Carine Giroud 1 21st IAEA Fusion Energy, Chengdu 16.10.2006 Carine Giroud 1 IAEA, Chengdu 16.11.2006 Progress in understanding impurity transport at JET

Carine Giroud 1 21st IAEA Fusion Energy, Chengdu 16.10.2006

Carine Giroud 1 IAEA, Chengdu 16.11.2006

Progress in understanding impurity transport at JET

C. Giroud1,

C. Angioni2, G. Bonheure3, I. Coffey4, N. Dubuit5, X. Garbet5, R. Guirlet5, P. Mantica6, V. Naulin7, M.E. Puiatti8, M. Valisa8,

A.D. Whiteford9, K-D. Zastrow1, M.N.A. Beurskens1, M. Brix1,E. de la Luna10, K. Lawson1, L. Lauro-Taroni8, A. Meigs1,

M. O’Mullane9, T. Parisot5, C. Perez von Thun1, O. Zimmermann11 and the JET-EFDA Contributors.

1 2 3 4 5

6 7 108 9 11


• Within ITB region:Transport can be close to neoclassical predictions

Rest plasma regionInward velocity V~Vneo Impurity D >> DneoIter physics group NF 99

• Inside core region W accumulation observed with peaked density profile without central wave heating in ASDEX

• Global observation:plasma with peaked density are prone to accumulation of highly charged impurities

Picture of impurity transport in present devices


Content

• Observation of anomalous impurity transport at JET– Reduction of Nickel peaking by electron heating

• Brief description of recent development in the turbulent transport theory

• Comparison of experiment with theoretical predictions– A transition of the dominant instability driving the transport could explain the difference in Nickel peaking– Experimental test of Z dependence predicted by turbulent transport theory


• Linear relationship assumed between impurity flux and density gradient

• In steady-state conditions and with edge source the local impurity density gradient length:

Experimental determination of transport coefficients

zzzz VnDΓ

rnz

z

z

z

z

DV-

rnn0

z

z

DVR-

Diffusion coefficient

Convection coefficientVz >0 outwards

Peaking factor

R major radius device


Experimental determination of transport coefficients

• Intrinsic impurities such as C: direct measurement of density profile

– measured density gradient determines –RV/D

• Extrinsic impurities injected by laser ablation (Ni) or gas injection (Ne, Ar).

− D and V determined individually by modelling of time evolution of spectroscopic data

− Ni: soft x-ray and VUV − Ne and Ar: soft x-ray and VUV and also from charge exchange spectroscopy


Effect of electron heating on Ni transport

#58144, dominant ion#58149, dominant electron

• Two similar ELMy H-modes:

• Two heating schemes:

•Different gradient lengths:

[M-E. Puiatti PoP 13 2006]

q0>10.1 <eff <0.2 (low collisionality) Bt=3.28T, q95=5.9, 3MW ICRH, 12-14MW NBI

ICRH dominant ion heating: 8 % 3HeICRH dominant electron heating: 20% 3He

Density gradient length R/LnTe gradient length R/LTeTemperature ratio Te/Ti Ti gradient length R/LTi

3.94

0.956.6

5.261

6.6


Two very different Ni profiles

ICRH dominant ion heatingPeaked Ni profile

ICRH dominant electronSlightly hollow Ni profile

[M-E. Puiatti PoP 13 2006]Steady-state profile calculated from D and V


Due to change in Ni transport


– Diffusion increased in centre– Convection reversed at mid-radius

While neoclassical transport unchanged

Reduction in Ni peaking due to anomalous transport

Neoclassical x10

ICRH dominant ion heating

ICRH dominant electron heating

Measurement


Recent development in turbulent transport theory

• Two main electrostatic micro-instability considered ITG/TEM

Microinstability Ion temperature gradient ITG

Trapped electron mode TEM

Direction of propagation

Ion diamagnetic

Electron diamagnetic

Destabilised by R/LTi R/LTe & R/Ln


Recent development in turbulent transport theory

• Three main mechanisms have been identified

Curvature pinch1 Compressibility of ExB drift velocity

Independent on Z and A

Thermodiffusion pinch2

Compression of the diamagnetic drift velocity

Dependent of 1/Z

Pinch connected to the parallel dynamics of the impurity3

Compression of parallel velocity fluctuations produced along the field line by the fluctuating electrostatic potential

Dependent on Z/ATzTz

2[X. Garbet PoP 12 2005]2,3[C. Angioni C PRL. 96 2006]

1[J. Weiland NF 29 1989]1[X. Garbet PRL 91 2003]1[M. B. Isichenko PRL 1996]

1[D.R. Baker PoP 5 1998]1[V. Naulin Phys Rev. E 2005]2[M. Frojdh NF 32 1992]


Pinch mechanisms in theory of turbulent impurity transport

All contribute to the total turbulent pinch

propagation: ITG ion diamagnetic direction TEM electron diamagnetic direction


Illustration of complex Z dependence of turbulent transport

• D and V calculated with the linear version of the gyrokinetic code GS2: - trace impurity considered.- only the fastest growing mode is taken in the quasi–linear model- no neoclassical transport included.

• Complex trend in Z of turbulent transport

specific calculation needed for studied discharge

GS2 [R/LTi=7, R/LTe=6, Te/Ti=0.88]

[C. Angioni]


Peaked Ni profileICRH dominant ion

Slightly hollow Ni profile ICRH dominant electron


R/Ln=3.9R/LTe=4

Te/Ti=0.95R/LTi=6.6

R/Ln=5.2R/LTe=6.Te/Ti=1.

R/LTi=6.6

Different dominant instability for peaked and flat Ni density

ITG dominated TEM dominatedGS2

Steady-state profile calculated from D and V


Ni pinch reversal found for a ITG to R/LTe driven TEM transition

[C. Angioni PRL. 96 2006][M-E. Puiatti PoP 13 2006]

• Investigate transition from ITG to R/LTe driven TEM– Stabilised R/Ln driven TEM: R/Ln=2.– gradually decreasing R/LTi towards stabilisation of ITG modes.

• Reproduce a pinch reversal as observed experimentally

Real

freq

uenc

y of

mos

t un

stab

le m

ode

(cs/R

)

ITG

TEM

V<0V>0

Te/Ti=0.95, R/Ln=2


First results on measured Z dependence of impurity peaking

#66134

Neoclassic

measure-ment

measure-ment

Neoclassic

r/a =0.15

r/a =0.55

Negative C peakingHollow profile

Peaking lower than neoclassical

Stronger Z dependence of peakingin core than at mid-radius

• Ne, Ar and Ni injected in ELMy H-modeq0>1, 0.1 <eff <0.2Bt=2.9T, q95=7, 2MW ICRH, 8.6MW NBI


GS2 peaking in same range as measurements

GS2

Anomalous part:-R(V-Vneo)/(D-Dneo)

GS2 w/o Thermodiffusion

Discharge ITG dominatedR/LTi~5.8, R/LTe~6.3, R/Ln~0.3 and Te/Ti~1.1, *~0.10

GS2

Measurement

Neoclassic


Summary

• JET experiments confirm earlier observations that neoclassical transport is not sufficient to describe impurity transport in bulk plasma

• First comparison between turbulent impurity transport theory and experiments show encouraging results:− A transition in the dominant instability driving the transport could explain the observed reversal of Ni convection− Same range of peaking as calculated by linear gyrokinetic calculation are measured : no strong increase of V/D as a function of Z. Turbulent transport could give the means for controlling heavy impurity peaking in ITER

• JET is set out to systematically compare theoretical predictions with experiment in coming campaign using JET upgraded CXRS capability..


Spare slides


Reduction of Ni peaking calculated with linear GS2

Condition for discharge with peaked Ni densities

• For increasing R/Ln and R/LTe– reduction of peaking calculated

Condition for discharge with flat Ni densities

Transition from ITG to R/Ln driven TEMReduction of the pinch predicted but

no reversal of the pinch[M-E. Puiatti PoP 2006]


58143 58149

58142 58144 58141

LBO Data : inverted SXR emissivity profiles.

slower penetration and more peaked profiles

MC

MH

time [s]


Simulation: line & SXR brightnessess

MH

MCno

rm. b

right

ness

Solid: experimental; dashed:simulation

(core)

(edge)

(core)

(edge)


Effect of electron heating on Ni transport

Pio

ns (M

W.m

-3)

#58144 #58149 • two similar discharges– q0>1– low collisionality 0.1 <eff <0.2

– 2 ICRH heating schemes were applied

dominant ion heating: 8 % 3He conc. dominant electron heating: 20% 3HeBt=3.28T, q95=?, 3MW ICRH, 12-14MW NBI

• Ni transport probed

Documents

Carine Giroud 1 21st IAEA Fusion Energy, Chengdu 16.10.2006 Carine Giroud 1 IAEA, Chengdu 16.11.2006 Progress in understanding impurity transport at JET