Visible imaging of turbulence in the SOL of the Alcator C-Mod tokamak J.L. Terry a, * , R. Maqueda b , C.S. Pitcher a , S.J. Zweben c , B. LaBombard a , E.S. Marmar a , A.Yu. Pigarov a , G. Wurden b a Plasma Science and Fusion Center, Massachusetts Institute Technology, Room NW 17-176, 175 Albany Street, Cambridge, MA 02139, USA b Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA Abstract Measurements of ¯uctuations in the scrape-o 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 DR; DZ 3:4 mm, RD/ 20) mm) of density ¯uctuations. The RMS emission ¯uctuation amplitudes, normalized by the average emission, ranged from 10% to 60%, depending on distance from the separatrix, and increased sharply beyond q from LCFS 20 mm. These intensity ¯uctuations imply that radial pro®le of the normalized RMS density ¯uctuations varies from 20% to 90%. Typical ¯uctuation 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 ¯uctuations is 7 mm. No essential dierences were seen in the SOL ¯uctuations when plasmas with `Enhanced D a ' EDA) H-mode con®nement were compared with those having L-mode con®nement. In addition, ¯uctuations in `natural' D a 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 ¯uctuations. Snapshots of D a emission were obtained with exposures of 2 and 10 ls. Typical ¯uctuation size in the poloidal dimension was 0.01±0.03 m. These striations/¯uctuations were randomly located in the successive gated frames taken at 1000 Hz, as expected for edge turbulence. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scrape-o layer; Turbulence; Fluctuations; Alcator C-Mod; Helium 1. Introduction This work reports on the characteristics of the tur- bulence, in this case density ¯uctuations, measured in the scrape-o layer of Alcator C-Mod [1] discharges. The diagnostic technique employed uses light emission from a stream of He atoms as the probe of the turbu- lence. General motivation for studies of edge turbulence comes from observations that edge transport barriers can improve energy con®nement in the core, e.g. in so- called H-mode con®nement regimes, as well as from the role played by turbulence in determining the SOL power-handling capability. These observations indicate the importance of diagnosing, understanding, and ulti- mately controlling the anomalous radial particle and energy transport at the plasma edge. The area concerned with SOL turbulence is a mature ®eld of investigation with numerous studies in the literature. An excellent review by Endler [2], and references therein) discusses the current state of understanding. Anomalous edge transport is believed to be due primarily to broadband turbulence, and the dominant electrostatic turbulence results in density ¯uctuations [3,4]. The wavelengths of the ¯uctuations are such that k perp q i < 1. Langmuir probes have been and remain the primary diagnostic Journal of Nuclear Materials 290±293 2001) 757±762 www.elsevier.nl/locate/jnucmat * Corresponding author. Tel.: +1-617 253 8637; fax: +1-617 253 0627. E-mail address: terry@psfc.mit.edu J.L. Terry). 0022-3115/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0022-311500)00453-0