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L-H transition at low density in ASDEX Upgrade
US-EU TTF Workshop April 5- 9 2011 San Diego
P. Sauter, F. Ryter, T. Pütterich, E. Viezzer, E. Wolfrum, D. Coster, R. Fischer and ASDEX Upgrade Team
• Motivation
• Experimental approach
• Results
Max-Planck-Institutfür Plasmaphysik
Max-Planck-Institut für PlasmaphysikEURATOM Association, Garching, Germany
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Motivation
• Aim: investigate respective role of electron and ion channels in L-H transition mechanism
• Requires decoupling: low density dominant electron heating (or ion heating)
• Corresponds to low density branch of L-H threshold
0
0.5
1
1.5
2
2.5
3
1 2 3 4 5 6 7 8
ECHH-NBIECHD-NBI
P thr
[MW
]ne [1019 m-3]
4He
D
[F. Ryter NF 2009]
ne,min
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Motivation
• Aim: investigate respective role of electron and ion channels in L-H transition mechanism
• Requires decoupling: low density dominant electron heating (or ion heating)
• Corresponds to low density branch of L-H threshold
0
0.5
1
1.5
2
2.5
3
1 2 3 4 5 6 7 8
ECHH-NBIECHD-NBI
P thr
[MW
]ne [1019 m-3]
4He
D
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Motivation
• Aim: investigate respective role of electron and ion channels in L-H transition mechanism
• Requires decoupling: low density dominant electron heating (or ion heating)
• Corresponds to low density branch of L-H threshold
0
0.5
1
1.5
2
2.5
3
1 2 3 4 5 6 7 8
ECHH-NBIECHD-NBI
P thr
[MW
]ne [1019 m-3]
4He
D
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Experimental approach
• Electron heating: ECRH up to 3 MW
• L-H transition reached by: PECRH steps at fixed density Density steps at fixed PECRH
• Edge measurements: Te from ECE and TS Ti and rotation from CXRS with NBI blips (10ms), boron line
• edge toroidal and poloidal systems (1.8 ms) • core (4 ms)
ne: Li beam, TS, interferometer
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Two examples: high Te/Ti at low density
png:
L-Mode DitheringI-Phases
H-Mode
I-Phase: "Intermediate Phase", see G. Conway this workshop, PRL 2011
Low density: 1.5 1019 m-3 High density 6 1019 m-3
Dithering ≈50 ms
26358 25747
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Operation diagram: explored ne and Pheat ranges
• Range of explored density varies by factor of ≈ 6
• Low density: large variation of Pheat because Pthres high
• Higher density: limited PECRH range because Pthres lower
• I-Phases (dithering) somewhat below Pthres (see also G. Conway this workshop and PRL 2011)
ne,ped,min ≈ 3 1019 m-3
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Edge profiles at different times
Profiles have been analyzed in:
L-Mode low and high power
I-Phase
H-Mode
L L I H
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Edge profiles in L-Mode low and high PECH
Strong increase of edge Te with PECRH : clear Te pedestal in L-ModeTi weakly affected
PEC 1.5 MW
PEC 3 MW
ne19 ≈ 1.5
ne19 ≈ 1.5
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I-Phase and H-mode: Ti pedestal forms
Constant PECRH ≈ 3 MW : Te pedestal almost unchangedTi pedestal develops: steeper edge ∇Ti
ne19 ≈ 1.5
ne19 ≈ 1.9PEC 3 MW
PEC 3 MW
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Edge profiles at constant PECRH ≈ 3 MW
Te pedestal unchanged within experimental uncertaintiesTi pedestal develops from L to H-Mode
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Pedestal data: temperatures versus density
• Te,ped at L-H transition increases towards low density: factor of 3
• Te,ped in H-mode only somewhat higher than at L-H
• Ti,ped at L-H transition increases only by ≈1.6 towards low density
• Ti,ped in H-mode clearly higher than at L-H
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Pedestal Te and Ti can be decoupled
Fixed ne,ped ≈ 1.5 1019 m-3 Increase of PECRH up to L-H
Linear increase of Te,ped with Pheatin L-mode up to L-H
Ti,ped seems to saturate
Density dependencefor POH < Pheat ≤ Pthres
L-H
Low density:Te/Ti up to 3 at L-H (PECRH ≈ 3 MW)
Higher density:Te/Ti ≈ 1.5 at L-H (PECRH ≈ 1 MW)
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Edge Te Ti decoupling
Fixed ne,ped ≈ 1.5 1019 m-3 Increase of PECRH up to L-H
Linear increase of Te,ped with Pheatin L-mode up to L-H
Ti,ped seems to saturate
Density dependencefor POH < Pheat ≤ Pthres
L-H
Low density:Te/Ti up to 3 at L-H (PECRH ≈ 3 MW)
Higher density:Te/Ti ≈ 1.5 at L-H (PECRH ≈ 1 MW)
Te/Ti decrease in H-Mode
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Edge pe and pi at L-H versus ne,ped different
• Edge electron pressure at L-H:
30 % variation at most no clear trend increase toward high density?
• Edge ion pressure at LH:
linear increase with edge density => factor of 4 variation
pe,ped ≈ pi,ped at L-H at high density
Both pe,ped and pi,ped higher in H-Mode
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Edge pressure profiles at low and high density
Low density at L-Hpe > pi and ∇pe > ∇pi
High density at L-Hpe ≈ pi and ∇pe ≥ ∇pi
pe
pi
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∇pe,ped and ∇pi,ped at L-H versus density
∇pe,ped and ∇pi,ped at L-Htaken at their maximumconverge with increasingdensity, as expected
Diamagnetic contribution to Erfrom ∇pi/n at L-H transition insame range as yielded byDoppler reflectometry in similardischarges (see G. Conway)
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Analysis of L-mode Te pedestal with SOLPS
Why using SOLPS ?
• Transport calculated in edge and SOL
• Profiles for radial transport (χe) adjusted to match experiment
• Transport along field lines in SOL calculated
determines Te at separatrix no explicit boundary conditions on Te
• Density and Ti also calculated
PECRH = 1.2 MW
Edge ∇Te in L-Mode requires transport barrier in χe
Can only be clear at low densityand if Pthres high
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Edge transport barrier in H and L modes
Transport barrier in H-Mode only somewhat deeper than in L-Modeat same power
PECRH = 1.8 MW
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for ne
x2(±1)x2(±1)x2x430%x1.5x3
∇pi/n∇pi∇pepipeTiTe
Summary
• Dependence of edge parameters investigated in low density with dominant electron heating in L-mode up to L-H and in H-mode • Strong decoupling of Te and Ti achieved at L-H: edge Te/Ti up to 3
• Te in L-mode exhibits clear pedestal at low density and high PECRH edge transport barrier
• Ti pedestal develops clearly in I-Phase and H-Modes
• Overview of parameters variations at L-H transition in density scan: