized by the major radius ofmagnetic axis Rax, the pitch parameter , and thequadrupole field component Bq. On experimental
results shown here,  and Bq are fixed to standard
ones, namely 1.254 and 100%, respectively. Bychanging the major radius magnetic axis, Rax, theamount of helical ripple can be changed.
HIBP system has been developed in many yeas.
By using tandem accelerator, the gold ion (Au+),
maximum energy of which is 6MeV, is generated
and is injected to plasma as primary beam. The
secondary beam (Au2+) is produced by the collision
with plasma, of which energy is analyzed by thetandem energy analyzer. The three dimensional
orbit of probe beam is controlled by two 8-poleelectrostatic deflectors. The radial profile ofpotential is obtained by sweeping the beam in 10
Hz. The spatial resolution (the scale of samplevolume) is about a few cm [2]. The primary andsecondary beam current intensity is about a few A
and a few nA respectively. In order to detect thislevel of secondary beam current, the very sensitive
current detector, micro channel plate (MCP), isused.
3. Experimental result
Density evolution of a typical shot is shown inFig.1, when the radial profile of plasma potentialwith HIBP was measured. The magnetic fieldstrength, Bt was 1.5 T, and the major radius of

00

1.0

2.0
Time (s)3.0

3.0 #81079

ne ( x 1019 m-3)
0.19 A
0.24 B
0.30 C
0.44 D
0.48 EA

(kV)

2.0

1.0

-0.5

0

0.1

0.2

0.3



0.4

Electron root

6
Electron root

42C

00

0.1

0.2



0.3

0.4

0.5-2

Ion root
00.1

0.2



0.3

0.4

0.5

Fig.3 Comparison of measured Er with theoreticalprediction in case A (a) and in case C (b)E0.02#81079 Er
Calculation
Experiment

8A

4-2D

0.56(b)#81079 Er0BC

1.5

10Calculation
Experiment

Er (kV/m)

D E

Fig.1 Temporal evolution of line averaged
density and heating methods

2.5(a)

8Er (kV/m)

A B Cne (1019110#81079

NBI#1
NBI#3-3m )

ECH

0.5

Fig.2 Potential profile measured with HIBP

magnetic axis, Rax was 3.75 m. The line averaged
density gradually increased from 0.2  1019 m-3 to0.5  1019 m-3 by gas puff control. The central
electron temperature was about 2.5 keV at the line
averaged density of 0.19  1019 m-3 and ~1.0 keV atthe line averaged density > 0.30  1019 m-3. Ineach period marked by alphabets A~E, the potential
profile was measured with HIBP as shown in Fig.2.
As the density increased, the potential decreased in
the core region. In low density and high electron
temperature case, the potential at the center tends tobe positive. In high density and low electron
temperature case, the potential at the center tends tobe negative or nearly zero.
By using a polynomial function of  as a fitting
function, radial electric field profile was obtained
from the experimental data.
Obtained radial
electric field profiles are compared with thoseestimated by GSRAKE code [4-5]. In Fig.3 (a)and (b), the comparison in period A and C areshown respectively. In the period A ( ne = 0.19 1019 m-3), from the theory the positive radial electricfield, so called electron root, is predicted. And
this prediction roughly coincides with theexperimental results.
19In the period C ( ne = 0.30-3 10 m ), the negative radial electric field, so
called ion root, in the core region is predicted.
And in this case, in the region  > 0.26, both rootsof electron and ion roots are expected. In the coreregion, the ion root calculated by the neoclassicaltheory coincides with the experimental result. In
the outer region where  ~ 0.6, the electron root oftheoretical prediction coincides with theexperimental result.
4. Summary
The radial potential profile was measured withHIBP in LHD. The radial electric field, Er, wasobtained from measured radial potential profile.
Based on the condition of neoclassical ambiporarity,
Er is estimated theoretically and compared withexperimental results.
Theoretical predictions
roughly coincide with the experimental results.
Thus, we clarified that physics of neoclassicalcontext is almost dominant in Er formation. The
results will provide a physical basis to consider
further improved confinement scenarios utilizing Er
in helical plasmas.
Acknowledgement
This work was supported by MEXT Japan underGrants-in-Aid for Young Scientists (Nos. 18760640,20686062), NIFS/NINS under the project ofFormation of International Network for ScientificCollaborations,andNIFS09ULBB505,
NIFS09ULBB515.
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Fusion Res. 2 (2007) S1098-1.
[2] T. Ido et al., J. Plasma Fusion Res. 2 (2007) S1100.
[3] A. Shimizu, T. Ido, M. Nishiura et al., J. Plasma
Fusion Res. 5 (2010) S1015.
[4] C. D Beidler., W. D. Dhaeseller, Plasma Phys.
Control. Fusion 37 (1995) 463.
[5] M. Yokoyama et al., Nucl. Fusion 42 (2002) 143.