First principles analysis of interactions between fast ions and microturbulence*

By taking advantage of a high-energy expansion in gyrokinetics, several reduced models are found for the effect of energetic particles on microturbulence and vice versa. For example, fast ions have been known to have a strong stabilizing impact on thermal ion-scale turbulence [1]. However, a theoretical explanation consistent with all known features of this effect has until recently been lacking. A physically transparent reduced model has now been developed which explains fast ion stabilization in terms of a direct mapping to the ion-electron temperature ratio [2]. This first principles model quantitatively and qualitatively describes several known aspects of the stabilization phenomenon, including: reduction of the linear growth rates, enhancement of the nonlinear effect, the destabilization of electron-driven modes, the relatively weak effect of alpha particles in burning plasmas, and more. 

Conversely, similar approximations are utilized to make predictions about energetic particle transport by microturbulence. The classical slowing-down distribution has been extended in a simple analytic form to include novel features discovered in more robust and complete simulations [3]. The strength of turbulence relative to collisions comprises a single dimensionless parameter that characterizes how strongly turbulent transport may affect the phase space distribution of fast ions. This redistribution has implications for reactor heating and Alfvén eigenmode stability. 

[1] Citrin, et al. "Nonlinear stabilization of tokamak microturbulence by fast ions." Physical Review Letters 111:155001 (2013)
[2] Wilkie, et al. "First principles of modelling the stabilization of microturbulence by fast ions." Nuclear Fusion 58:082024 (2018)
[3] Wilkie. "An analytic slowing-down distribution as modified by turbulent transport." Journal of Plasma Physics. Accepted. arxiv:1808.01934 (2018)