Deviations from omnigeneity and the transition to non-linear Alfvenic Eigenmode behaviorA. Lachmann, E. Paul, A. Knyazev

Energetic particles (EP) maintain fusion by heating the plasma core. EP confinement in stellarators is sensitive to deviations from omnigeneity and Alfvénic Eigenmodes (AEs) [1]. Wave particle interactions between AEs and EP distribution functions can create amplitude and phase modulations of the AE, changing the experienced resonances and transport induced by resonance overlap [2]. Damping can shrink resonances, creating a hole-clump pair in phase space [2]. It has been shown previously that stochastically diffusive behaviors, i.e. microturbulence, suppress chirping/bursting phase space structures [4]. Deviations from omnigeneity result in uniquely trapped particle orbits, with drifts that may disrupt phase space structures, mitigating hole-clump pairs and chirping/bursting [3].

In this work, we explore non-linear AE saturation, bursting, and chirping in 3D fields using M3D-C1-K, a 3D MHD solver coupled to a particle-in-cell kinetic simulation by pressure and current [5]. We study the contribution of 3D orbit drifts to the onset of chirping and bursting and evaluate the relationship between the macroscopic equilibrium and the smaller length scale of chirping and bursting.

[1] Paul,E.J., et al. JPP. 89(2023).

[2] Bierwage,A., et al. Plasma & Fus. Res. 16,(2021):1403087.

[3] Paul,E.J., et al. NF. 62, n.12(2022):126054.

[4] Duarte,V., et al. PoP 24, 122508(2017).

[5] Liu,C. et al. Comp. Phys. Commun. 275(2022):108313.

We acknowledge funding through the U.S. DoE, under contract DE-SC0024548.