Session CI2A - Edge Transport.
INVITED session, Monday afternoon, November 15
Room Chatham C, SCC
Experimental observations and nonlinear numerical simulations strongly suggest that the radial transport of plasma through the scrape-off layer (SOL) is strongly related to turbulent fluctuation structures, called `blobs', which propagate coherently into the far SOL. These intermittent transport events are not only important for confinement properties of fusion devices but have also a strong impact on divertor heat loads and wall recycling phenomena. Thus, a detailed understanding of the dynamical properties and scaling dependencies of blobs is required. The present work characterizes turbulent SOL fluctuations using turbulence imaging in Alcator C-Mod, along with comparisons to similar imaging data from NSTX. A fast camera system, which records Dalpha light fluctuations of the edge and SOL plasma in the poloidal plane, shows that blobs form close to the separatrix in the high pressure gradient region and propagate into the SOL, thereby causing high radial correlation lengths. The optical measurement in combination with Langmuir probe measurements shows that the radial propagation is a result of the self-consistent plasma potential structure associated with the blobs. Correlations are found not to be limited to the blob propagation region but extend across the separatrix into the edge plasma. The experiments are compared to the results of nonlinear global numerical simulations covering the edge plasma and SOL [1]. The phase relation between plasma density and potential of blobs is consistent with the experimental findings. In the simulation radial correlations across the separatrix are found to be a result of the global edge plasma respond to the blob transport showing a close interplay between the edge and SOL plasma.
[1] O. E. Garcia et al., Phys. Rev. Letters 92(16), 2004.