H. Weisen 1 21st IAEA FEC, Chengdu 2006

Peaked Density Profiles in Low Collisionality H-modes in JET, ASDEX Upgradeand TCV

H. Weisen, C. Angioni, M. Maslov, A. Zabolotsky, M. Beurskens, C. Fuchs, L. Garzotti, C. Giroud, P. Mantica, D. Mazon, L. Porte, J. Stober, EFDA-JET contributors, ASDEX Upgrade Team and TCV Team 21st IAEA FEC 20.10.2006

TCV

H. Weisen 2 21st IAEA FEC, Chengdu 2006

OUTLINE

• Motivation• AUG-JET combined density profile database• Most relevant and significant variables governing density

profile peaking• Regressions and ITER extrapolations• Density profiles under intense electron heating• Impact on fusion performance• Conclusions

Peaked Density Profiles in Low Collisionality H-modes in JET, ASDEX Upgrade and TCV

H. Weisen 3 21st IAEA FEC, Chengdu 2006

Importance of density peaking

Peaked density profile more fusion powerPfusnDnTv n2T2 p2 for 7T20 keV

Peaked density profiles more bootstrap current.

Peaked density profiles higher core density for given edge density.

Peaked density profiles may compensate for lower than expected density limit in ITER (Borrass, NF 2004)

Peaked density profiles prone to neoclassical impurity accumulation at high Z and/or at low anomalous transport (e.g. C.Giroud, EX / 8-3)

H. Weisen 4 21st IAEA FEC, Chengdu 2006

Separate studies in AUG & JET

• Density profiles in ELMy H-mode more peaked at low collisionality eff=10-14ZeffR0 neTe

-2 (SI,eV)

C. Angioni et al, PRL 90 (2003) 205003 H. Weisen et al, NF 45 (2005) L1-L4

10−1 1

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

eff at r=0.5

n e(0

.4)/n e

(0.8

)

2<q95

<33<q

95<4

4<q95

<55<q

95<6

6<q95

<7

ITER referencescenario (inductive)

COMBINED DATABASE (2006)

277 JET H-modes, 343 AUG H-modes

Reduced colinearities between physics variables

eff

at r=0.5

H. Weisen 5 21st IAEA FEC, Chengdu 2006

Dimensionless physics variables

• Fundamental parameters from drift wave theoryr* = 4.3710-3 (meff Te)1/2/(aBT )

eff = 210-14 ne R0/Te2

= 4 10-3p/BT2 (as used by ITPA)

• Dimensionless NBI source term from diffusion-convection equation in steady state

• Additional variables: NGR ,q95,Te(r=0.2)/Te, R0)• Flux due to edge neutrals in core region poorly known,

but typically one order of magnitude below NBI flux (Zabolotsky NF 2006, Valovic NF lett. submitted). Not included here.

)(00 VnD

R

n

nR

)(1

VnDn

n

)(

1V

nDn

n

)(

1V

nDn

n

dr

dT

T

R

Q

Q

QDT

nD

R

TOT

NBI

NBI

00 2*

H. Weisen 6 21st IAEA FEC, Chengdu 2006

Consistent definition for peaking

• Peaking factor: ne(r=0.2)/ne• But different diagnostics & different analysis on JET and AUG

systematic errors large errors on regressions• Method: JET density profiles from interferometry remapped

onto (virtual) AUG interferometer geometry, JET & AUG inverted using same geometry and same set of basis functions

• JET original and remapped/inverted agree within 2%, validates virtual interferometer method.

• Systematic errors may exist for other variables. – Introduced R0 as a device label. – If regressed variable scales with R0 that may indicate possible

systematic errors in variables or inadequate choice thereof

H. Weisen 7 21st IAEA FEC, Chengdu 2006

Bivariate correlations

• Wide variety of discharges conditions, with and without beam fuelling

• Correlation of lneff with NGR=ne/nGR is strong

• Correlations of lneff with * and r* in combined database are weak

0.1 0.1 0.110 10 101 1 1

0.2

1 0.01 0.25

00

ITER

ITER

ITER

H. Weisen 8 21st IAEA FEC, Chengdu 2006

Bivariate correlations

• Density peaking increases as eff drops, even in absence of NBI fuelling

• Greenwald fraction nearly as correlated with density peaking as ln(eff)

• Peaking in NBI-only discharges correlates with source parameter

• Correlations with r*, q,Te(0.2)/Te, and ceE are insignificant

0.1 10eff

1 0.2 1

1 1 1

2 2 2

0 0.3

0.52

ITE

R

ITE

R

ITE

R

H. Weisen 9 21st IAEA FEC, Chengdu 2006

Multivariate regressions

• ne2/ne=a0+iaiXi and ne2/ne=a0iXiai

• 1 ne2/ne 2 both forms equivalent• Tested many combinations of variables• Criteria*:

1. Statistical relevance of variable i StRi=aiSTD(Xi)/STD(ne2/ne)

(How much does the variation of variable i contribute to the variation of ne2/ne?)

2. Statistical significance StSi=ai /STD(ai)(How well is the coefficient of variable i determined?)

3. RMSE of fit(How good is the fit?)

* O. Kardaun, Classical Methods of Statistics, Springer Verlag, 2005

H. Weisen 10 21st IAEA FEC, Chengdu 2006

Multivariate regressions

• Strong correlation between eff and NGR

regress with only one and both, with and without device label R0

(details: see poster/paper EX/8-4 or C.Angioni NF lett. submitted)

• Summary of multivariate study:

eff is the most relevant whenever included in a fit (mostly also most significant)

* is relevant and significant whenever included

NGR, R0 and/or r* become significant and relevant only if eff is excluded

may be significant or not depending on other variables. Small contribution.

q95,Te(r=0.2)/Te, always insignificant and irrelevant

H. Weisen 11 21st IAEA FEC, Chengdu 2006

Multivariate regressions

ne2/ne = 1.350.015 –(0.120.01)lneff +(1.170.01)* – (4.30.8)ITER: 1.45

• All fits including eff predict peaked profile for ITER ne2/ne >1.4

• All fits excluding eff predict flat profile for ITER ne2/ne ~1.2

• However theory (dimensionless scaling) and appearance of strong R0 dependence when eff omitted, suggest that it is wrong to exclude eff.

• JET/AUG study therefore suggests that ITER will have ne2/ne >1.4

EXAMPLE

H. Weisen 12 21st IAEA FEC, Chengdu 2006

Other parameters

• Combined database does not (yet) have Ti and local shear, nC, but subset of JET data does.

• Impurities generally not more peaked than electrons, carbon even significantly less (Giroud EX/8-3).

• No correlation of ne2/ne and li or local magnetic shear from polarimetry (at odds with theory and with L-mode results elsewhere)

• No correlation between ne2/ne and Te2/Te (at odds with theory)

• Evidence for thermodiffusion :weak dependence on Ti/Te in JET subset

• Ti/Te influence qualitively consistent with theory: low Ti/Te flatter profiles

• Coefficient for source in fit for R/Ln at mid-radius provides experimental value for /D~1.5 consistent with anomalous transport theory (Garbet, 2004)

Rne/ne=0.970.34-(0.650.1)lneff+(1.460.63)D +(0.650.4)Ti/Te

ITER: Rne/ne2.6, ne2/ne 1.46

fnb=Pnbi/Ptot

H. Weisen 13 21st IAEA FEC, Chengdu 2006

Peaked, purely electron heated H-modes with N=2 in TCV

• Theory suggests that strong TEM may reduce or completely remove density peaking by outward thermodiffusion (Garbet, PPCF 2004)

• Flattening with core ECRH observed in several devices, i.e. TCV L-modes (Zabolotsky EX/P3-7) and often attributed to TEMs.

• Suggest -heated ITER may have flat density profile• Flattening not seen in JET ICRH H-modes, possibly due to low power

• Recent 1.5 MW ECRH-heated TCV H-modes at eff0.4 are peaked despite Te/Ti 2 at N 2

(L. Porte, EX/P6-20)

• Weak Te/Ti influence on JET and these TCV results suggest thermodiffusive density flattening not significant in ITER, which will be closer to equipartition.

OH

ECRH

TCV H-modes

H. Weisen 14 21st IAEA FEC, Chengdu 2006

Implications for fusion power

• Pfus increases by 30% for ne2/ne=1.46 (ITER)at constant and nD,T

• For inductive reference Q=10 scenario (Polevoi 2003), auxiliary heating can be reduced from 40MW with flat profile to 15MW.

Q~30 if E unchanged!

• No correlation between ne2/ne and dimensionless global energy confinement time ceE:

we expect current confinement predictions for ITER to hold, even if density peaking has is not explicitly accounted for in scaling laws for E

Ti profile as in inductive reference scenario assumed

Improvement less strong if Ti profile is broader

H. Weisen 15 21st IAEA FEC, Chengdu 2006

Pressure profile merit factor

• Fusion power for fixed maximized for dlnv/dlnTi=2 (around 10keV)

pfus p2, Pfus p2dV =p2V Pressure profile merit factor p2/p2

• Density profile contribution to merit factor is22

22

TT

pp

p2/p2 and density contribution increase towards lower collisionalities• Effect of density peaking not cancelled by temperature flattening• Regression for ITER : p2/p2 ~1.55 and p2T2/(p2T2)1.25

JET

ITE

R

ITE

R

H. Weisen 16 21st IAEA FEC, Chengdu 2006

Conclusions

• Dominant contribution to density peaking in H-mode is anomalous

• Collisionality is most significant variable

• NBI fuelling in JET and AUG also significant

• Scaling with eff , Te/Ti and beam source consistent with D/eff~2/3

• Scaling does not follow simple theoretical expectations with q95,li,Te(0.2)/Te

• NBI-free, ECH heated H-modes in TCV with N=2 and Te/Ti=2 show that

peaking is not suppressed by electron heating

• Extrapolations to ITER predict ne2/ne 1.4

• Pressure profile merit factors p2/p2 and p2T2/(p2T2) increase towards low eff similarly to ne2/ne

• ~30% extra fusion power due to density peaking in ITER inductive reference scenario (fixed and nD,T)