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Plasma Physics (Physics/ECE/NE 922) Seminars

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Event Number 4779

  Monday, March 5th, 2018

MHD modeling of DIII-D QH-mode discharges and comparison to observations
Time: 12:00 pm
Place: 2241 Chamberlin Hall
Speaker: Dr. Jacob King, Tech-X Corporation
Abstract: It is desirable to have an ITER H-mode regime that is quiescent to edge-localized modes (ELMs). ELMs deposit large, localized, impulsive, surface heat loads that can damage the divertor. One such regime is quiesent H-mode (QH-mode) with edge harmonic oscillations or broadband MHD as observed on DIII-D, JET, JT-60U, and ASDEX-U [Burrell et al. Phys Plasmas 2012, Garofalo et al. Nucl Fusion 2012 and references within]. These ELM-free discharges have the edge-plasma confinement necessary for burning-plasma operation on ITER. QH-mode is characterized by perturbations with small toroidal-mode numbers (n '1-5) where measurements from beam-emission spectroscopy, electron-cyclotron emission, and magnetic probe diagnostics show density, temperature and magnetic oscillations. These measurements demonstrate that the perturbations are a saturated macroscopic mode localized within the edge pedestal region. The particle transport is enhanced compared to standard H-mode discharges with ELMs, leading to essentially steady-state
profiles in the pedestal region. Finally, the operation regime of the QH-mode is dependent on the rotation profile, and QH-mode discharges are produced with an applied torque through either coor counter-neutral-beam injection and/or neoclassical toroidal viscosity from plasma interaction with non-resonant magnetic fields.
Extended-MHD modeling of DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 2002] QH-mode discharges with nonlinear NIMROD [C. R. Sovinec et al., JCP 195, 355 2004] simulations saturates into a turbulent state, but does not saturate when the steady-state flow inferred from measurements is not included. This is consistent with the experimental observations of the quiescent regime on DIII-D. The simulation with flow develops into a saturated turbulent state where the n=1 and 2 toroidal modes become dominant through an inverse cascade. Each mode in the range of n=1-5 is dominant at a different time. Consistent with experimental observations during QH-mode, the simulated state leads to large particle transport relative to the thermal transport. Analysis shows that the amplitude and phase of the density and temperature perturbations differ resulting in greater fluctuation-induced convective particle transport relative to the convective thermal transport. Comparison to magnetic-coil measurements shows rotation frequencies differ between the simulation and experiment which indicates that more sophisticated extended-MHD two-fluid modeling is required.
This work was supported by the DOE Office of Science (Office of Fusion Energy Sciences
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