Theoretical astrophysics and plasma physics at RPC

Abstract:

Cosmic rays (CRs) play a pivotal role in shaping the thermal and dynamical properties of astrophysical environments, such as galaxies and galaxy clusters. Recent observations suggest a stronger confinement of CRs in certain astrophysical systems than predicted by current CR-transport theories. Here, we show that the incorporation of microscale physics into CR-transport models can account for this enhanced CR confinement. We develop a theoretical description of the effect of magnetic microscale fluctuations originating from the mirror instability on macroscopic CR diffusion. We confirm our theory with large-dynamical-range simulations of CR transport in the intracluster medium (ICM) of galaxy clusters and kinetic simulations of CR transport in micromirror fields. We conclude that sub-teraelectronvolt CR confinement in the ICM is far more effective than previously anticipated on the basis of Galactic-transport extrapolations. The transformative impact of micromirrors on CR diffusion provides insights into how microphysics can reciprocally affect macroscopic dynamics and observable structures across a range of astrophysical scales.

Abstract:

Efficient control of turbulent heat transport is crucial for magnetic confinement fusion reactors. This work discusses the complex interplay between density gradients and microinstabilities, shedding light on their impact on turbulent heat transport in different fusion devices. In particular, the influence of density gradients on turbulent heat transport is investigated through an extensive inter-machine study, including various stellarators such as W7-X, LHD, TJ-II and NCSX, along with the Asdex Upgrade tokamak (AUG) and the tokamak geometry of the Cyclone Base Case (CBC). Linear and nonlinear simulations are performed employing the δf-gyrokinetic code stella across a wide range of parameters to explore the effects of density gradients, temperature gradients, and kinetic electrons. A strong reduction in ion heat flux with increasing density gradients is found in NCSX and W7-X due to the stabilization of temperature-gradient-driven modes without significantly destabilizing density-gradient-driven modes. In contrast, the tokamaks exhibit an increase in ion heat flux with density gradients. Notably, the behavior of ion heat fluxes in stellarators does not align with that of linear growth rates, if only the fastest-growing mode is taken into account. Additionally, this study provides physical insights into the microinstabilities, emphasizing the dominance of trapped-electron-modes (TEMs) in CBC, AUG, TJ-II, LHD and NCSX, while both the TEM and the passing-particle-driven universal instability contribute significantly in W7-X.