Blob sizes and velocities in the Alcator C-Mod scrape-off layer

A new blob-tracking algorithm for the GPI diagnostic installed in the outboard-midplane of Alcator C-Mod is developed. I t tracks large-amplitude fluctuations propagating through the scrape-off layer and calculates blob sizes and velocities. We compare the results of this method to a blob velocity scaling from a simple blob-model for sheath-connected blobs. We further present initial results from a fully three-dimensional blob model that features plasma resistivity as a free parameter

Fast imaging of filaments in the X-point region of Alcator C-Mod

A rich variety of field-aligned fluctuations has been revealed using fast imaging of Dαemission from Alcator C-Mod's lower X-point region. Field-aligned filamentary fluctuations are observed along the inner divertor leg, within the Private-Flux-Zone (PFZ), in the Scrape-Off Layer (SOL) outside the outer divertor leg, and, under some conditions, at or above the X-point. The locations and dynamics of the filaments in these regions are strikingly complex in C-Mod. Changes in the filaments’ generation appear to be ordered by plasma density and magnetic configuration. Filaments are not observed for plasmas with n/nGreenwald≲ 0.12 nor are they observed in Upper Single Null configurations. In a Lower Single Null with 0.12 ≲ n/nGreenwald ≲ 0.45 and Bx∇B directed down, filaments typically move up the inner divertor leg toward the X-point. Reversing the field direction results in the appearance of filaments outside of the outer divertor leg. With the divertor targets “detached”, filaments inside the LCFS are seen. These studies were motivated by observations of filaments in the X-point and PFZ regions in MAST, and comparisons with those observations are made. Keywords: Alcator C-Mod; Turbulence; Divertor; X-point; Filament

Scaling of radial propagating structures in the scrape-off layer of the National Spherical Torus Experiment (NSTX)

The radial propagation of spatiotemporal turbulent structures in the scrape-off layer of the National Spherical Torus Experiment [M. Ono, M.G. Bell, R.E. Bell, et al. Plasma Phys. Control. Fusion 45, A335 (2003)] is investigated. Two-dimensional spatiotemporal imaging of the Dα emission intensity is used to observe the fluctuation structures in the poloidal plane perpendicular to the ambient magnetic field. Turbulent structures are extracted and the individual properties such as velocity, spatial scale and amplitude are determined. The typical poloidal scale of the structures is kϑ = 0.5 cm−1. The poloidal and radial structure velocities are < 5 km/s and 1 km/s, respectively. The radial velocities do not vary significantly with their spatial size within this set of data

Measurements of the Motion of Plasma Filaments in a Plasma Ball

Measurements were made of the motion of the filamentary structures in a plasma ball using high speed cameras and other optical detectors. These filaments traverse the ball radially at ~106 cm/sec at the driving frequency of ~26 kHz, and drift upward through the ball at ~1 cm/sec. Varying the applied high voltage and frequency caused the number, length, and diameter of the filaments to change. A custom plasma ball was constructed to observe the effects of varying gas pressure and species on the filament structures