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Title: Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod

Abstract

This paper describes 2D imaging measurements of plasma turbulence made in the scrape-off layer of the Alcator C-Mod tokamak simultaneously at two different poloidal locations, one near the outer midplane and the other near the divertor X-point region. These images were made with radial and poloidal resolution using two gas puff imaging diagnostics not directly connected along a B field line. The turbulence correlation structure has a significantly different tilt angle with respect to the local flux surfaces for the midplane and X-regions, and a slightly different ellipticity and size. The time-averaged turbulence velocities can be different in the midplane and X-regions, even within the same flux surface in the same shot. The structures are partially consistent with a magnetic flux tube mapping model, and the velocities are compared with various models for turbulence flow.

Authors:
; ; ; ;  [1]; ; ; ; ;  [2];  [3];  [4];  [5];  [6]
  1. Princeton Plasma Physics Laboratory, Princeton, New Jersey 08540 (United States)
  2. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
  3. Consorzio RFX, Associazione Euratom-ENEA sulla fusione, C.so Stati Uniti 4, I-3512 Padova (Italy)
  4. Max Planck Institute for Plasma Physics, EURATOM Association, D-17491 Greifswald (Germany)
  5. Lodestar Research Corporation, 2400 Central Ave., Boulder, Colorado 80301 (United States)
  6. General Atomics, P.O. Box 85608, San Diego, California 92186-5608 (United States)
Publication Date:
OSTI Identifier:
22227948
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 20; Journal Issue: 7; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ALCATOR DEVICE; COMPARATIVE EVALUATIONS; CORRELATIONS; DIVERTORS; MAGNETIC FLUX; MAGNETIC SURFACES; MAGNETOHYDRODYNAMICS; PLASMA CONFINEMENT; PLASMA DIAGNOSTICS; PLASMA SCRAPE-OFF LAYER; THERMONUCLEAR REACTORS; TURBULENCE

Citation Formats

Zweben, S. J., Davis, W. M., Diallo, A., Ellis, R. A., Stotler, D. P., Terry, J. L., Golfinopoulos, T., Hughes, J. W., LaBombard, B., Landreman, M., Agostini, M., Grulke, O., Myra, J. R., and Pace, D. C. Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod. United States: N. p., 2013. Web. doi:10.1063/1.4813758.
Zweben, S. J., Davis, W. M., Diallo, A., Ellis, R. A., Stotler, D. P., Terry, J. L., Golfinopoulos, T., Hughes, J. W., LaBombard, B., Landreman, M., Agostini, M., Grulke, O., Myra, J. R., & Pace, D. C. Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod. United States. doi:10.1063/1.4813758.
Zweben, S. J., Davis, W. M., Diallo, A., Ellis, R. A., Stotler, D. P., Terry, J. L., Golfinopoulos, T., Hughes, J. W., LaBombard, B., Landreman, M., Agostini, M., Grulke, O., Myra, J. R., and Pace, D. C. Mon . "Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod". United States. doi:10.1063/1.4813758.
@article{osti_22227948,
title = {Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod},
author = {Zweben, S. J. and Davis, W. M. and Diallo, A. and Ellis, R. A. and Stotler, D. P. and Terry, J. L. and Golfinopoulos, T. and Hughes, J. W. and LaBombard, B. and Landreman, M. and Agostini, M. and Grulke, O. and Myra, J. R. and Pace, D. C.},
abstractNote = {This paper describes 2D imaging measurements of plasma turbulence made in the scrape-off layer of the Alcator C-Mod tokamak simultaneously at two different poloidal locations, one near the outer midplane and the other near the divertor X-point region. These images were made with radial and poloidal resolution using two gas puff imaging diagnostics not directly connected along a B field line. The turbulence correlation structure has a significantly different tilt angle with respect to the local flux surfaces for the midplane and X-regions, and a slightly different ellipticity and size. The time-averaged turbulence velocities can be different in the midplane and X-regions, even within the same flux surface in the same shot. The structures are partially consistent with a magnetic flux tube mapping model, and the velocities are compared with various models for turbulence flow.},
doi = {10.1063/1.4813758},
journal = {Physics of Plasmas},
number = 7,
volume = 20,
place = {United States},
year = {Mon Jul 15 00:00:00 EDT 2013},
month = {Mon Jul 15 00:00:00 EDT 2013}
}
  • This paper describes 2-D imaging measurements of plasma turbulence made in the scrape-off layer of the Alcator C-Mod tokamak simultaneously at two different poloidal locations, one near the outer midplane and the other near the divertor X-point region. These images were made with radial and poloidal resolution using two gas puff imaging (GPI) diagnostics, which were not directly connected along a B field line. The turbulence correlation structure has a significantly different tilt angle with respect to the local flux surfaces for the midplane and X-regions, and a slightly different ellipticity and size. The time-averaged turbulence velocities can be differentmore » in the midplane and Xregions, even within the same flux surface in the same shot, and in most cases the fluctuations in poloidal velocity in these two regions were not correlated. These structures are partially consistent with a magnetic flux tube mapping model, and the velocities are compared with various poloidal flow models.« less
  • A region of steep plasma gradients, the so-called ”narrow-feature”, has been found in the near scrape-off layer (SOL) of inner-wall limited (IWL) discharges. Dedicated IWL discharges were carried out in Alcator C-Mod [E.S. Marmar et al., Nucl. Fusion 55, (2015)] to study this phenomenon, allowing detailed observations of the plasma profiles and fluctuations. Langmuir probe (LP) measurements show a clear two decay length n e and T e profile structure at the outer midplane. The Gas-Puff Imaging (GPI) diagnostic shows large turbulent fluctuations across the last closed flux-surface, hence supporting the hypothesis that turbulent phenomena play a role in settingmore » the profile steepness. We have carried out the flux-driven non-linear turbulence simulations of two C-Mod discharges which allows a three-way comparison between LP, GPI, and simulation data. Observations and simulations correlate the steep gradient region characterizing the narrow feature with sheared poloidal flows and a deviation of the plasma potential from its floating value. Furthermore, the E x B shear rate exceeds the linear ballooning growth rate, indicating that the narrow feature could result from the effects of sheared flows, although causality could not be established. The fluctuation level in the narrow feature remains of order unity across the entire SOL, indicating that the transport reduction in the near-SOL cannot result from a simple quench rule.« less
  • This paper describes quantitative comparisons between turbulence measured in the scrape-off layer (SOL) of Alcator C-Mod [S. Scott, A. Bader, M. Bakhtiari et al., Nucl. Fusion 47, S598 (2007)] and three dimensional computations using electromagnetic gyrofluid equations in a two-dimensional tokamak geometry. These comparisons were made for the outer midplane SOL for a set of inner-wall limited, near-circular Ohmic plasmas. The B field and plasma density were varied to assess gyroradius and collisionality scaling. The poloidal and radial correlation lengths in the experiment and computation agreed to within a factor of 2 and did not vary significantly with either Bmore » or density. The radial and poloidal propagation speeds and the frequency spectra and poloidal k-spectra also agreed fairly well. However, the autocorrelation times and relative Da fluctuation levels were higher in the experiment by more than a factor of 2. Possible causes for these disagreements are discussed. 2009 American Institute of Physics.« less