Correlations between Quasi-coherent Fluctuations and the Pedestal Evolution during the Inter-ELM Phase on DIII-D
Authors: A. Diallo, R.J. Groebner, T.L. Rhodes, D.J. Battaglia, D.R. Smith, T.H. Osborne, J.M. Canik, W. Guttenfelder, and P.B. Snyder
Abstract: Direct measurements of the pedestal recovery during an edge-localized mode cycle provide evidence that quasi-coherent fluctuations (QCFs) play a role in the inter-ELM pedestal dynamics. Using fast Thomson scattering measurements, the pedestal density and temperature evolutions are probed on sub-millisecond time scales to show a fast recovery of the density gradient compared to the temperature gradient. The temperature gradient appears to provide a drive for the onset of quasi-coherent fluctuations (as measured with the magnetic probe and the density diagnostics) localized in the pedestal. The amplitude evolution of these QCFs tracks the temperature gradient evolution including its saturation. Such correlation suggests that these QCFs play a key role in limiting the pedestal temperature gradient. The saturation of the QCFs coincides with the pressure gradient reaching the kinetic-balloooning mode (KBM) critical gradient as predicted by EPED. Furthermore, linear microinstability analysis using GS2 indicates that the steep gradient is near the KBM threshold. Finally, the modeling supports the observations and together suggest that QCFs may be KBMs.
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Submitted to: Physics of Plasmas
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Download PPPL-5096 (pdf 2.6 MB 42pp)
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