Toward Integrated Multi-Scale Pedestal Simulations

X.Q. Xu, Lawrence Livermore National Laboratory

January 11, 4:00pm, SERF 383

Abstract:  The high-fidelity BOUT++ two-fluid code suite has demonstrated significant recent progress toward integrated multi-scale simulations of tokamak pedestal, including Edge- Localized-Mode (ELM) dynamics, evolution of ELM cycles, and continuous fluctuations, as observed in experiments. Nonlinear ELM simulations show three stages of an ELM event: (1) a linear growing phase; (2) a fast crash phase; and (3) a slow inward turbulence spreading phase lasting until the core heating flux balances the ELM energy loss and the ELM is terminated. The BOUT++ simulations are used to study the linear and nonlinear characteristics of edge-localized mode at different collisionality and radial electric field. By increasing collisionality, nonlinear simulations show that (a) power spectrum becomes broad;  (b) the dominant mode increases from n=6 to n=35. Bispectrum analysis shows that nonlinear mode coupling becomes stronger at high collisionality, especially for the high-n modes with n > 20, resulting in the lack of dominant filamentary structures and reduced ELM energy loss. The impact of radial electric field Er on peeling and ballooning modes is different. The increase Er significantly enhances the linear growth rate of low-n peeling modes, while the linear growth rates of ballooning modes remain almost the same. Bispectrum analysis indicates that the increase Er can enhance the nonlinear coupling, and shorten the phase coherence time of the linear growth, which is a key nonlinear criterion for the occurrence of ELM crash.

A new coupling/splitting model has been developed to perform simulations of multi- scale ELM dynamics. Simulation tracks five ELM cycles for 10000 Alfven times for small ELMs. The temporal evolution of the pedestal pressure is similar to that of experimental measurements for the pedestal pressure profile collapses and recovers to a steep gradient during ELM cycles. To validate BOUT++ simulations against experimental data and develop physics understanding of the fluctuation characteristics for different tokamak operation regimes, both quasi-coherent fluctuations (QCFs) in ELMy H-modes and Weakly Coherent Modes (WCMs) in I-modes have been simulated using three dimensional 6-field 2-fluid electromagnetic model. The H- mode simulation results show that (1) QCFs are localized in the pedestal region having a predominant frequency at f ≈300−400kHz and poloidal wavenumber at k_q≈0.7/cm, and propagate in the electron diamagnetic direction in the laboratory frame. The overall signatures of simulation results for QCFs show good agreement with C-Mod and DIIID measurements. (2) The pedestal profiles giving rise to QCFs are near the marginal instability threshold for ideal peeling-ballooning (P-B) modes for both C-Mod and DIII-D, while the collisional electromagnetic drift-Alfven wave appears to be dominant for DIII-D. (3) Particle diffusivity is either smaller than the heat diffusivity for DIII-D or similar to the heat diffusivity for C-Mod. Key I-mode simulation results are that (1) a strong instability exists at n\geq 20 for resistive ballooning mode and drift-Alfven wave; (2) the frequency spectrum of nonlinear BOUT++ simulation features a peak around 300kHz for the mode number n=20, consistent with a reflectometer measurement at nearby position; (3) the calculated particle diffusivity is larger than the calculated heat diffusivity, which is consistent with a key feature of the I-mode pedestal with no particle barrier.