Large Helical Device

On transport characteristics of the edge ergodic layer of LHD

M. Kobayashi, Y.Feng*, S. Morita, K. Sato, M. B. Chowdhuri**,
S. Masuzaki, M. Shoji, Y. Nakamura, N. Yamamoto, M. Tokitani, N.
Ohyabu, M. Goto, T. Morisaki, I. Yamada, K. Narihara, N. Ashikawa, H.
Yamada, A. Komori, O. Motojima and the LHD experimental group
National Institute for Fusion Science, Toki 509-5292, Japan*Max-Planck-Institute fuer Plasmaphysik, Greifswald, Germany**Graduate University for Advanced Studies, Toki 509-5292, Gifu, Japan1
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceImplication of impurity screening at high density operation
Large Helical Device

Even at very high density operation
with more than
1020 m-3& peaked density profile,no significant impurity
accumulation
Zeff < 2

In this paper, a possible
mechanism of impurity retention(screening) is discussed in termsof edge transport.

2
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceLarge Helical Device

Outline of the talk
¾Condition for impurity retention in SOL¾Impurity transport analysis in ergodic layer
3D impurity transport analysis
Comparison with experiments¾ Implication for high Z impurity¾ Summary

3ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceCondition for impurity retention in SOL
Large Helical Device

1D impurity transport model along B field lines
dominant termsmz

V − Vz∂T∂T∂ Vz
1 ∂ Ti nz=−+ mz i+ ZeE// + 0.71Z 2 e + 2.6Z 2 i + ....
nz ∂ s∂t∂s∂sτsion thermal forcefriction

In force balance, impurity velocity becomes,mz

2Z ∇ //Ti

dTi/dsCondition for impurity retention

Vi // > Ciτsmz

RecyclingViIIZ 2∇ //Ti

corefriction>1 Î impurity retention
thermal force⎛ 5 / 2niTiVi //⎞nVii//⎜~⎟∝⎜ κ i T 2.5∇ T T 1.5∇ T ⎟0 i// ii// i ⎠⎝ITC/ISHW 2007, Oct. 15-19, 2007 Toki

TitargetVz // ≈ Vi // + C i

τsLCFS

SOLs

Impurity retention :dense and cold plasma, flow acceleration.
4National Institute for Fusion ScienceBasic field structure: island overlapping → stochastic
Large Helical Device

Connection length / mradialpoloidal
radialLow-orderislands
m/nDivertor legs*

10/2
Edge surface
Layers*10/3
10/4

StochasticRegion*

10/5
10/6
10/7

ConfinementRegion*

10/8

poloidal5*Terminology: N. Ohyabu et al., Nucl. Fusion, Vol. 34, No.3 (1994)
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceLarge Helical Device

3D Impurity transport analysis in ergodic layer

6ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion Science3D modelling of LHD edge region (EMC3-EIRENE)
Large Helical Device

Computational mesh, configuration and installations

wallLCMSPSOLSOLCore plasma

Cross-field transport coefficients•χe=χi=3D roughly holds•spatially constant (global transport)•determined experimentally
ITC/ISHW 2007, Oct. 15-19, 2007 Tokiget•Power entering the SOL•Density on LCMS
•Sputtering coefficient

Divertor
legs

Boundary conditions

C0H,H2tar

Physics model
zStandard fluid equations of mass,
momentum, ion and electron energy
zTrace impurity fluid model (Carbon)
zKinetic model for neutral gas (Eirene)

Example of LHD
helical divertor

EMC3Core, CX-neutral transport, particle source
SOL, EMC3 simulation domain
Vacuum of plasma*

7National Institute for Fusion ScienceFrom thermal-force to friction-dominated impurity transport
Large Helical Device

Thermal force dominant

inward flow
Carbon density distribution

Impurity flow

Increase nenLCMS=2×1019 m-3

Plasma ion
flow nvII(1018 m-3)
1.0

3×10190.1

0.01

4×1019Impurity flow
8friction dominant

ITC/ISHW 2007, Oct. 15-19, 2007 Toki

outward flow

National Institute for Fusion ScienceImpurity retention is effective in edge surface layer.
Divertor legs

Stochastic

Edge surfaceConfinementnz(r) / nzd

Large Helical Device

radial

9ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceSignificant increase of friction force in edge surface layers
Large Helical Device

0.8
60.6
0.4

40.2

20.060465
reff (cm)70

Flow acceleration in ESF due toionization source and particle sinkof short flux tubes

0200

32100

Reduction of T in ESF

10103

6065700ni M
5 / 2ni TiVi //friction~∝thermal force κ 0i Ti2.5∇ //Ti T ∇ //Ti

stochastic

102i101

Sudden increase of friction force in ESF.

100
10-16065
reff (cm)edge surface
70

10ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceImpurity density profiles of different charge states
Large Helical Device

1012

accumulation

Z ≥ +4 Z ≤ +3retention

C+1C(all)

C(all)

C+61011
C+4

C+4

C+1

C+5C+2C+5C+2

C+3

C+61010109

C+3nup=2e19 m-3
0.60

Ionization potential :CIII (C+2) : 48 eV
CIV (C+3) : 65 eV
CV (C+4) : 392 eV
CVI (C+5) : 490 eV

0.65

0.70

big gap

0.60

0.65

0.70

reff (m)reff (m)zClear separation of profile between C+1~C+3 & C+4~C+6

Impurity retention :C+1,C+2,C+3
ITC/ISHW 2007, Oct. 15-19, 2007 Tokiedge surface
layer

nup=4e19 m-3

C+4,C+5,C+6

11National Institute for Fusion ScienceQualitative difference btw w/ & w/o retention (friction term)
Large Helical Device

Modelling
2 104

Exp.

CIIIw/ friction termmonotonicincrease

CIICIV

1 104

AgreementCV00
0 10Exp.

CVI
23456Modelling<ne> (e19 m-3)

2 104

nearly constant

CVw/o friction term

CIIICIVCII
1 104

CVI

By S. Morita, M. Chowdhuri

00
0 1023456Experimental evidence of impurity
retention in ergodic layer

12<ne> (e19 m-3)
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceLarge Helical Device

Implication for high Z impurity

13ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceZ-dependence & neutral penetration lengthLarge Helical Device

Z-dependence

Vz // ≈ Vi // + C iSince τ s ∝ Ti

1.5

τsmz

Z 2∇ //Ti(n Z 2 ) , Vz// becomes Z-independent.

Neutral penetration lengthLocation of ionization source of impurity & bulk plasma :Friction force is effective only whensimp < sH

acceleration of Vi//starts at s=sH

simpsHs(SH, Simp : penetration length of neutral hydrogen& impurity)

divertor

14ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion SciencePenetration length of Hydrogen & Carbon
Large Helical Device

radialIonization profiles

CarbonHydrogen
poloidal

SH >> SC

15
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceImpurity retention will be effective also for high Z impurity (Fe)
Large Helical Device

neutral_penetration
100H10-1

C 50eV10-2

Fe 100eV
C 10eVFe 10eV10-3(ne=1e19 m-3)
10.0

100.0

1000.0Te (eV)

SH >> SC >> SFe
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

16National Institute for Fusion ScienceStrong reduction of FeXX, XXIII radiation was observed at high density.
Large Helical Device~1sflat-top
UP_73503_236 102

nLCFS~2e19 m-3
2#73503
FeXXIII_timetrace(338)_rev #73515#735083e19 m-3
4e19 m-3

5 102

1nLCFS=2e19 m-3

4 102

03 102__4e19 m-3

5accumulation
window?

2 102

4factor of 5decrease

3e19 m-3

3e19 m-3

3

4e19 m-3

1 102

2nLCFS~2e19 m-3

By K. Sato

10
1.0

0
0.5

1.0

1.5

2.0

1.2

1.4

1.6

1.8

2.0

time(s)time(s)

Implication of reduction of Fe+19 & Fe+22 @ high density
17Similar behavior was reported with lower power exp. in Y. Nakamura et al., NF 43 (2003) 219.)
ITC/ISHW 2007, Oct. 15-19, 2007 Toki

National Institute for Fusion ScienceSummaryLarge Helical Device

The impurity transport in the ergodic layer of LHD is analyzed, using the edge transport
code (EMC3-EIRENE) in comparison with experimental data.
¾The 1D impurity model indicates that when the plasma becomes dense and cold, theimpurity is retained at the edge region (screening).
¾The retention effect is demonstrated in the ergodic layer of LHD by the 3D code.
¾The edge surface layer plays an important role to switch on impurity retention.
¾Geometrical advantage of LHD :Ergodic layer surrounding plasma
The retention is effective for all poloidal & toroidal direction.
¾The carbon line radiation measurements in exp. show clear evidence of the impurity
retention in the ergodic layer.
¾The retention model could apply also for high Z impurity :Balance btw friction & thermal force is Z-independent
High Z impurity has shorter neutral penetration length than low Z impurity.
¾In experiments, FeXX & XXIII line radiation intensity decrease at higher density,indicating reduction of Fe in core.
Further experimental confirmation needed :TESPEL injection, in order to separate core & edge transport.
Ar, Ne puff by fixing source rate.
ITC/ISHW 2007, Oct. 15-19, 2007 Toki18

National Institute for Fusion Science