Simulations of Stability and Turbulence in Tokamak Fusion Reactors

Evan Davis, Massachusetts Institute of Technology

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Net energy production via controlled nuclear fusion requires the confinement of a hot, ionized gas known as plasma for sufficiently long times to allow appreciable fusion reactions to occur. Currently, the most promising approach to magnetic fusion is the tokamak. Tokamaks are toroidal vacuum vessels that use large magnetic fields (2-3 T) to suspend a hot plasma (about 100 million K) in space, preventing the hot core from touching any material walls. Energy confinement in tokamaks is strongly controlled by plasma turbulence and transport in the edge region, just millimeters inside the last closed magnetic flux surface. To prevent melting the walls, the plasma temperature must drop from hundreds of millions of degrees in the core to a few thousand at the reactor wall. Sharp pressure gradients and large current densities in this edge region can drive a host of instabilities. While ideal magnetohydrodynamics (MHD) can explain the constraint of many tokamaks’ default high-performance H-mode regime, it fails to explain the Enhanced D-Alpha (EDA) H-mode on MIT’s Alcator C-Mod tokamak. Stable to ideal MHD, the EDA H-mode always is accompanied by a beneficial edge fluctuation known as the quasi-coherent mode (QCM) that exhausts impurities, allowing for steady-state operation with excellent energy confinement. To better understand the EDA regime and the QCM’s origins, the nonlinear initial value code BOUT++ was used to simulate EDA stability over a wide variation in edge pressure gradient and current density. Ideal simulations quantitatively agree with ideal MHD codes, and the inclusion of resistivity and FLR effects excites a QCM-like mode. The properties of this mode are compared to recent experimental measurements from phase contrast imaging and Langmuir probes.

Abstract Author(s): E.M. Davis, M. Porkolab, J.W. Hughes, X.Q. Xu, P.B. Snyder