A Multi-Region Multi-Timescale Burning Plasma Dynamics Model for Tokamaks

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Liu, Zefang
Stacey, Weston M.
Biegalski, Steven R.
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Controlled thermonuclear fusion in tokamaks brings forth demands for burning plasma dynamics research. The deuterium-tritium fusion generates energetic alpha particles, which first transfer their energy to core electrons. The heated electrons and remaining fusion alpha particles then heat core ions through Coulomb collisions, which can increase the fusion reaction rate and may conceivably lead to a thermal runaway instability. Meanwhile, core electrons lose energy to the edge plasma and wall through electron cyclotron radiation, bremsstrahlung, impurity radiation, and transport; and core ions lose energy through transport to the edge and ion orbit loss. The various timescales of radiation and transport processes in different regions are vital to the determination of tokamak operation. A multi-region multi-timescale transport model is developed to simulate burning plasma dynamics in tokamaks. Regions including the core, edge, scrape-off layer, and divertor are modeled as separate nodes. Fusion alpha heating with a time delay between heating electrons and ions is considered. Ohmic and auxiliary heating methods are included. Radiations such as electron cyclotron radiation, bremsstrahlung, line radiation, and recombination radiation are involved. Coulomb collisional energy transfer is utilized for energy redistribution among species. Ion orbit loss is included as one edge plasma effect. Transport times between nodes are derived theoretically from the fluid model, where diffusivity parameters are computed numerically from experiment data by machine learning. The transport model is validated by application to DIII-D non-fusion plasmas in various auxiliary heating conditions. Both inductive and non-inductive ITER operation scenarios are simulated with the model. Simulation results with sensitivity analyses indicate the radiation and transport can promptly remove extra heat from the core plasma and thereby inhibit the thermal runaway instability from the fusion alpha heating. This model can be used for tokamak deuterium-tritium burn control studies in the future.
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