Dynamics and statistical properties of blob structures in scrape-off layer plasmas

Abstract

The velocity scaling of blobs, subject to dissipation by either electric currents to the sheaths, or dynamical friction, is studied by numerical simulations where the blob amplitude relative to the background plasma is a free parameter. When subject to dissipation by sheath currents, the radial velocity of a blob depends on the square of its cross field size l, vrad ∼ √l for small l. For large l, the radial velocity of the blob scales as vrad ∼ l−2. The radial blob velocity is maximal for an intermediate size and depends sensitively on the blob amplitude relative to the background density. In the case of dynamical friction, the blob velocity depends on its cross field size as vrad ∼ √l when friction is negligible. For strong friction the blob velocity is inversely proportional to the friction coefficient and size independent. The length scale for the transition between these velocity regimes depends sensitively on the relative blob amplitude. An algorithm to track plasma blobs in spatially resolved optical measurements is developed and applied to gas-puff imaging data obtained from the scrape-off layer of the Alcator C-Mod tokamak. For discharges where the line averaged plasma density is small relative to the Greenwald density, ne/nG ≾ 0.3, the observed radial blob velocities are well approximated by the sheath connected velocity scaling. In the case of ne/nG ≾ 0.4, the radial blob velocities are found to be larger than predictions by the sheath connected velocity scaling. Statistical properties of the scrape-off layer plasma in Alcator C-Mod are studied for a series of discharges where the line-averaged particle density was varied. Long time series of the ion saturation current and the floating potential are obtained from Langmuir probes dwelled in the outboard mid- plane scrape-off layer as well as from a set of Langmuir probes embedded in the lower divertor baffle. We find that the waiting times between burst events and the burst amplitudes are approximately exponentially distributed. The exponentially distributed burst waiting times are compatible with the assumption that the individual blob events are uncorrelated. The conditionally averaged burst shapes of the saturation current perturbations and floating potential are similar for both poloidal positions at low line-averaged particle densities. For a discharge with high line-averaged density we find that the electric potential, sampled at the divertor, looses coherence with increasing distance to the last closed flux surface. These results indicate that sheath dissipation is a robust mechanism that governs the dynamics of plasma blobs in Alcator C-Mod at low line-averaged particle densities, while at high densities the blob filaments are electrically disconnect from the sheaths.