24T1
Development of Large Helical Device Project
大型ヘリカル装置実験計画の研究展開
Hiroshi Yamada for the LHD Experiment Group
山田弘司、LHD実験グループ
National Institute for Fusion Science, 322-6 Orochi-cho, Toki-shi, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町322-6
The Large Helical Device Project has been playing a unique alternative and complementary role totokamaks and advancing fusion science by a virtuous circle of extension of operational regimes, discovery
and systematic understanding based on accurate analyses. The highlighted achievements in plasma
parameters are 5.1% of , 1.21021m-3 of density, steady-state operation beyond 1keV for 1 hour, and 20keV and 6.4 keV of electron and ion temperatures, respectively, etc. Developed diagnostics and numericalcodes, which can manage multi-dimensional geometry with high space and time resolutions, enable us tochallenge 3 N’s (Non-linear, Non-diagonal, Non-local) and 3-D plasma physics. Achievements and scopesin terms of both extrapolating capability towards a helical DEMO reactor and comprehensive
understanding of toroidal plasmas are discussed. The targeted goal of the Large Helical Device Project is
integration of high-performance plasma and steady-state operation, which leads to academic creation as
well as fusion energy development.

comprehensive understanding of toroidal plasmas.
The key issue to accomplish the mission
emphasized in these days is demonstration of highperformance plasma which convinces us that ahelical plasma can be ignited and clarification of3-D plasma physics which is inevitable and critical
for tokamaks as well.
2. Status of Machine Capability
The superconducting coil system provides themagnetic field up to 3T in steady-state. A heliotron
configuration generated by a pair of helical coils
accommodates a built-in divertor, which looks likea double-null divertor in tokamaks but rotates
poloidally along the toroidal excursion.
LHD employs three heating schemes; NBI (180and 40keV), ECH(77GHz) and ICH(25-100MHz).
Figure 2 shows their capability. The primaryheating source is provided by NBI with up to 29
MW. ECH and ICH play important complementaryroles in the experiment. In particular, ECH (up to 4
Injected Power (MW)

1. Introduction
The Large Helical Device (LHD) has produced
more than 110,000 plasma discharges to datesince the initial operation in 1998. Amongst
large-scale fusion experimental devices, this very
high availability is attributed to intrinsic physics
advantage of a net-current-free helical plasma andthe superconducting coil system [1].
Figure 1 shows a bird’s-eye view of the coils inLHD. LHD employs a pair of helical coils andthree sets of poloidal coils with top-down
symmetry, all which are superconducting andgenerate confinement magnetic surfaces without
any help of net toroidal currents. In addition,
LHD has ten pairs of perturbation coils on the top
and the bottom, which are used for error fieldcancellation and generation of Resonant MagneticPerturbations.

Fig.l A bird’s-eye view of coil systems in LHD.

NBIECH
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1010.1-1

The mission of the LHD project is establishmentof scientific basis for a helical DEMO reactor and0123

10101010Pulse Length (s)Fig.2 Heating capability as a function of pulse length
10MW) is remarkable in well-focused local heatingand power modulation to generate heat pulse. ICH(up to 2.7MW) plays the leading part in steady-state
operation.

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6Ti4T4e192-32n0

Density (10 m )Temperature (keV)

3. Experimental Achievements
The mission to extend plasma parameters andphysics study in depth and breadth aresynergetically related to each other [2]. Integrationof high performances has progressed in recent
experiments. For example, high  plasmas of about
5 % have been maintained in steady-state for morethan 100 times the energy confinement time.
Robustness of a helical plasma is pronounced in
extremely high density operation beyond 11021m-3
at the moderate magnetic field up to 3T. This
demonstration develops an innovative scenario tomitigate physics as well as engineering demands
Figure 3 shows the radial profiles of ion andelectron temperatures and electron density in a
typical high-ion-temperature plasma. It should be
noted that these fine measured profiles are projectedon the 3-D MHD equilibrium. The ion thermaltransport is improved close to the neoclassical level.
It has been found that enhanced ion temperature
gradient drives the outward convection of
impurities and toroidal rotation [3]. These
Non-diagonal contributions cannot be explained by
the neoclassical theory and have been quantified interms of impurity mass, 3-D geometrical effect(helical ripples), etc.

e-0.6-0.4-0.2 0 0.2 0.4 0.6
Effective minor radius (m)0Fig.3 Radial profiles of ion (closed circles) andelectron (open circles) temperatures andelectron density (open triangles) in a typicalhigh-ion-temperature plasma

Other two N’s, i.e., Non-linear and Non-local arealso highlighted research subjects. Experimental
observations of turbulence and MHD instability
have been compared with large scale non-linear
simulations, which lead to validation of thenumerical analyses and description of consistentphysics pictures. Non-local transport is anunresolved issue which is important to control
plasmas. Long-distance correlation [4], which
would be a major player in non-local transport, hasbeen identified in electron temperature fluctuations(see Fig.4).

Fig.4 Contour of correlation amplitude ofelectron temperature fluctuation on theplasma cross-section

Characterization of 3-D effect has beendeveloped in transport and MHD in the LHD
experiment. 3-D geometry affects viscosity andradial electric field which regulate mean flows andzonal flows, and consequently change neoclassicaland turbulent transports. 3-D effect is also
remarkable in topological change due to magnetic
islands and stochastization of magnetic fields.
4. Future Scope
The upgrade of LHD is planned to bring out its
maximum performance. Closed divertor withcryo-pump will start to work in 2012. Neutral
compression by the modified configuration withbaffles and a dome has been demonstrated in theprovisional experiment on LHD [5]. Heating
capability and the deuterium experiment are alsoplanned
Acknowledgments
The LHD project is conducted under theinitiative of the inter-university researchcooperation. The author is grateful to allcollaborators for their contributions to the project.
Tremendous effort by the LHD operational group isgratefully acknowledged. This work is supported by
the budget NIFS10UMLG001 of the National
Institute for Fusion Science.
References[1] A.Komori et al.: Fusion Sci. Technol. 58 (2010) 1.
[2] H.Yamada et al.: Nucl. Fusion 51 (2011) 094021.
[3] K.Ida et al.: Nucl. Fusion 50 (2010) 064007.
[4] S.Inagaki et al.: Phys. Rev. Lett. 107 (2011) 115001.
[5] S.Masuzaki et al.: Plasma Fusion Res. 6 (2011)
1202007.