BOUT++ Simulations of Edge Turbulence in Alcator C-Mod’s EDA H-Mode

Evan Davis, Massachusetts Institute of Technology

Energy confinement in tokamaks is believed to be strongly controlled by plasma transport in the edge region, just inside the last closed magnetic flux surface. A first-principles understanding of these edge processes is an active field of theoretical and experimental research. The Boundary-plasma Turbulence (BOUT++) code is capable of nonlinear fluid boundary turbulence analysis in a general geometry. As a fluid code, BOUT++ is particularly useful for simulating relatively collisional plasmas, such as Alcator C-Mod’s Enhanced D-Alpha (EDA) H-mode (ν* > 1). C-Mod’s EDA H-mode is always accompanied by edge fluctuations known as the quasi-coherent mode (QCM). The QCM is believed to reduce impurity confinement, allowing for steady-state H-mode operation with excellent energy confinement.

Using experimentally measured profiles as input, BOUT++ calculations show that typical C-Mod EDA H-modes are ideal MHD stable, but become linearly unstable when the pedestal resistivity is included (η > 10^-7 Ω-m). The computed resistive ballooning mode growth rate in such shots is shown to scale approximately as the 1/3 power of resistivity and the 2/3 power of the torodial mode number (n), consistent with theory. The inclusion of diamagnetic effects in linear simulations damps high toroidal mode numbers (n > 30) and produces mode propagation in the electron diamagnetic direction, in qualitative agreement with experimental observations of the QCM. Incorporation of experimentally measured flow profiles has allowed the self-consistent calculation of the edge radial electric field. Nonlinear simulations have reached turbulent steady state. The computed turbulence spectrum and radial transport coefficients will be compared with measurements of the QCM from relevant fluctuation diagnostics in C-Mod, such as phase contrast imaging (PCI), reflectometry, gas puff imaging (GPI), and magnetic probes.

*This work was performed under the auspices of the USDOE under awards DE-FG02-94-ER54235, DE-AC52-07NA27344, DE-AC52-07NA27344, and NNSA SSGF.

Abstract Author(s): E.M. Davis, M. Porkolab, J.W. Hughes, MIT PSFC; X.Q. Xu, LLNL