Experimental and theoretical studies of edge turbulence in the Alcator C-Mod tokamak

Evan Davis, Massachusetts Institute of Technology

Alcator C-Mod is a compact, high-field tokamak located at MIT’s Plasma Science and Fusion Center. Energy confinement, and thus potential fusion gain, in C-Mod and other tokamaks is believed to be strongly controlled by plasma parameters in the edge region, or so-called plasma pedestal, just inside the last closed magnetic flux surface. Thus, a comprehensive, first-principles model of edge turbulence is a topic of much contemporary research. Development of such a model requires an effective coupling between experiment and theory.

Phase Contrast Imaging (PCI) is an established diagnostic on C-Mod that is well suited to study edge turbulence. PCI is a type of internal interferometric technique that measures line-integrated electron density perturbations in the plasma with a carbon dioxide laser. C-Mod’s 32-chord PCI beam passes vertically through the plasma cross-section, collecting data from both the plasma core and edge. Unfortunately, as a line-integrated measurement, PCI inherently lacks localization along the beam path. Masking portions of the PCI phase plate allows for partially localized turbulent fluctuation measurements. An overview of C-Mod’s PCI system and the latest edge turbulence measurements will be presented.

Physical understanding of measurements made with PCI can be enhanced by the Boundary-plasma Turbulence (BOUT++) code. BOUT++ is a user-friendly, state-of-the-art, nonlinear fluid turbulence code capable of boundary turbulence analysis in a general geometry. Using BOUT++, linear instability growth rates, frequencies, and nonlinear saturation for particular C-Mod experimental parameters and plasma profiles have been preliminarily calculated. The computed turbulence spectrum and radial plasma transport coefficients will be compared with measurements from PCI and other relevant diagnostics. Further work will help validate BOUT++ against edge turbulence measurements under different energy confinement regimes (i.e. H-mode, I-mode, and L-mode) in C-Mod.

*C-Mod PCI project and BOUT++ code development supported by U.S. DOE.

Abstract Author(s): E. M. Davis, M. Porkolab, P. Ennever, N. Tsujii, Plasma Science and Fusion Center, MIT, and X. Q. Xu, Lawrence Livermore National Laboratory.