Long Quasi-Linear Simulations of Fast Ion Slowing Down in ITER steady-state plasma with Alfvén Instabilities

We investigate the slowing down of MeV-range energetic ions in ITER steady-state plasma scenarios in the presence of Alfvén eigenmode (AE) instabilities using the Resonance-Broadened Quasilinear (RBQ) code. This work is based on a revised quasilinear theory. Our results show that AE activity can influence both neutral beam ions and alpha particles, though fast-ion transport remains modest assuming classical slowing down distribution.

Energetic particle (EP) relaxation is analyzed using a suite of tools: NOVA for linear stability of the sub-cyclotron Alfvénic spectrum, NOVA-C for drift-kinetic effects, RBQ for predictive transport, and NUBEAM for global particle simulations within TRANSP. These self-consistent simulations reveal EP depletion near the plasma core on timescales comparable to the slowing-down time (N.N. Gorelenkov et al., IAEA 2023). The temporal evolution of fast-ion beta, \beta_{f}\left(t\right), is included by scaling AE-driven diffusion coefficients from RBQ as D_xyf(Γ:t)=D_xyf0(Γ:t₀)(β_f/β_f0)^4/3, assuming AE growth rates scale with fast-ion (f) pressure. These results suggest that while AE-driven transport may be moderate, Alfvénic modes could impact current drive and thus pose a challenge to maintaining ITER’s self-sustained steady-state plasma operation.