Edge pedestal profile characteristics of h mode discharges in asdex upgrade
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Joint European Research Doctoral Network in Fusion Science and Engineering. Edge Pedestal Profile Characteristics of H-Mode Discharges in ASDEX Upgrade. Philip Schneider Special Thanks to E. Wolfrum, J. Boom, B. Kurzan and the ASDEX Upgrade Team. Why is the plasma edge so important?

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Edge pedestal profile characteristics of h mode discharges in asdex upgrade

Joint European Research Doctoral Network in Fusion Science and Engineering

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

Philip Schneider

Special Thanks to

E. Wolfrum, J. Boom, B. Kurzan

and the ASDEX Upgrade Team

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


  • Why is the plasma edge so important? and Engineering

  • 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?

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


H mode
H-MODE and Engineering

  • Typical for H-Mode:

    • steeper gradients

    • reduced turbulent transport in the steep gradient region

    • Er well increased

H-Mode

L-Mode

  • Various models available

  • Most models explain gradient with turbulence reduction by ExB shear

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Core temperature depends on edge temperature
Core Temperature Depends on Edge Temperature and Engineering

  • 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Elms limit the gradients possible in the pedestal region
ELMs limit the Gradients possible in the Pedestal Region and Engineering

  • 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

pressure-driven ballooning mode

ELM crash

current-driven peeling mode

  • Both instabilities depend on the edge values of Te, ne and their gradients

Connor et al. Phys. Plasmas 1998

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


My work as a physicist
My Work as a Physicist and Engineering

analyse raw data

build database for measured

plasma parameters (Te, Ti, ne, …)

design and conduct

new experiments

find new

compare with existing

empirical models

theory

empirical models

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


The h mode is characterized by a steep gradient at the edge

Pedestal Top and Engineering

Pedestal / Edge Transport Barrier

Pedestal Width

Gradient

Pedestal Bottom

The H-Mode is Characterized by a Steep Gradient at the Edge

  • 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)

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Robust criteria needed to describe pedestal
Robust Criteria Needed to Describe Pedestal and Engineering

  • The edge pedestal is described with:

  • 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Edge diagnostics at aug
Edge Diagnostics at AUG and Engineering

  • 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)

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Equilibrium reconstruction increases uncertainties
Equilibrium Reconstruction Increases Uncertainties and Engineering

  • 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Definitions of tanh and 2line fit

shift on r-axis and Engineering

height

ped-width

offset

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-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-width

ped-bottom

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Both tanh and 2line fit can be good
Both Tanh- and 2Line-Fit can be good and Engineering

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Tanh fit is strongly influenced by data outside pedestal

line fit and Engineering

Tanh-fit is Strongly Influenced by Data 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Line fit reproduces pedestal width better than tanh fit
LINE Fit Reproduces Pedestal Width Better Than TANH Fit and Engineering

  • 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


T e increases with p heat
T and Engineeringe Increases with Pheat

Pedestal top:

  • Te increases with Pheat

  • Te decreases with ne at constant Pheat

⇒ very important to distinguish between various dependencies,

here Te(Pheat,ne,…)

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Increasing the density does not increase the pressure
Increasing the Density does not Increase the Pressure and Engineering

  • constant Ip=1.0MA, Bt=-2.5T, Pheat=8.5MW, shape

  • variing gas fuelling level

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Summary and outlook
Summary and Outlook and Engineering

  • 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Peeling ballooning
peeling & ballooning and Engineering

  • ballooning mode:

    • pressure driven

    • ballooning stability parameter:

  • peeling mode:

    • edge current driven

    • bootstrap current:

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Different equilibria result in radial shifts of the data
Different Equilibria Result in Radial Shifts of the Data and Engineering

24163: t =1.90-2.05s during Raus-scan

ECE measured frequencies

FPP, EQI, EQH

R, z

FPP, EQI, EQH

p

  • 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

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Thomson scattering: and Engineering

ne, Te

New edge CXRS:

Ti

Reflectometry:

ne (HFS, LFS), ne

Doppler Reflectometry:

Er

~

Li-beam optics, passive He II:

Er

Li-beam:

ne, Ti, ni, ne

~

ECE:

Te

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Understand physics by looking at many discharges db
Understand physics by looking at many discharges -> DB and Engineering

  • obvious dependencies

    • heating vs. Te top

    • fuelling vs. ne top

    • delta Te vs. heating

    • delta felm vs elmtype in same Raus-scan

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


Phase in r aus scan does not influence the pedestal width
Phase in R and Engineeringaus-Scan Does Not Influence the Pedestal width

Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics


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