Predicting Performance and Stability of Tokamak Plasmas Using Flexible, Integrated Modeling

Future tokamaks (e.g., EXCITE, FPP) require predictive, integrated models to optimize performance while avoiding transients like disruptions. The STEP (Stability, Transport, Equilibrium, and Pedestal) workflow has been developed in OMFIT to predict stable equilibria self-consistently with core-transport and pedestal calculations. STEP couples theory-based codes to integrate a variety of physics, including MHD stability (DCON, GATO), transport (ONETWO, NEO, STRAHL, TGYRO), equilibrium (EFIT, CHEASE), pedestal formation (EPED), and current-drive, heating, and fueling (CHEF). STEP interfaces the input/output of each code with a centralized ITER-IMAS data structure, allowing codes to be run in arbitrary order and enabling open-loop, feedback, and optimization workflows. This paradigm simplifies the integration of new codes, making STEP highly extensible. Core-pedestal calculations with STEP have been successfully validated against the equilibria and profiles of individual DIII-D discharges and across >500 discharges of the H98,y2 database, with a mean error in confinement time from experiment <18% across three orders of magnitude. STEP has also reproduced results in more exotic DIII-D scenarios, including a U-shaped dependence for stored energy on positive and negative triangularities and an assessment of ideal-MHD stability in negative-central-shear plasmas. Predictive ITER modeling with STEP has shown that pellet fueling enhances fusion gain, with Q≈13 found in the baseline and Q≈9 in the advanced-inductive scenario. Finally, STEP is used to make predictions for the next-step EXCITE tokamak, including a high-pressure (~400 kPa), 80%-bootstrap fraction scenario. In the near-term, STEP calculations will allow for the optimization of heating and current drive to maximize plasma pressure while maintaining MHD stability.