Reduced model describing resonance overlap threshold for fast ion transport by toroidal Alfvén eigenmodes

Abstract This work introduces a reduced model to predict the ‘resonance overlap threshold’ governing transport of fast ions (FIs) by toroidal Alfvén eigenmodes (TAEs) in tokamak plasmas. TAE-FI resonance occurs within distinct ‘resonance regions’ of particle phase space, which grow wider when the TAE mode amplitude increases. If these resonance regions are separate then FI transport is limited and localised; if multiple resonance regions overlap then large-scale stochastic transport of FIs can occur, jeopardising confinement. Predicting the resonance overlap threshold between these two scenarios is an important problem in the field of FI transport modelling. Current workflows rely on computationally expensive orbit-following codes; the new reduced resonance overlap model provides a much simpler and faster alternative, for ease of implementation in integrated models. In this paper, we apply this approach to passing particles and find good agreement with more detailed numerical modelling, including HALO code simulations. Models based on this approach could provide a useful step in increasing efficiency of predictive modelling for next-generation fusion reactors.