Description
Accurate prediction of energetic-particle transport remains a major challenge because existing EP transport models often rely on simplified quasilinear or diffusive assumptions and, more generally, are restricted to representations of transport processes in configuration space, which do not fully capture resonant, non-Maxwellian phase-space dynamics. Reliable prediction of energetic-particle (EP) transport is a key requirement for reactor-relevant burning plasmas, where EP-driven fluctuations and zonal electromagnetic fields can strongly modify equilibrium profiles.
We present a transport framework based on the concept of Zonal State evolution, a nonlinear equilibrium consistent with a finite level of fluctuations, composed of phase-space zonal structures (PSZS), \emph{i.e.}, the distribution-function component, and the associated zonal electromagnetic fields (ZFs). Starting from the general PSZS governing equation in action-like coordinates, we discuss a hierarchical reduction of transport dynamics, from nonlinear global gyrokinetics to reduced quasilinear descriptions, and its role in multiscale workflows.
We present diagnostics for PSZS evolution in HMGC, ORB5, GTC, and XTOR-K: in HMGC through a proper poloidal average of toroidally symmetric distribution components; in ORB5 through a finite-element projection in $(P_\phi, E, \mu)$ space; and in GTC/XTOR-K through a direct projection in the same space. The EP-com workflow, allowing initialization of gyrokinetic PIC simulations from PSZS-based initial conditions, is also discussed.
Finally, we outline the use of the ATEP workflow as a general phase-space transport model and compare its predicted PSZS evolution against gyrokinetic results for EP mode dynamics, including ITER-relevant scenarios. The overall objective is to establish a consistent, code-to-code validated pathway to long-timescale EP transport modeling that accounts for resonant and non-Maxwellian phase-space distortions. These effects are of crucial importance for accurately describing the nonlinear dynamic evolution of burning plasmas and, therefore, constitute a key element for reliable prediction of reactor-relevant fusion plasma operations.