Edge Pedestal Profile Characteristics of H-Mode Discharges in ASDEX Upgrade

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1/17 Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics 19.04.23

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

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Edge Pedestal Profile Characteristics of H-Mode Discharges in ASDEX Upgrade