“Kinetic Instabilities in Extreme Plasma Physics: Laboratory and astrophysical dynamics

Extreme plasma physics explores regimes where strong electromagnetic fields, intense radiation, and quantum electrodynamics (QED) effects fundamentally alter the behavior of matter. These conditions are found in some of the most energetic astrophysical environments, such as pulsars, black holes, and relativistic shocks, and are increasingly accessible in laboratory experiments using high-intensity lasers and particle beams. This Thesis investigates how radiation reaction, through synchrotron and betatron cooling, reshapes phase space and triggers kinetic instabilities across a range of such extreme plasma conditions.

First, we show that synchrotron cooling in strongly magnetized plasmas generically leads to anisotropic, ringshaped momentum distributions that are unstable to the electron cyclotron maser instability (ECMI). Radiation reaction sustains population inversion and enables prolonged coherent emission beyond classical saturation. Second, we demonstrate that betatron radiation in plasma wakefields produces similar structuring in high-energy beams, forming "donut beams"" with pitch-angle anisotropies.

These features are confirmed through analytical theory and largescale three-dimensional simulations. Finally, simulations in the context of the Fireball experiment at CERN demonstrate how relativistic electron-positron beams develop collective instabilities under realistic laboratory conditions, providing the first direct analogues of astrophysical pair-plasma dynamics. Together, these studies represent two interconnected threads of extreme plasma physics, radiative cooling and pair-plasma kinetics, and lay the groundwork for a kinetic theory of radiatively structured plasmas, bridging theory, simulation, and experiment, and opening new paths toward probing high-energy astrophysical processes in the laboratory