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 emissions from a local gas jet of He atoms were imaged along radially separated, toroidally viewing chords and provided local measurements (ΔR,ΔZ=3.4 mm, RΔφ∼20) mm) of density fluctuations. The RMS emission fluctuation amplitudes, normalized by the average emission, ranged from 10% to 60%, depending on distance from the separatrix, and increased sharply beyond ρfrom LCFS=20 mm. These intensity fluctuations imply that radial profile of the normalized RMS density fluctuations varies from ∼20% to 90%. Typical fluctuation spectra show turbulence out to frequencies of ∼150 kHz, although most of the spectral power is below 20 kHz. The radial correlation length for the <20 kHz fluctuations is ∼7 mm. No essential differences were seen in the SOL fluctuations when plasmas with `Enhanced Dαʹ (EDA) H-mode confinement were compared with those having L-mode confinement. In addition, fluctuations in `naturalʹ Dα 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 Dα emission were obtained with exposures of 2 and 10 μs. Typical fluctuation size in the poloidal dimension was 0.01–0.03 m. These striations/fluctuations were randomly located in the successive gated frames taken at 1000 Hz, as expected for edge turbulence.