23D07
Configuration dependence of the transition criterion to an improved
confinement mode using a biased electrode in the LHD
LHDにおける電極バイアスによる閉じ込め改善モード遷移に対する
磁場リップル構造依存性
Hiromi Takahashi1), Sumio Kitajima2), Keiichi Ishii2), Mamoru Kanno2), Yu Satoh2),
Jo Tachibana2), Suguru Masuzaki1), Mamoru Shoji1), Naoko Ashikawa1), Masayuki Tokitani1),
Masayuki Yokoyama1), Yasuhiro Suzuki1), Shinsuke Satake1), Shigeru Inagaki3) and Mamiko Sasao2)
高橋裕己1), 北島純男2), 石井啓一2), 菅野守2), 佐藤優2), 立花丈2), 増崎貴1), 庄司主1),
芦川直子1), 時谷政行1), 横山雅之1), 鈴木康浩1), 佐竹真介1), 稲垣滋3), 笹尾眞實子2)1)

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
2) Department of Quantum Science and Energy Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
3) Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan1)

2)

核融合科学研究所 〒509-5292 岐阜県土岐市
東北大学量子エネルギー工学専攻 〒980-8579 宮城県仙台市
3)
九州大学応用力学研 〒816-8580 福岡県春日市

In order to examine the role of the neoclassical poloidal viscosity for the transition to improved
confinement mode, the configuration dependence of the transition criterion was investigated in the Large
Helical Device (LHD) using the biasing electrode. The electrode biasing experiments were carried out forthree configuration cases with different magnetic axis position. The critical driving force required for thetransition was found to be large in the outward-axis configuration. This result qualitatively agreed with thetheoretical predictions based on the Shaing model.

1. Introduction
In the neoclassical theory, the poloidal ion
viscosity has local maxima against the rotation
velocity [1]. When the driving force in poloidal
direction exceeds a critical value, the poloidal
rotation velocity increases rapidly and the plasma
makes transition to the H mode.
The electrode biasing experiment has theadvantage of ability to control radial electric field
externally by controlling the electrode voltage
and/or the electrode current and to estimate thedriving force from the electrode current [2, 3].
In LHD, electrode biasing experiments have been
performed in the standard configuration and thetransition to an improved mode was successfully
obtained [4]. Once the transition occurred, thedecrease of electrode current, the suppression of thedensity fluctuation and the improvement of theenergy confinement time were observed.
In the research, we carried out the biasing
experiments for three configuration cases in LHD.
It was expected that the difference of the transition
criterion due to the configuration was clearlyobserved by choosing the target configurations
appropriately because the magnetic ripple structureof LHD widely changes depending on theconfiguration especially in the plasma edge region.

2. Experimental Setup
The LHD is the largest heliotron device with R =3.9 m, a = 0.6 m, and has a pole number of 2 and atoroidal period of 10. The heliotron configurations
are produced by a set of helical winding coils andthree sets of poloidal field coils, which are allsuperconducting magnets. The magnetic field on
the axis is ~2.9 T. The disc-shaped electrode (φ =100 mm, l = 40 mm) made of isotropic graphite wasinserted from the lower port and set at the positionat the normalized minor radius of ~0.85.
3. Experimental Results
Figure 1 shows the temporal behaviors of (a) thebias voltage VE and the electrode current IA, (b) theline-averaged density and (c) the spectrum of theelectron-density fluctuation. The target plasma wasproduced and sustained by 280-kW ECRH under
the configuration of Rax = 3.53 m/ Bt = +2.705 T.
The electrode was biased against the vacuum vesselby DC power supply. In this study, the electrodevoltage was ramped up to +620 V from t = 4.51 s to
4.66 s and was ramped down after t = 4.66 s. As canbe seen in Fig. 1, the electrode current dropped andthe plasma showed negative resistance in 4.65 s < t< 4.71 s. The region was considered to be the4(g)Hysteresis

IE [A]

32ForwardReverse

100

100 200 300 400 500 600 700

VE [V]
Fig. 2 The characteristic curve of IE during the discharge.

4. Discussion and Conclusion
Figure 2 shows the relation between VE and IE
during the discharge. The characteristic curve
illustrated N-shape and IE has the local maximum
and the local minimum against VE. Moreover,hysteresis was observed in 520 V < VE < 580 V.
This implies that the plasma makes transition tohigh (low) confinement state when the torque inpoloidal direction JrxBt exceeds to the localmaximum of the poloidal ion viscosity (fall down to
the local minimum). Similar results were observedin the Tohoku University Heliac [3].
Figure 3 shows the configuration dependence ofthe poloidal driving force required for the transition
and the value of the local maximum in theneoclassical poloidal viscosity calculated using
Shaing model [1]. The critical driving forces wereevaluated using the value of the electrode current at
the timing that the negative resistance occurred in
the voltage-ramped-up phase. As can be seen fromthe figure, the critical driving force required for thetransition was found to be larger for theconfiguration of Rax = 3.6 m than that for Rax = 3.53
m. Although the poloidal damping force, whichbalances with the poloidal driving force, includes
the contribution of the friction force, the tendencyof the configuration dependence is qualitativelyagreed with the theoretical prediction. The
experiments for further outward configuration with

0.08

0.3

0.06
0.2
0.04
0.1
0Πp,n_LM

30.1trtransition area and the density fluctuation
significantly suppressed in wide frequency region.
The energy confinement time slightly increased
after the transition.

0.4

Critical J xB [N/m ]Fig. 1 The temporal behaviors of (a) VE and IA, (b) theline-averaged density and (c) the spectrum of theelectron-density fluctuation.

0.02

3.5

3.6

3.7

3.8

3.9

0Rax [m]Fig. 3 The configuration dependence of the critical
driving force and the value of the local maximum
in the poloidal viscosity.

Rax = 3.75 m was carried out but the transition was
not triggered under the capability of the DC power
supply (650 V/ 23 A). This implies that the larger
driving force is required for outward configurations
and is consistent with the theory.
In the future work, the transition criterion will becompared with that of the other toroidal devices toadvance understanding of the dependence of thepoloidal ion viscosity on the ripple structure.
This work was supported by a NIFS grant
NIFS09KLPR002 and by a grant for scientificresearch from the Ministry of Education, Scienceand Culture of Japan (23760812).
References[1] K. C. Shaing: Phys. Rev. Lett. 76 (1996) 4364.
[2] R. J. Taylor et al.: Phys. Rev. Lett 63 (1989) 2365.
[3] H. Takahashi et al.: Plasma Phys. Control. Fusion 48(2006) 39.
[4] S. Kitajima et al.: Nucl. Fusion 51 (2011) 083029.