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Plasma Physics (Physics/ECE/NE 922) Seminar
Schnack Memorial Seminar: Simulating Transient Excitation of Neoclassical Tearing Modes
Date: Monday, April 17th
Time: 12:00 pm - 6:00 pm
Place: 2241 Chamberlin Hall
Speaker: Eric Howell, Tech-X Corporation
Abstract: Disruption avoidance and mitigation strategies are needed for ITER and future fusion pilot plants. Neoclassical tearing modes (NTMs) are a leading physics cause of disruptions in present-day experiments, and predictive NTM models are vital for the success of burning plasma tokamaks. NTM growth is driven by a helical bootstrap current deformation that results from pressure flattening across an island. NTMs are metastable states that require a seed island to grow. The seed islands are generated by ELMs, sawteeth, and other transients. Nonlinear simulations study the transient excitation of NTMs. Simulations use reconstructions of DIII-D ITER baseline scenario discharges. The dominant neoclassical effects, including the bootstrap current drive, are modeled using heuristic fluid closures for neoclassical stresses. Magnetic perturbations are transiently applied to excite a growing NTM. These perturbations act as surrogates for ELMs. A Goldilocks effect is observed with regard to the applied perturbation amplitude. If the amplitude is too small, no NTMs grow. If the amplitude is too large, a 2/1 NTM grows robustly immediately following the pulse. If the amplitude is just right, then the 2/1 NTM grows in two phases: an initial slowly growing phase followed by a fast robustly growing phase. An energy transfer analysis shows that the early slow growth phase is partially driven by nonlinear interactions with higher-n core modes. The later robust growth phase is described by the modified Rutherford equation.

The work is funded by the US Department of Energy
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