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

1. 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

2. 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? Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

3. H-MODE • 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

4. Core Temperature Depends on Edge 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 Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

5. ELMs limit the Gradients possible in 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 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

6. My Work as a Physicist 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

7. Pedestal Top 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

8. Robust Criteria Needed to Describe Pedestal • 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

9. Edge 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) Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

10. Equilibrium Reconstruction 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 Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

11. shift on r-axis 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

12. Both Tanh- and 2Line-Fit can be good Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

13. line fit 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

14. LINE Fit Reproduces Pedestal Width 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 Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

15. Te 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

16. Increasing the Density does not Increase the Pressure • constant Ip=1.0MA, Bt=-2.5T, Pheat=8.5MW, shape • variing gas fuelling level Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

17. Summary 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 Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

18. peeling & ballooning • ballooning mode: • pressure driven • ballooning stability parameter: • peeling mode: • edge current driven • bootstrap current: Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics

19. Different Equilibria Result in Radial Shifts of the Data 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

20. Thomson scattering: 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

21. Understand physics by looking at many discharges -> DB • 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

22. Phase in Raus-Scan Does Not Influence the Pedestal width Philip Schneider - JERDNiFSaE - Edge Pedestal Profile Characteristics