Phil Snyder, Eric Bass, Yueqiang Liu, Brendan Lyons, Joey McClenaghan, and Lang Lao represented the GA Theory Group at the 27th IAEA Fusion Energy Conference in Ahmedabad, India this week. Dr. Snyder presented work on “High Fusion Performance in Super H-Mode Experiments on Alcator C-Mod and DIII-D.” Eric Bass showed results from the work on “Predictions of alpha-Particle and Neutral-Beam Heating and Transport in ITER Scenarios.” In the work on 3D effects, Yueqiang Liu presented results on the “Role of NTV Particle Flux in Density Pumpout during ELM Control by RMP,” and Brendan Lyons on “Predict-First Analysis and Experimental Validation of MHD Equilibrium, Stability, and Plasma Response to 3D Magnetic Perturbations.” Joey McClenaghan showed the recent work on “Transport at High Betap and Development of Candidate Steady State Scenarios for ITER.”

An integrated modeling workflow capable of finding the steady-state solution with self-consistent core transport, pedestal structure, current profile, and plasma equilibrium physics has been developed and tested against DIII-D discharges. Key features of the achieved core-pedestal coupled workflow are its ability to account for the transport of impurities in the plasma self-consistently, as well as its use of machine-learning-accelerated models for the pedestal structure and for the turbulent transport physics. Notably, the coupled workflow has been implemented as part of the STEP (Stability Transport Equilibrium Pedestal) module within the OMFIT framework [http://gafusion.github.io/OMFIT-source], a module that makes use of the ITER integrated modeling and analysis suite (IMAS) data structure as a central repository for exchanging data among physics codes. This technical advance has been facilitated by the development of the OMAS numerical library [https://gafusion.github.io/omas/]. Initial testing shows that the workflow is capable of reproducing with high fidelity the experimentally measured kinetic profiles of DIII-D plasma discharges from the plasma axis to the separatrix (including the carbon impurity density). The new workflow provides an important advancement towards our ability to perform high-fidelity self-consistent simulations with rapid turnaround time.

A verification benchmark has been carried out between the M3D-C1 and NIMROD extended-magnetohydrodynamics (ExMHD) codes for simulations of impurity-induced disruption mitigation. Disruptions are a significant concern for future tokamaks, and high-fidelity simulations are required in order to ensure the success of disruption mitigation techniques (e.g., shattered-pellet injection) in large-scale fusion reactors. Both MHD codes have been coupled to the KPRAD code for impurity dynamics. The codes show excellent agreement in four axisymmetric, nonlinear simulations, particularly during the thermal quench. This agreement is seen in the time histories of important, 0-D plasma quantities such as thermal energy, radiated power, and electron number, as well as 1-D profiles and 2-D contours of temperature and current density. The simulations predict that, given the same number of atoms injected, argon quenches the plasma roughly twice as fast as neon. Furthermore, the inclusion of temperature-dependent Spitzer-like resistivity causes the current to diffuse and decay, inducing axisymmetric instabilities that eventually result in a current quench. This work represents an important verification of the impurity and MHD models implemented in M3D-C1 and NIMROD, giving greater confidence in the ability of both codes to perform more sophisticated disruption-mitigation simulations.

Work continued on the development of a new tool that will advance our predictive modeling capabilities. Recent efforts have focused on applying Modelprofiles to DIII-D H-mode discharges using the neural-net versions of TGLF and EPED (TGLF-NN and EPED-NN). A small subset of discharges from a database of H-modes was used for applying and testing Modelprofiles for fast “cold start” transport predictions and for equilibrium reconstruction. A current scan and a triangularity scan were chosen from a DIII-D experiment devoted to validating the EPED model (Groebner, et al, Nucl. Fusion, 2009). In the subset of discharges chosen, the plasma current was varied from 0.8 to 1.5MA and the triangularity from 0.06 to 0.33. The normalized toroidal beta was held fixed at 2.0. The TGLF-NN results for the two scans was found to be similar to the previous H-mode validation study using experimental boundary conditions at r/a ~ 0.80. For the entire database of 30 discharges, we still find that TGYRO/TGLF-NN more accurately predicts the energy confinement time than the ITER98y2 empirical scaling.