Visible imaging of turbulence in the SOL of the Alcator C-Mod tokamak

Measurements of fluctuations in the scrape-off layer (SOL) of Alcator C-Mod plasmas were made using visible emission from the plasma edge. Line emissi ons from a local gas jet of He atoms were imaged along radially separated, toroidally viewing chords and provided local measurements (DeltaR, DeltaZ = 3.4 mm, R Delta phi similar to 20 mm) of density fluctuations. The RMS emi ssion fluctuation amplitudes, normalized by the average emission, ranged fr om 10% to 60%, depending on distance from the separatrix, and increased sha rply beyond rho (from) = 20 mm. These intensity fluctuations imply that rad ial profile of the normalized RMS density fluctuations varies from similar to 20% to 90%. Typical fluctuation spectra show turbulence out to frequenci es of similar to 150 kHz, although most of the spectral power is below 20 k Hz. The radial correlation length for the <20 kHz fluctuations is <similar to>7 mm, No essential differences were seen in the SOL fluctuations when pl asmas with 'Enhanced D-alpha' (EDA) H-mode confinement were compared with t hose having L-mode confinement. In addition, fluctuations in 'natural' D-al pha light, due to recycling at an outboard poloidal limiter, were imaged by a gated fast-framing camera and provided measurements of the poloidal size scale of the SOL fluctuations. Snapshots of D-alpha emission were obtained with exposures of 2 and 10 mus. Typical fluctuation size in the poloidal d imension was 0.01-0.03 m. These striations/fluctuations were randomly locat ed in the successive gated frames taken at 1000 Hz, as expected for edge tu rbulence. (C) 2001 Elsevier Science B.V. All rights reserved.