Description
Extreme plasma physics concerns regimes in which strong electromagnetic fields, intense radiation, and matter–antimatter pair production fundamentally alter kinetic plasma behaviour. Such conditions arise in pulsar and black-hole magnetospheres, relativistic shocks, and are increasingly accessible in laboratory experiments. This thesis addresses two central components of this field: radiative plasma kinetics and electron–positron pair-plasma dynamics.
A kinetic model for radiatively dominated plasmas is developed using analytical theory and large-scale particle-in-cell simulations. Synchrotron cooling in strongly magnetized collisionless plasmas is shown to drive phase-space bunching, generically producing anisotropic ring-shaped momentum distributions with population inversion, i.e., ∂ f /∂ p⊥ > 0. These non-equilibrium states are unstable to the electron cyclotron maser instability (ECMI). Radiation reaction sustains the phase-space bunching, maintaining coherent emission beyond classical saturation. An analogous radiation-reaction–driven mechanism also arises in plasma wakefield accelerators, where betatron cooling induces amplitude bunching and ring-like phase-space structures in relativistic beams, enabling coherent emission via the ion-channel laser instability.
The thesis further investigates the collective behaviour of relativistic electron–positron plasmas, with contributions to the first laboratory generation of high-density, quasi-neutral pair beams at CERN’s SPS and led three-dimensional kinetic simulations demonstrating that realistic energy spread and angular divergence suppress current-filamentation growth. Additional non-ideal effects, including charge imbalance and multi-species components, were also analysed. This improved understanding of pair-beam instabilities establishes experimental constraints relevant to γ-ray–induced pair cascades and limits on the intergalactic magnetic field.
Together, these studies advance a unified kinetic description of extreme plasmas encompassing both radiative effects and pair-plasma dynamics.