Experimental investigation of the parallel structure of fluctuations in the scrape-off layer of Alcator C-Mod

Abstract

The parallel structure of blob filaments in the scrape-off layer of Alcator C-Mod is investigated by combining electrostatic probe measurements with localized measurements of Dα or He I line emission. The spatially resolved emission measurements were made in the outboard midplane region, while probes sampled fluctuations both close to the magnetic X-point and at the outer divertor target. Time-delay correlation analysis reveals that blobs form highly elongated filaments along the ambient magnetic field from the midplane to the divertor target. Those filaments appear to be fairly unaffected by the strong magnetic shear region close to the X-point with maximum correlation amplitudes of approximately 75% between near-midplane and near-X-point fluctuations, as well as between near-midplane and divertor-target fluctuations. However, the filaments do not form at the same time everywhere within a flux tube, but appear, on average, first nearer the midplane and show diffusion along the magnetic field. This parallel propagation occurs on a time-scale that is much faster than expected from particle diffusion, but agrees with fundamental dispersion estimates.