A Comprehensive Gyrokinetic Framework for Energetic Particles

The effects of energetic particles on the turbulence and transport of a tokamak plasma are once again of great

interest as the community designs scenarios for current and future Fusion Pilot Plant designs. These effects range

from the interaction of energetic particles with existing turbulence improving confinement [A. Di Siena et al 2023

Nucl. Fusion 63 036003] to extensive integrated work modelling energetic-particle-driven instabilities. Gyrokinetic

codes are routinely used for these studies so the time is ripe to confirm the underlying theoretical basis for using these

tools.

In this work the multiscale gyrokinetic framework of [Abel et. al. Rep. Prog. Phys. 2013] is extended to include

a collisionless species, whose collisions occur on the transport timescale. This extension allows for non-Maxwellian

distribution functions, such as those produced by neutral beam and radio-frequency heating in current fusion devices

or alpha particles in future burning plasmas. In performing this extension, we provide the theoretical basis in which

the numerical studies of [Wilkie et. al. Phys Plasmas 2016 & Plasma Phys. and Control. Fus. 2017] were grounded.

The issue of parallel flows of these collisionless species is also discussed, and gives rise to a self-consistent inclusion

of current drive within the multiscale framework. We also examine the fully-general system in the limit where

the transport of collisionless particles is subdominant. This leads to a formalism including a tail distribution that

thermalizes before undergoing turbulent transport.

We demonstrate the utility of this framework by placing several well-known aspects of energetic particle physics

within it. These examples include passive transport of energetic particles by microturbulence, stabilisation of ITG by

energetic particles, and the effects of energetic particles on the magnetic equilibrium.