Model prediction of impurity retention in ergodic layer and comparison with edge carbon emission in LHD

(Impurity retention in the ergodic layer of LHD)

M. Kobayashi, Y. Fenga, S. Morita, M.B. Chowdhuri, M. Goto, K. Sato, S. Masuzaki, I. Yamada, K. Narihara, H. Funabaa, N. Tamura, Y. Nakamura, T. Morisaki, N. Ohyabu, H. Yamada, A. Komori, O.

Motojima and the LHD experimental group

National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japana Max-Planck-Institute fuer Plasmaphysik, Wendelsteinstrasse 1, D-17491 Greifswald,

Germany

18th PSI 26-30 May 2008, Toledo, Spain

2

Implication of impurity screening at high density operation

Even at very high density operation with more than

1021 m-3 & peaked density profile,

no significant impurity accumulation Zeff < 2

Implication of a possible mechanism of impurity retention (screening) in the edge region.

Neo classical transport model in core region can not interpret it.

3

1D impurity transport model along B field lines

iii

iii

TT

VTn

//5.2

0

//2/5~

>1 impurity retention

Introduction : Condition for impurity retention in 1D model

iz

siiz TZm

CVV //2

////

Condition for impurity retention

forcethermal

forcefriction score SOL

LCFS

targ

et

Ti

dTi/ds

ViII

Recycling

friction ion thermal force

dominant terms

....6.271.01 22

//

s

TZ

s

TZZeE

VVm

s

nT

nt

Vm ie

s

ziz

zi

z

zz

In force balance, impurity velocity becomes,

iz

sii TZ

mCV //

2//

Impurity retention : dense and cold plasma, flow acceleration, suppress //-grad T.

Z-independent

Outline of the talk

1. Introduction : IDB plasma with Zeff < 2

2. 3D impurity transport modelling in LHD

Geometrical effect on impurity transport

3. Comparison with experiments

Density dependence of carbon emission

4. Summary

5

Basic field structure: island overlapping stochastic

poloidal

radial

10/8

10/710/6

10/2

10/3

10/4

10/5

Low-order islands m/n

ConfinementRegion*

StochasticRegion*

Edge surfaceLayers*

*Terminology: N. Ohyabu et al., Nucl. Fusion, Vol. 34, No.3 (1994)

Connection length / m

radial

poloidal

6

3D modelling of LHD edge region (EMC3-EIRENE)

Boundary conditionsBohm condition at divertor platesPower entering the SOLDensity on LCMSSputtering coefficient

SOLPSOL

Core plasma

LCMS

wall

targ

et

EMC3

Div

erto

rle

gs

C0

H,H2

Computational mesh, configuration and installations Core, CX-neutral transport, particle source SOL, EMC3 simulation domainVacuum of plasma*

Cross-field transport coefficients e=i=3D roughly holdsspatially constant (global transport) determined experimentally

Example of LHD helical divertor

Physics modelStandard fluid equations of mass, momentum, ion and electron energyTrace impurity fluid model (Carbon)Kinetic model for neutral gas (Eirene)

7

From thermal-force to friction-dominated impurity transport

Plasma ion flow nvII

Thermal force dominant inward flow

friction dominant outward flow

Carbon density distribution

Impurity flow

Impurity flow

nLCMS=2×1019 m-3

3×1019

4×1019

(1018 m-3)1.0

0.1

0.01

Incr

ease

ne

1010

1011

1012

1013

1014

60 65 70

rad_P08_D0.5_SHI1.5_C3_NUP4

reff (cm)

stochastic

edge surface

0.0

0.2

0.4

0.6

0.8

0

2

4

6

60 65 70

rad_P08_D0.5_SHI1.5_C3_NUP4

reff (cm)

0

1

2

3

4

0

100

200

60 65 70

rad_P08_D0.5_SHI1.5_C3_NUP4

reff (cm)

Edge surface layers : effective retentionStochastic region : flat profile (suppression of thermal force)

Edge surface

Stochastic

0.1

1

10

60 65 70r

eff (cm)

stochastic edge surface

2e19 m-3

3e19 m-3

4e19 m-3

Carbon density

thermal

friction

9

poloidal

radi

al

Te/eV

0

190

Island structure affects plasma transport

m/n=10/3

10/5

10/7

R / cm

Te

/ e

V

* Thomson (t=1.24,1.27,1.3)EMC3/EIRENE*

EMC3-EIRENE

10

Reduction of II-conductive heat flux by -transport

i252 q

dr

dTT

dr

dTn i

iii

i/

1D radial energy transport for ions:

i

i

iT

n

2

25

/

If << 1 (long connection length),-heat conduction makes significant contributions.

Condition for significant reductionof parallel ion heat conduction:

(Y .Feng, Nucl. Fusion 46 2006)

IIdl

dr

rcore SOL

LCMS

targ

et

Ti w

w/o

qII

q II

upstream

downstream

q

~10-4 (LHD)

11

109

1010

1011

1012

0.60 0.65 0.70

rad_imp_P08_NUP4.0

reff (m)

C+1

C+2

C+3

C+4

C+5

C+6

C(all)

109

1010

1011

1012

0.60 0.65 0.70

rad_imp_P08_NUP2.0

reff (m)

C+1

C+2

C+3

C+4

C+5

C+6C(all)

4Z 3Z

nup=4e19 m-3nup=2e19 m-3

edge surface layer

accumulation retention

Clear separation of profile between C+1~C+3 & C+4~C+6

Impurity retention : C+1,C+2,C+3 C+4,C+5,C+6

Impurity density profiles of different charge states

Ionization potential :CIII (C+2) : 48 eVCIV (C+3) : 65 eVCV (C+4) : 392 eVCVI (C+5) : 490 eV

big gap

Spectroscopy

Density dependence of carbon emission : indication of impurity retention

CIII: 977Å, 1S22S2-1S22S2P (VUV monochromators)CIV: 1548Å, 1S22S-1S22P (VUV monochromators)CV: 40.27Å, 1S2-1S2P (EUV spectrometer)

Absolutely calibrated100

101

0 2 4 6 8

CIII/neCIV/neCV/ne

lin. avg. ne (e19 m-3)

Exp.

(a)

101

102

0 2 4 6 8

CIII/neCIV/neCV/neCIII/ne w/o frictionCIV/ne w/o frictionCV/ne w/o friction

ne (1019 m-3)

Model

(b)

CV/ne

CIII/ne

CIV/neCIII/ne w/o fric.

CIV/ne w/o fric.

CV/ne w/o fric.

Other works : Impurity transport analysis of ergodic layer

D.Kh. Morozov et al., POP 2 (1995) 1540.

Importance of friction of background plasma to exhaust impurity.M.Z. Tokar, POP 6 (1999) 2808

Core carbon content (C6+) reduction with decreasing edge Te in Tore Supra.Y. Corre et al., NF 47 (2007) 119

Y. Feng et al. NF 46 (2006) 807 (W7-AS)M. Kobayashi et al. CPP 48 (2008) 255 (LHD)

Prediction of impurity retention @ high density with 3D modelling

Viscosity terms related to thermal flux.

14

Summary

The impurity transport in the ergodic layer of LHD is analyzed, using the edge transport code (EMC3-EIRENE) in comparison with edge carbon emission.

Magnetic field structure of the ergodic layer in LHD : Edge surface layer : short and long flux tubes coexist.Stochastic region : remnant islands with Lc > ~ 100 m.

Plasma transport, particle and energy, follows the island structure as confirmed by the 3D modelling as well as experiments.3D code predicts impurity retention in the ergodic layer :

Flow acceleration in edge surface layersCross-field transport prevents large // T drop suppress thermal

force.Good agreement in carbon emission btw 3D modelling & exp. :

Indication of impurity retention in ergodic layer by friction force.The retention model could apply also for high Z impurity :

Balance btw friction & thermal force is Z-independentHigh Z impurity has shorter neutral penetration length than low Z impurity.