UCLA. Broadband Magnetic and Density Fluctuation Evolution Prior to First ELM in DIII-D Edge Pedestal. Presented by G. Wang a, In collaboration with

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1 Broadband Magnetic and Density Fluctuation Evolution Prior to First ELM in DIII-D Edge Pedestal Presented by G. Wang a, In collaboration with W.A. Peebles a, P.B. Snyder b, T.L. Rhodes a, E.J. Doyle a, T.H. Osborne b, G.J. Kramer c, R. Nazikian c, M.A. Van Zeeland b, L. Zeng a, X. Nguyen a, K.H. Burrell b, R.J. Groebner b, A.W. Leonard b, E.J. Strait b, M.E. Fenstermacher d a University of California, Los Angeles, CA b General Atomics, San Diego, CA c Princeton Plasma Physics Laboratory, Princeton, NJ d Lawrence Livermore National Laboratory, Livermore, CA Presented at the 48th Annual Meeting of the Division of Plasma Physics American Physical Society Philadelphia, Pennsylvania October 3-November 3, 26 UCLAUCLA UCLA

2 Motivation Understanding H-mode edge pedestal is a key fusion plasma issue - Pedestal height strongly impacts core confinement and consequently overall fusion performance - High priority for ITER Recent progress shows that Peeling-Ballooning model can explain ELM onset and provide constraints on pedestal [P.B. Snyder, H.R. Wilson, et al., Phys. Plasmas 9 (22) 237 & Nucl. Fusion 44 (24) 32.] However, pedestal transport and therefore its evolution is not understood - Experimental turbulence measurements needed for pedestal and SOL transport models The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 2

3 Summary of results Broadband density fluctuations localized to the edge pedestal have been observed to increase significantly prior to first ELM This broadband density fluctuation increase shows signature of correlation with observed increase in magnetic fluctuations measured by magnetic loops Increased density and magnetic fluctuations both exhibit a ballooning character Increase in pedestal density fluctuations correlated with increased total pressure gradient and reduced dp e /dt of electron pedestal pressure P e ELITE MHD stability code analysis indicates edge pressure gradient exceeds nominal ideal MHD ballooning stability limit at same time as observed magnetic turbulence increase The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 3

4 Broadband localized density fluctuations in pedestal and magnetic fluctuations increase significantly prior to first ELM δn/n (%) Reflectometer detection location at outboard midplane, in the steep P e region Contour of reflectometer phase fluctuation spectrum Contour of magnetic fluctuation spectrum from magnetic loops 42 4e Reflection located ~ 1cm inside the separatrix, ±.25 cm - Measured phase fluctuations from reflectometer in conjunction with PPPL 2-D reflectometer code provide quantitative value of n/n Magnetic fluctuation is from outboard midplane magnetic loops 3 Density (1 19 m -3 ) Dα (a.u.) time (ms) The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 4

5 Increased broadband density fluctuation in pedestal shows signature of correlation with magnetic fluctuations γ2 Auto-power (a.u.) Reflectometer phase fluctuation Magnetic fluctuation average over ms average over ms time (ms) Coherence ( 2 ): frequency resolved linear correlation between two signals. E.g., 2 =1: perfectly correlated, 2 =: no correlation Coherence of phase fluctuation and magnetic fluctuation increased in the frequency range where enhanced fluctuations are observed - Phase fluctuation down-sampled to 2 khz sampling rate of magnetic signal Contour of reflectometer phase fluctuation spectrum Contour of magnetic fluctuation spectrum from magnetic loops 42 4e-6 12 The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 5

6 Increased density and magnetic fluctuations both exhibit a ballooning character Magnetic loops Outboard midplane Inboard midplane 125 Interferometer denv1 denv Contour of reflectometer phase fluctuation spectrum Contour of Interferometer density fluctuation (denv2) Dα time (ms) Power spectrum (a.u.) average over ms denv2 denv1 denv2 denv The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 6

7 Increase in pedestal density fluctuations correlated with increased total pressure gradient and reduced dp e /dt of electron pedestal pressure P e (1 19 m -4 kev) (1 19 m -3 kev) δn/n (%) Contour of reflectometer phase fluctuation spectrum Dα (a.u.) Maximum P tot gradient Pedestal Pe time (ms) 42 2e-5 Pedestal pressure determined by averaging over 3 adjacent Thomson and/or CER data points in time P tot shows an increase nearly coincident with change in pedestal turbulence - consistent with P tot drive of ballooning type modes Reduced dp e /dt of pedestal P e also coincident with change in pedestal turbulence - consistent with increased electron transport from increased turbulence The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 7

8 Edge pressure gradient exceeds nominal ideal MHD ballooning stability limit at same time as observed magnetic turbulence increase Edge current [(jmax+jsep)/2<j>] Normalized pressure gradient (α) δn/n (%) nominal ideal MHD ballooning stability limit Dα (a.u.) time (ms) Stability boundary Operating point Edge pressure gradient exceeds nominal ideal MHD ballooning stability limit ELITE code used to evaluate Peeling-Ballooning stability Suggest that as edge pressure gradient exceeds nominal ideal MHD ballooning stability limit, turbulence modes with magnetic fluctuation components, e.g. maybe kinetic/resistive ballooning modes, are driven unstable The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 8

9 Summary Broadband density fluctuations localized to the edge pedestal have been observed to increase significantly prior to first ELM This broadband density fluctuation increase shows signature of correlation with observed increase in magnetic fluctuations measured by magnetic loops Increased density and magnetic fluctuations both exhibit a ballooning character Increase in pedestal density fluctuations correlated with increased total pressure gradient and reduced dp e /dt of electron pedestal pressure P e ELITE MHD stability code analysis indicates edge pressure gradient exceeds nominal ideal MHD ballooning stability limit at same time as observed magnetic turbulence increase The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 9

10 Backup VGs The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 1

11 Quantitative, edge pedestal density fluctuation measurements from fluctuation reflectometer system Launch Isolator Isolator Mixer Isolator Receive LO Directional Coupler DIII-D reflectometer system: high time (<1 s) and spatial resolution (sub-cm),high sensitivity, non-perturbative, k < 6 cm -1, launch and receive antennas at outboard midplane RF E(t)=A(t)e jφ(t) Gunn Oscillator - Phase fluctuations of received microwave electric field provide qualitative indicator of evolution of localized density fluctuations - PPPL 2-D reflectometer code utilized to obtain quantitative value of n/n from measured microwave electric field The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 11

12 The Peeling-Ballooning Model and ELITE Peeling-Ballooning model for ELM onset and constraints on pedestal ELMs caused by intermediate wavelength (n~3-3) MHD instabilities Both current and pressure gradient driven, non-local Complex dependencies on, shape etc. ELITE code developed to efficiently evaluate P-B stability Extensively benchmarked against other MHD codes, includes rotation Though stability is non-local, can roughly characterize it with local p -j diagrams Extensive validation against experiment (>1 discharges) Discharge generally stable well before ELM, crosses P-B bound before ELM occurs [P.B. Snyder, H.R. Wilson, et al., Phys. Plasmas 9 (22) 237 & Nucl. Fusion 44 (24) 32.] The 48th Annual Meeting of the APS-DPP Philadelphia, PA Oct. 3 - Nov. 3, 26 Wang 12