Edge Pedestal Profile Characteristics of H-Mode Discharges in ASDEX Upgrade
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Transcript of Edge Pedestal Profile Characteristics of H-Mode Discharges in ASDEX Upgrade
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Edge Pedestal Profile Characteristics of Edge Pedestal Profile Characteristics of H-Mode Discharges in ASDEX UpgradeH-Mode Discharges in ASDEX Upgrade
Philip Schneider
Special Thanks to E. Wolfrum, J. Boom, B. Kurzan and the ASDEX Upgrade Team
Joint European Research Doctoral Network Joint European Research Doctoral Network in Fusion Science and Engineeringin Fusion Science and Engineering
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Why is the plasma edge so important?
What is the edge pedestal?
How do we access the edge pedestal at AUG?
What is done to evaluate the data?
Which conclusion can be drawn from the analysis?
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H-MODEH-MODE
Typical for H-Mode: ↳ steeper gradients↳ reduced turbulent transport in
the steep gradient region
↳ Er well increased
Various models available Most models explain gradient with
turbulence reduction by ExB shear
L-Mode
H-Mode
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Core Temperature Depends on Core Temperature Depends on Edge TemperatureEdge Temperature
The core plasma performance depends on the edge temperature
⇒ reaching fusion relevant temperatures in the plasma core is coupled with the edge performance
W.Suttrop et al., PPCF 1997
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ELMs limit the Gradients possible ELMs limit the Gradients possible in the Pedestal Regionin the Pedestal Region
Edge localized modes (ELMs) are present in “every” H-Mode discharge Peak power load on the wall Limit to the pressure gradient achieved at the plasma edge
⇒ understanding of ELMs is crucial ↳ to run ITER without destroying the first wall↳ to predict the pedestal of ITER
Best theory for ELMs at hand ⇒ peeling-ballooning-theory
current-driven peeling mode
pressure-driven ballooning modeELM crash
Both instabilities depend on the edge values of Te, ne and their gradients
Connor et al. Phys. Plasmas 1998
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My Work as a PhysicistMy Work as a Physicist
analyse raw data
build database for measuredplasma parameters (Te, Ti, ne, …)
compare with existing
theory empirical models
empirical models
find new
design and conduct
new experiments
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The H-Mode is Characterized by a The H-Mode is Characterized by a Steep Gradient at the EdgeSteep Gradient at the Edge
Pedestal Top
Pedestal / Edge Transport Barrier
Pedestal Bottom
Pedestal Width
Gradient
Experimentally challenging: ↳ 2 orders of magnitude in Te, Ti (10 – 103 eV)
↳ 1 order of magnitude in ne (0.5 – 10 · 1019 m-3)
↳ small spatial scale of few cm (1.5 – 2.5 cm)
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Robust Criteria Needed to Robust Criteria Needed to Describe Pedestal Describe Pedestal
pedestal top value (Te,ped)
pedestal bottom value (Te,sep)
pedestal width (Te,ped)
maximum gradient (Te,ped)
⇒ robust criteria are needed to define these parameters for a wide range of different discharge properties
The edge pedestal is described with:
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Edge Diagnostics at AUGEdge Diagnostics at AUG
Li-Beam: ne
(active measurement with Li atoms)
↳ (+) 1 kHz, sees the whole pedestal
↳ (–) ne<9·1019m-3
ECE Radiometer: Te
(passive measurement of electron cyclotron emission)
↳ (+) 32 kHz, whole radius↳ (–) shinethrough, cut-off
Thomson Scattering: Te & ne
(active measurement with laser beams)
↳ (+) Te & ne at the same radial position
↳ (–) 20-120Hz, sees not always the whole pedestal
Edge Charge Exchange: Ti
(active measurement with NBI heating beams)
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Equilibrium Reconstruction Equilibrium Reconstruction Increases Uncertainties Increases Uncertainties
Alignment of the diagnostics is very important ↳ calculate quantities (βp, Pe, …), which depend on several measured parameters
(Te, ne, Ti, …)
Aligning these diagnostics can be done with equilibrium reconstruction
Thomson Scattering (VTS) is used for aligning Te and ne ↳ only possible with an acceptable accuracy for few dedicated discharges
↳ not perfect: 0 - 5mm shift between diagnostics~ 0.25 Te,ped
⇒ pedestal definition must not be based on the equilibrium reconstruction
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ped-width
2Line-fit↳ (+) good reproduction of
pedestal top and gradients↳ (o) pedestal bottom predefined
↳ (+) Te,bot = Te,sep = 100eV
A. Kallenbach et al., JoNM 2005
↳ (-) ne,bot ?
↳ (-) not good for modeling
Definitions of Tanh- and 2Line-FitDefinitions of Tanh- and 2Line-Fit
Tanh-fit↳ (+) yields width without further
assumptions↳ (+) smooth function for modeling↳ (-) symmetric function – data is
not always symmetric↳ (-) fit to pedestal is influenced
by data in the SOL
ped-top
ped-bottom
ped-width
3
210 tanh
c
rcccfit
offset
height shift on r-axis
width
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Both Tanh- and 2Line-Fit can be Both Tanh- and 2Line-Fit can be goodgood
tanh line
Te,ped
[keV]0.43±0.02 0.45±0.04
Te,ped
[cm] 2.1±0.1 1.7±0.3
Te,ped
[keV/m]-23±10 -21±4
tanh line
ne,ped
[1019m-3] 6.5±0.3 6.7±0.4
ne,ped [cm] 2.1±0.2 1.8±0.2
ne,ped
[1019m-4]-311 -307
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Tanh-fit is Strongly Influenced by Tanh-fit is Strongly Influenced by Data Outside PedestalData Outside Pedestal
TANH is a symmetric function <--> the data is often not symmetric
⇒ tanh is combined with polynomials outside the pedestal↳ this cannot always compensate for asymmetry in the data
⇒ tanh-fit can lead to “wrong” results for the pedestal Boundary conditions always influence tanh -> more scatter
tanh line
ne,ped 8.5 · 1019m-3 8.0
ne,ped 2.8 cm 1.6
ne,ped -330 · 1019m-4 -380
line fit
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LINE Fit Reproduces Pedestal LINE Fit Reproduces Pedestal Width Better Than TANH FitWidth Better Than TANH Fit
Pedestal width measured with↳ LINE Fit
Te,ped [1.5,2.3] cm
ne,ped [1.4,2.1] cm
↳ TANH Fit Te,ped [1.2,2.9] cm
ne,ped [1.2,2.6] cm
⇒ LINE Fit yields less scatter in measurements of discharges with the same properties
here: Ip=1.0MA, Bt=-2.5T, Pheat=6.5MW, constant gas fuelling and plasma shape
Since the pedestal top values are pretty accurate, this is also true for gradients
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TTee Increases with P Increases with Pheatheat
Pedestal top:
Te increases with Pheat
Te decreases with ne at constant Pheat
⇒ very important to distinguish between various dependencies,
here Te(Pheat,ne,…)
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Increasing the Density does not Increasing the Density does not Increase the PressureIncrease the Pressure
constant Ip=1.0MA, Bt=-2.5T, Pheat=8.5MW, shape variing gas fuelling level
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Summary and OutlookSummary and Outlook
Any systematical comparison between the pedestal of different discharges has to be independent of the equilibrium
The fit with two straight lines has advantages over the tanh fit concerning the reproduction of pedestal widths and gradients(scatter reduced by a factor of ~2)
Feed the database Do better filtering Include more measurements of the pedestal
↳ especially Ti and vtor
Compare with scalings found at other machines↳ e.g. ped √βp (P. Snyder 2009)
Modeling = test data against theory
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peeling & ballooningpeeling & ballooning
ballooning mode:↳ pressure driven↳ ballooning stability parameter:
peeling mode:↳ edge current driven↳ bootstrap current:
ePdr
dp
B
Rq
2
2
2
PB
jBS
1
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ECE measured frequencies
FPP, EQI, EQH
R, z
FPP, EQI, EQH
p
Different Equilibria Result in Different Equilibria Result in Radial Shifts of the Data Radial Shifts of the Data
radial shift due to different equilibria of up to 4mm (~0.2 Te,ped)
shape is preserved⇒ pedestal definition must not be based
on the equilibrium or p
24163: t =1.90-2.05s during Raus-scan
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New edge CXRS: Ti
Thomson scattering: ne, Te
ECE: Te
Li-beam: ne, Ti, ni, ne
Li-beam optics, passive He II: Er
Reflectometry: ne (HFS, LFS), ne
Doppler Reflectometry: Er
~
~
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Understand physics by looking at Understand physics by looking at many discharges -> DBmany discharges -> DB
obvious dependencies ↳ heating vs. Te top↳ fuelling vs. ne top↳ delta Te vs. heating↳ delta felm vs elmtype in same Raus-scan
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Phase in RPhase in Rausaus-Scan Does Not -Scan Does Not Influence the Pedestal widthInfluence the Pedestal width