22F07
Comparison study between micro turbulence measured by two dimensional
phase contrast imaging and gyro kinetic simulation in LHD
LHDにおける二次元位相コントラストイメージングにより計測した乱流揺動とジャイロ運動論シミュレーションの比較
K. Tanaka1, C. A. Michael2, L.N. Vyacheslavov3 , D. R. Mikkelsen4, J. Baumgaertel4, G.
Hammett4, M. Nunami1, T.H.Watanabe1, H. Sugama1, LHD experiment group1 et al.,田中謙治1, C. A. Michael2，L.N. Vyacheslavov3 , D. R. Mikkelsen4, J. Baumgaertel4, G. Hammett4,沼波政倫1,渡邉智彦1, 洲鎌英雄1,LHD実験グループ1,他
1National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町 322-6
2Culham Centre for Fusion Enerrgy, Abingdon , Oxfordshire, OX14 3D United Kingdom
カラム核融合エネルギーセンター アビンドン オクスフォードシャー 英国
3Budker Institute of Nuclear Physics, 630090, Novosibirsk, Russia
ブドカー核物理研究所 ノボシビルスク ロシア
4Princeton Plasma Physics Laboratory, Princeton, NJ 08543 USA
プリンストンプラズマ物理研究所 プリンストン ニュージャージー州 米国
Micro turbulence of ion temperature gradient mode (ITG) and trapped electron mode (TEM) regime weremeasured by two dimensional phase contrast imaging (2D-PCI) in Large Helical Device (LHD). Itscharacteristics at ρ=0.4-0.7 were compared with experimentally measured density profile and linear andquasi linear gyro-kinetic calculation by GS2 code. The measured fluctuation is likely to be dominated by
TEM in the peaked density profile, while it is dominated by ITG in the hollowed density profile. GS2
showed ITG dominant for both case, however, larger contribution of TEM was found in the peaked density
profile. Zero flux condition agreed qualitatively between experiment and simulation.

1. Introduction
Control of density profiles is one of theimportant issues for future reactor operation inorder to optimize fusion output power andstabilize magnetohydrodynamic instability andmicroinstabilities. Therefore, it is of utmost
importance to understand the mechanisms
governing particle transport. In this paper, thephysical mechanisms of two clear different
density profiles of LHD are investigated. One ispeaked density profile, which was obtained at
Rax=3.5m, the other is hollowed density profile at
Rax=3.6m. Here, Rax is the magnetic axis position.
Since, the neoclassical properties are almost
comparable at both Rax, the difference of thedensity profile can be caused by the difference of
the anomalous behavior[1]. For this study, twoapproaches are tried. One is fluctuation
measurements by using 2D-PCI [2].. The other isgyrokinetic linear and quasi-linear calculation byusing GS2 code [3]. Linear stability anddependence of particle flux on density gradientwere studied around experimentally achieved
density gradient.

2. Experimental Results
Figure 1 shows radial profiles of Te, Ti , ne,fluctuation phase velocity and their amplitudes atRax=3.5 and 3.6m. The heating power was kept
almost the same, namely powers for Rax=3.5 and3.6 m were 8.1 and 7.4 MW, respectively. Even
though heating powers were almost the same, theresulting profiles were quite different. As shown inFig. 1 (a-1), (a-2), (b-1), and (b-2), the achieved Teand Ti were higher at Rax=3.6 m than at Rax=3.5 m,
and density profiles were peaked at Rax=3.5 m andhollowed at Rax=3.6 m.
In Fig. 1 (c-1) and (c-2), spatial profiles of phasevelocity are shown. Since measured wavenumbers
were dominated by poloidal components,
propagations in ion and electron diamagnetic
directions in the Lab. frame can be identified. In Fig.
1 (d-1) and (d-2), fluctuation amplitudes integrated
over k are shown.
As shown in Fig.1 (c-1,2) and (d-1,2), two peaks
are visible with one being at around |ρ| = 0.4-0.7,
and the other at around |ρ| =1.0. The former
propagates to e-dia. direction at Rax=3.5m and toi-dia. direction at Rax=3.6m. The plasma poloidal
rotation measured by CXRS at closest point isnear zero, thus, the progation direction measured by
PCI is likely to be same as in the plasma frame. Thee-dia. propagating components at Rax=3.5m can beindication of TEM, while i-dia. propagatingcomponents at Rax=3.6m can be attributed to ITG.
The fluctuation around |ρ| = 0.4-0.7 would take animportant effect on the different density profile.
3. Comparison with gyrokinetic calculation
Figure 2 (a) shows growth rate (γ) and real
frequency (ωr ) of peaked density profile and forhollowed density profile. The calculation is done
for the electrostatic fluctuation with nonadiabaticelectrons and finite collisionality. Calculated
location is ρ=0.6 for peaked density profile andρ=0.65 for hollowed density profile. The
normalized density gradient (-1/ne dne/dr) is
positive for Rax=3.5m and negative for Rax=3.6m atthese locations. The calculations were done forkρi=0.1~1, where k is poloidal wavenumber and ρi
is ion Larmor radius. As shown in Fig.2 (a), γ wasclearly higher at Rax=3.5m (peaked density profile)than at Rax=3.6m (hollowed density profile). In
latter case, only limited region with kρi=0.37-0.68
was unstable. The real frequency is both i-dia.

directed for both cases, however, ωr is smaller at
peaked density profile indicating larger contributionof TEM. Figure.1 suggests that dominat turbulencemode is TEM at Rax=3.5 and ITG at Rax=3.6. Thisqualitatively agrees with gyro kinetic calculation.
The particle source is localized at the very edge
region. The Monte Carlo simulation of neutral
penetration showed the peak of the particle source
rate was outside of the last closed flux surface [1].
Thus, the particle balance in the steady state
condition results in the zero particle flux in thesource free core region, which is likely to be insideof ρ=0.9. The calculation was linear, therefore,
saturated fluctuation level cannot be estimated.
However, linear calculation with nonadiabatic
electrons can estimate phase difference between
density and potential, then, direction of the particle
flux can be known. If the target fluctuations, which
are calculated by GS2, determine the particle
transport in source free region, estimated flux by
GS2 should be zero. In order to determine zeroparticle flux condition, the quasi linear particle
fluxes are estimated scanning normalized density
gradient around experimental value. As shown in
Fig.2 (b), particle flux is directed outward at lower
normalized density gradient and inward at higher
normalized density gradient. Zero flux is obtainedin the negative gradient at Rax=3.6m, and positive
gradient at Rax=3.5m. The signs of the gradientagree between experiments and simulation,although there are quantitative differences.
References[1] K. Tanaka et al., : Fusion Sci. Tech. (2010)[2] K.Tanaka, C. A. Michael, L. N. Vyacheslavov et al., :Rev. Sci. Instrum., 79, 10E702 (2008)[3] http://sourceforge.net/apps/mediawiki/gyrokinetics/

Fig.1 Comparison of ne, Te, Ti , fluctuation profiles andspectra for Rax=3.5 m (a-1)-(c-1) and 3.6 m (a-2)-(c-2).
(a-1) and (a-2) are Te and Ti profiles, (b-1) and (b-2) arene profiles, (c-1) and (c-2) are phase velocity profile. In(c-1) and (c-2), poloidal rotation velocity measured byCXRS are shown by white line. and (d-1) and (d-2) are
profiles of fluctuation amplitudes profiles In (d-1) and(d-2), noise levels are shown by thin lines .

Fig.2 Comparison of gyrokinetic calculation (a) growth rate
and real frequency and (b) density gradient dependence ofquasi-linear particle flux. kρi=0.5