Characterization of density fluctuationsduring ELMs in the DIII-D tokamak

Abstract

Bursts of turbulence associated with ELMshave been studied systematically in DIII-D with a multichannelphase contrast imaging (PCI) diagnostic, which is sensitive to the longpoloidal wavelength components of the density fluctuations in the outeredge of the tokamak. A comparison of the temporal dynamics of theturbulence with the signature Dα signal from the divertor has revealedsystematic differences between type I and type III ELMs: even thoughprecursor fluctuations are sometimes seen before type I ELMs, the PCIsignal level remains high until the peak in the Dα signal; bycontrast, in type III ELMs the fluctuation burst precedes theDα peak by 0.4-0.6 ms. Type I ELMs can generate `echoes',i.e. secondary bursts, in the scrape-off layer. Coherent modes areobserved during type III ELMs only. The radial and temporal correlationstructures and the spectral properties of the turbulence during thetransient ELM phase have been reconstructed by averaging over multipleELMs, in order to improve the statistical accuracy. ELM turbulence isfound to share many properties with L mode turbulence, including themain qualitative features of radial wavenumber and frequency spectraand radial dispersion relations. However, features unique to ELMturbulence are also identified.