High Fidelity Predictions of Performance in SPARC, ARC, and ITER: Energy confinement, density peaking, and implications for modeling of burning plasmas

Nonlinear gyrokinetics (CGYRO) has been used to predict the profiles (n_e, T_e, and T_i) and performance of ITER, SPARC, and ARC over a wide range of anticipated operational scenarios. More than 20 cutting-edge predictions, enabled by GPU acceleration and machine learning techniques, provide a picture of the underlying physics that should play a crucial role in setting transport, confinement, and performance in burning plasmas. Despite significant differences in engineering parameters between these 3 devices, similarities between different burning plasmas are striking. All conditions exhibit dominant Ion Temperature Gradient driven (ITG) turbulence with potential for unstable Kinetic Ballooning Modes near axis. High fidelity workflows suggest that energy confinement may fail to follow the traditional tau_98,y2 confinement scaling, leading to potentially reduced overall performance. Such breakdown is attributed to the presence of extremely stiff, ITG dominated core transport which limits significant modification of the temperature profile at high performance. Burning plasmas are also found to exhibit reduced density peaking compared with empirical scalings derived by Angioni [Angioni NF 2009]. The physics of these observations is linked to both reduction in the overall attainable peaking due to finite beta but is also found to result from the presence of strongly ITG plasma conditions operating at low collisionality. Gyro-Bohm scaling of heat transport and the dependence of ITG turbulence on collisions is shown to inevitably lead to a saturation of density peaking in conditions relevant for burning plasmas. This collection of results sets limits on the attainable performance in high performance plasmas by pinning core gradients for a given pedestal condition. The implications of this work on the prediction of burning plasma performance and potential avenues for enhancing performance in FPP relevant plasma conditions will be discussed.