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Modulations in amplitude and phase of Alfvénic eigenmodes (AEs) may occur due to wave particle interactions with energetic particle (EP) distribution functions [1]. Damping can shrink the resonance with the mode, creating a hole-clump pair in the region of the shrunken resonance [1]. Hole-clump pairs in phase space are associated with phase modulations (chirps) of modes and propagate through phase space, increasing transport [1], and have experimentally observed in chirping fishbone modes to expel EPs to the wall in phase with the mode [2]. Previous work has proved that stochastically diffusive behaviors, such as microturbulence, can suppress chirping modes, most likely by diffusing the relevant hole-clump pair [3]. Additionally, 3D error fields in NSTX have been shown to suppress chirping in AEs, inducing mode saturation, suggesting the use of 3D fields for ‘phase space engineering,’ to intentionally suppress targeted modes [4]. Imperfections in omnigenous fields in stellarators, and 3D effects more generally, can result in new classes of trapped particles with unique convective drifts [5]. These drifts can introduce chaos around resonances, introducing a stochasticity that can sometimes act as an effective diffusion. In this work, we study the diffusive effects of error in omnigenous fields and 3D fields on energetic particles, we measure phase space stochasticity using Weighted Birkhoff Averaging (WBA), introduce a numerically computed effective diffusion coefficient, and compare to the stochastic effects of pitch angle scattering.
[1] Bierwage,A., et al. Plasma & Fus. Res. 16,(2021):1403087.
[2] Heidbrink, W., et al. Plasma Phys. Control. Fusion. 53, (2011) 085028.
[3] Duarte,V., et al. PoP 24, 122508(2017).
[4] Bortolon, et al. PRL. 110, 265008 (2013).
[5] Paul,E.J., et al. NF. 62, n.12(2022):126054.