MHD modeling of a DIII-D low-torque QH-mode discharge and comparison to observations (1703.02584v1)

Abstract: Extended-MHD modeling of DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] quiescent H-mode (QH-mode) discharges with nonlinear NIMROD [C. R. Sovinec et al., J. Comput. Phys. 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 that rotation frequencies differ between the simulation and experiment, which indicates that more sophisticated extended-MHD two-fluid modeling is required.

• The paper presents extended MHD modeling using NIMROD simulations to reproduce low toroidal mode number dynamics in DIII-D low-torque QH-mode discharges by including steady-state flow profiles.
• Simulations reveal that density transport exceeds thermal transport due to phase differences, explaining the experimentally observed density pump-out characteristic of QH-mode.
• The findings highlight the importance of plasma flow for maintaining stable QH-mode and suggest that more advanced two-fluid modeling is needed to improve predictive capabilities for future tokamaks like ITER.

Overview of MHD Modeling of Low-Torque QH-mode Discharge in DIII-D Tokamak

The paper by J.R. King et al. presents an extended magnetohydrodynamic (MHD) modeling of a quiescent H-mode (QH-mode) discharge in a DIII-D tokamak. Using nonlinear NIMROD simulations, the paper contributes significantly to understanding QH-mode dynamics under conditions of low external torque. The investigation emphasizes the reproduction of key phenomena observed experimentally in DIII-D, namely, the dominance of low toroidal mode numbers in QH-mode and the associated particle transport mechanisms.

Key Findings and Methodology

The investigation employs extended-MHD modeling, with focus on understanding turbulence saturation and its impact on transport phenomena. The distinctive feature of the paper is its ability to replicate the saturation at low toroidal modes ($n_\phi=1$ and $2$) via simulations that include steady-state flow profiles. This contrasts with simulations lacking flow, where high-$n_\phi$ dynamics dominate, failing to reach a saturating turbulent state. Such findings are in agreement with experimental observations that relate QH-mode accessibility to control over edge shearing flows.

The simulations shed light on several core phenomena:

• Mode Dynamics and Turbulence Saturation: Incorporation of steady-state toroidal and poloidal flows results in a sustainable saturated turbulence, with an inverse cascade prominently featuring low-$n_\phi$ modes in the dynamical evolution. Conversely, absence of flow simulation results in ELM-like high-$n_\phi$ disruptions.
• Transport Characteristics: The paper finds a mismatch in density and thermal perturbation-induced transport. Density transport exceeds thermal transport, aligning with experimentally observed density pump-out. Analysis of pressure and current profiles suggests that profile flattening due to turbulent state enhances particulate transport.
• Phase Analysis: Detailed examination of fluctuations revealed significant phase discrepancies between density and temperature perturbations relative to normal flow, explaining the dominant role of particle transport in the dynamics. The difference between density and thermal flux can be attributed to these phase differences and the inherent inequalities in perturbation amplitudes.

Implications and Future Outlook

The findings have profound implications for devising operational strategies in tokamaks, particularly in optimizing plasma performance without ELM-induced loads. The research underscores the necessity for further studies encompassing two-fluid dynamics, which are hypothesized to reconcile observed discrepancies such as mismatched rotation frequencies between the simulation and experimental observations. Integration of such modeling enhancements will potentially reconcile existing differences and improve predictive capabilities.

Additional refinements in computational practicality and model fidelity are essential moving forward. Enhanced computational resources to accommodate full two-fluid models, alongside realistic dissipation parameters, will expand the accuracy and applicability of MHD simulation frameworks. The results invite further exploration into operational regimes across different tokamaks, potentially extending insights towards ITER's operational scenarios.

Conclusion

King et al.'s work advances the fundamental understanding of QH-mode dynamics under low-torque conditions in tokamak pedestals. By demonstrating the critical role of steady-state flows in regulating mode dynamics and resulting transport phenomena, the paper exemplifies how computational MHD models can be strategically leveraged to guide experimental tokamak operations and the design of future reactors. The insights yielded pave the way for refining modeling approaches to enhance the predictability and stability of fusion plasmas, crucial for the realization of sustainable fusion energy.