Proceedings of ITC/ISHW2007

Improvement of Ion Confinement in Core Electron-RootConfinement (CERC) Plasmas in Large Helical Device
Y. Takeiri, M. Yokoyama, K. Nagaoka, K. Ida, S. Kubo, T. Shimozuma, H. Funaba,
M. Osakabe, K. Ikeda, K. Tsumori, Y. Oka, M. Yoshinuma, S. Morita, M. Goto, K. Narihara,
I. Yamada, K. Tanaka, O. Kaneko, A. Komori, and LHD Experimental Group
National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan(Received 2 November / Accepted 2 November)
An increase in the ion temperature has been observed in the high-energy NBI-heated plasmas with superpositionof the centrally focused ECRH in Large Helical Device. The Ti rise is accompanied by the formation of theelectron-ITB. The transport analysis shows that the ion transport as well as the electron transport is improved withthe reduction of the anomalous transport. The neoclassical ambipolar flux calculation shows the positive
radial-electric field (Er) in the region of the Ti rise, and the Er should suppress the great enhancement of the ripple
transport due to the Ti-rise. These analyses indicate the ion transport improvement in the core electron-rootconfinement (CERC) plasmas. The toroidal rotation is driven in the co-direction by adding the ECRH, and thetoroidal rotation velocity is increased with the Ti rise. A correlation between the Ti rise and the toroidal rotation issuggested.
Keywords: core electron-root confinement, neoclassical transport, anomalous transport, radial electric field,
ion transport, toroidal rotation, electron-ITB

1. Introduction
The helical devices have non-axisymmetric magnetic
field configuration, which induces neoclassical ripple
transport. The neoclassical ion and electron fluxes arestrongly dependent on the radial electric field (Er), thevalue of which is determined by the ambipolarity conditionof these fluxes. The Er also has a great influence on theplasma properties related to both the ripple transport andthe anomalous transport. In major middle-sized helicaldevices such as CHS, W7-AS, and TJ-II, the improvementof the electron transport is commonly observed in the coreelectron-root confinement (CERC) plasmas [1,2], in whichlarge positive Er is observed in the core region. The CERC
plasmas are obtained when the central electrons arestrongly heated with the ECRH (electron cyclotronresonance heating), and they are also observed in Large
Helical Device (LHD) [3-5]. In the CERC plasmas, theelectron internal transport barrier (electron-ITB) with asteep gradient on the Te profile is formed in the core region,and the core electron transport is improved in theneoclassical electron root with the positive Er [1,2,6]. As aresult, high Te is achieved in the CERC plasmas.
The confinement improvement in the neoclassicalelectron root is specific to the helical system, and the iontransport should also be improved in the CERC plasmas inauthor’s e-mail: takeiri@nifs.ac.jp

the theoretical prediction. We have observed a Ti rise in theNBI (neutral beam injection) plasma by applying thecentrally focused ECRH in LHD, in which the Te profileshows the electron-ITB formation. It is thought that the Ti
rise is ascribed to the ion transport improvement in theCERC plasma. The transport analysis and the comparison
with the neoclassical calculation are carried out, and theresults are discussed. In the following, the plasma
properties of the CERC plasmas, which are realized byapplying the ECRH to the NBI plasmas, are presented witha view of the ion confinement improvement.

2. NBI and ECRH Systems
The Large Helical Device (LHD) is the world’s
largest superconducting helical device [7], and it isequipped with NBI and ECRH systems for the plasma
heating. The NBI system consists of three high-energy
negative-ion-based NB injectors [8,9] and a low-energy
positive-ion-based NB injector [10]. The arrangement ofthe NBI systems is illustrated in Fig. 1. The injection
direction of the negative-NBI is tangential while that of thepositive-NBI is perpendicular to the magnetic axis. The
injection energy of the negative-NBI is as high as 180keV
and the plasma electrons are dominantly heated. The total
injection power achieved is 14MW in the negative-NBI. To

Proceedings of ITC/ISHW2007

Electron Temperature (keV)

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increase the ion heating power effectively with thenegative-NBI, high-Z discharges are utilized for thehigh-Ti experiments with Ar/Ne gas puffing [11]. As aresult, the Ti is increased with an increase in the ion heatingpower, and reached 13.5keV [12]. For efficient ion heatingin the hydrogen plasmas, the positive-NBI has been
recently constructed with the injection energy of 40keV,
which dominantly heats the plasma ions, and 6MW of theinjection power was achieved [10]. The positive-NBI is
also utilized for the Ti-profile measurement with the CXS(charge-exchange spectroscopy) along a toroidal line ofsight [13,14], which is better for the measurement in thecentral region than that along a poloidal line of sight. With
this arrangement, the toroidal rotation is also measured.
The ECRH system employs 168GHz, 84GHz and82.7GHz gyrotrons [15]. Each microwave is injected as ahighly focused Gaussian beam using the vertical andhorizontal antenna systems with quasi-optical mirrors. The
beam-waist radius at the focal point is 15-30mm. The
focused location can be changed at 3.5-3.9m of the major
radius on the equatorial plane. The total injection power
achieved is 2.1MW. In the experiments, thesecond-harmonic heating with 84GHz and 82.7GHz
microwaves is utilized at around 1.5T of the magnetic fieldstrength on the axis.

3. Ion Temperature Rise in the CERC plasmas
The core electron-root confinement (CERC) is a
specific feature commonly observed in helical systems, and0.2

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Toroidal Rotation Velocity (km/s)

Fig.1 Arrangement of three high-energy tangential
NB injectors (BL1, BL2 and BL3) and alow-energy perpendicular NB injector (BL4). A
toroidal line of sight from the BL3 injection
port for the CXS system, which utilizes theBL4 beam, is also indicated.

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Fig.2 (a) Electron temperature profiles, (b) ion
temperature profiles, and (c) toroidal
rotation velocity profiles, for plasmas heatedwith the counter-NBI (BL2) and theperpendicular-NBI (BL4) with and without
the ECRH superposition.
no counterpart in tokamaks. In LHD, by adding thecentrally focused ECRH to the NBI plasma, a strongly
peaked Te profile is observed in the core region. Such akind of the electron-ITB (internal transport barrier)formation indicates the improvement of the electron

Proceedings of ITC/ISHW2007

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χ i/Ticonfinement, and the reduction of the electron thermal
diffusivities is recognized together with positive radial
electric field (Er). This means that the improvement of theelectron transport in the core region occurs in theneoclassical electron root. The ion transport is also
expected to be improved in the electron root, and theECRH was applied to the plasma heated by thenegative-NBI and the positive-NBI.
Figures 2(a) and (b) show the electron and ion
temperature profiles with and without the ECRH
superposition, respectively. The electron density is
0.4x1019m-3 and the target plasma is sustained by thecounter-NBI (BL2) and the perpendicular-NBI (BL4). The
applied ECH power is 0.68MW and is focused at aroundρ=0.15. By adding the ECRH the Te profile indicates asteep gradient inside ρ=0.4, which shows the improvementof the electron confinement in the electron root. The Ti isalso increased in association with the Te rise, as shown inFig. 2(b). The ion temperature is measured with the charge
exchange emission of CVI, and the carbon impurity profilein the core region is strongly hollowed due to the impurity
pump-out effect when the ECRH is applied. As a result, thecentral Ti profile is not measured. Although it is unknown
how much the central Ti is increased by the ECRH
superposition, the Ti rise is observed in the mid-radiusregion. It seems that the Ti-rise location is different fromthe Te-rise location.
The toroidal rotation is also measured with the CXS
in the tangential line of sight. Figure 2(c) shows thetoroidal rotation velocity, Vt, for the plasmas with andwithout adding ECRH. It is found that the toroidal rotation
is driven in the co-direction at the Ti-rise location byadding the ECRH. The increase in the toroidal rotation
seems to be correlated with the Ti rise, and the spontaneous
toroidal rotation due to the ECRH applying is suggested tocause the confinement improvement.
The transport analysis based on the power balance
was carried out for the plasmas with and without theECRH applying shown in Fig. 2. Since there is no Ti datain the central region, the Ti profiles are parabolically fittedusing the measured data, as shown in Fig. 2(b). Thus, theion transport in the central region is not discussed here. Asfor the electron transport, since the heat exchange between
the electrons and the ions is almost neglected in such a
low-density plasma, the electron thermal diffusivity isobtained in the whole region. Figures 3(a) and (b) show thethermal diffusivities for the electrons and ions normalized
by the gyro-Bohm factor of Te3/2 and Ti3/2, respectively. Theχe/Te3/2 and the χi/Ti3/2 are regarded as a measure of thedegree of the anomalous transport. As shown in Fig. 3, byadding the ECRH the thermal diffusivity normalized by thegyro-Bohm factor for the ions is reduced in the region of
ρ>0.5 while that for the electrons is not changed in thectr-NBI (BL2)ne=0.4x1019m-3
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1Fig.3 Profiles of (a) the electron thermal
diffusivities normalized by Te3/2, χe/Te3/2, and(b) the ion thermal diffusivities normalized byTi3/2, χi/Ti3/2, for NBI plasmas with andwithout the ECRH superposition shown inFig. 2.
same region. In the central region of ρ<0.5, the χe/ Te3/2 isreduced by adding the ECRH although the change of theion transport in the central region is unknown. Theseresults suggest that the location of the transport
improvement by adding the ECRH is different between theelectrons and ions.
The neoclassical calculation was also carried out forthe plasmas shown in Fig. 2. The used code is theGSRAKE code, and the calculated ambipolar Er is shownin Fig. 4(a). Positive Er is found at around ρ=0.6, whichcorresponds to the Ti-rise location. The value of thepositive Er is not so large, and the obtained electron root isa single solution. The calculated ion thermal diffusivities,
χi, including the Er effect, are shown in Fig. 4(b), and it isfound that the neoclassical χi is not increased with the Tirise. The ripple transport is greatly enhanced without the Er,
and the positive-Er effect suppresses the enhancement ofProceedings of ITC/ISHW2007
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confinement). Therefore, it is thought that the Ti-increase isobserved in the CERC conditions. The location of the Tirise seems to be different from that of the Te rise. The
transport analysis shows that the ion transport
improvement is ascribed to the reduction of the anomaloustransport. The neoclassical ambipolar calculation showsthat the Er is positive at the location of the Ti-rise. Thissuggests that the ion confinement is improved in the CERC
plasma. Considering that the neoclassical ion thermal
diffusivity is not so changed, the experimental ion transport
is dominated by the anomalous transport. The toroidal
rotation is measured with the CXS in a toroidal line ofsight, and a correlation is recognized between the toroidal
rotation and the Ti-rise by adding the ECRH.
The improvement of the ion transport in the CERC isa possible scenario for increasing the ion temperature in thehelical system, and this is experimentally demonstrated inLHD.

GSRAKE (Z=3)χi (m2/s)

Acknowledgements
The authors are grateful to the technical staff in theLHD for the excellent operation of the LHD and the NBI
and ECRH systems. This work has been supported by theNIFS under NIFS07ULBB501.

1ctr-(BL2) with ECH
0.10References

ne=0.4x1019m-3(b)0.20.4ρ

0.6

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1Fig.4 Results of the neoclassical calculation using
the GSRAKE code for plasmas with andwithout the ECRH superposition shown inFig. 2. (a) Radial electric-field profiles, and(b) ion thermal-diffusivity profiles.

the ripple transport due to the Ti rise. Considering that theneoclassical χi is not so changed by the ECRH applying,
the Ti-rise is thought to be ascribed to the reduction of theanomalous transport.

4. Summary
An increase in the ion temperature has been observedin the NBI-heated plasmas with the superposition of thecentrally focused ECRH in LHD. The target plasma isproduced with high-energy NBI, and low-energy NBI isalso utilized for the ion heating and the Ti-profile
measurement with CXS. By adding the ECRH to the NBI
plasma, an increase in Te is observed with a steep
Te-gradient. That indicates the formation of theelectron-ITB, in which the improvement of the electrontransport is recognized in the CERC (core electron-root[1]M. Yokoyama, et al., Fusion Sci. Technol. 50, 327(2006).
[2] M. Yokoyama, et al., Nucl. Fusion 47, 1213 (2007).
[3] Y. Takeiri, et al., Phys. Plasmas 10, 1788 (2003).
[4] T. Shimozuma, et al., Plasma Phys. Control. Fusion 45,1183 (2003).
[5] K. Ida, et al., Phy. Rev. Lett. 91, 085003 (2003).
[6] Y. Takeiri, et al., Fusion Sci. Technol. 46, 106 (2004).
[7] O. Motojima, et al., Nucl. Fusion 47, S668 (2007).
[8] O. Kaneko, et al., Proc. 16th Int. Conf. on Fusion Energy
1996 (Montreal, 1996) Vol. 3 (Vienna, IAEA) p. 539.
[9] Y. Takeiri, et al., Nucl. Fusion 46, S199 (2006).
[10] M. Osakabe, et al., this conference.
[11] Y. Takeiri, et al., Nucl. Fusion 45, 565 (2005).
[12] Y. Takeiri, et al., Nucl. Fusion 47, 1078 (2007).
[13] K. Nagaoka, et al., this conference.
[14] M. Yoshinuma, et al., this conference.
[15] S. Kubo, et al., Plasma Phys. Control. Fusion 47, A81(2005).