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Energetic Particle Diffusion In Critically Balanced Turbulence

Observations and modelling suggest that the fluctuations in magnetised plasmas exhibit scale-dependent anisotropy, with more energy in the fluctuations perpendicular to the mean magnetic field than in the parallel fluctuations and the anisotropy increasing at smaller scales. The scale-dependence of the anisotropy has not been studied in full-orbit simulations of particle transport in turbulent plasmas so far. In this paper, we construct a model of critically balanced turbulence, as suggested by \cite{GoSr1995}, and calculate energetic particle spatial diffusion coefficients using full-orbit simulations. The model uses an enveloped turbulence approach, where each 2-dimensional wave mode with wavenumber $k_\perp$ is packed into envelopes of length $L$ following the critical balance condition, $L\propto k_\perp^{-2/3}$, with the wave mode parameters changing between envelopes. Using full-orbit particle simulations, we find that both the parallel and perpendicular diffusion coefficients increase by a factor 2, compared to previous models with scale-independent anisotropy.