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Adaptive Mesh Simulations of Multi-Physics Processes During Pellet Injection in TokamaksPresenter: Ravi Samtaney, PPPL |
It was experimentally established that pellet injection, wherein a small pellet of frozen Deuterium is injected into the plasma, is an effective technique to refuel tokamaks. The refueling of burning plasma reactors, such as ITER, by pellet injection has been identified to be critical importance. High-fidelity simulations of the refueling process require the development of innovative mathematical and numerical methods to capture the nonlinear coupling between physics at small scales dominated by pellet ablation processes and the physics at the device scale modeled with magnetohydrodynamics (MHD). The physical processes of pellet injection in high temperature tokamaks span several decades of space-time scales, which has prevented effective simulations of these events in the past. The large disparity between pellet size and device size, the large density differences between the pellet ablation cloud and the ambient plasma, and the heat transport by long mean-free-path electrons all pose severe numerical challenges.
We present the results of fully 3D adaptive mesh MHD simulations of fueling pellets injected into tokamaks. The Chombo framework for block structured local adaptive mesh refinement(AMR), extended to use the equilibrium magnetic coordinates, is employed to mitigate the problems due to the large range of spatial scales. Generalized upwinding techniques are employed to deal with sharp gradients. A critical component is the modeling of the highly anisotropic energy transfer from the background hot plasma to the pellet ablation cloud via long mean-free-path electrons along magnetic field lines. The modeling includes a semi-analytical kinetic treatment of the transport of electron energy flux which drives the ablation. We discuss the phenomenology of the mass redistribution processes involving the density equilibrating along field lines and transport across surfaces (in the large-major-radius direction) due to a nonlinear manifestation of an interchange instability driven by the large local pressure at the pellet. The clear benefit of high-field-side injection relative to low-field-side injection is demonstrated and explained.
R. Samtaney and S. C. Jardin were supported at PPPL under USDOE Contract no. DE-AC020-76-CH03073.