Collisional effects and attosecond diagnostics in laser-generated plasmas

Doktorsavhandling, 2022

When matter is radiated by laser light of extreme intensity, it is rapidly ionized, thereby forming a plasma. Such laser-generated plasmas can be used as sources of energetic particles and radiation, or to study astrophysically relevant phenomena in the laboratory and the behavior of matter under extreme conditions. This thesis considers the dynamics and diagnosis of laser-induced plasmas, with focus on the effect of Coulomb collisions on electrostatic shocks and laser-energy absorption, as well as ultra-rapid plasma diagnostics using attosecond pulses.

Electrostatic shocks in plasmas have the potential to accelerate ions with a very narrow energy spread. First, collisional effects on electrostatic shocks are studied in two regimes of low and high collisionality. In the former, we show that even rare collisions can significantly affect the structure of the electrostatic shock over long time scales due to an accumulation of trapped ions. The high-collisionality case was studied using particle-in-cell simulations of laser foil targets. Effective ion acceleration by electrostatic shocks relies on a high electron temperature. Heating of the upstream ions, through collisions with the shock-accelerated ions, creates a self-amplifying process that increases the fraction of accelerated ions. However, this unstable condition rapidly depletes the energy of the shock, which transitions into a blast wave, unable to accelerate ions.

An additional study of the same laser--solid interaction shows that, unlike the commonly held knowledge, collisions may dominate the energy absorption of ultraintense laser pulses through inverse bremsstrahlung, and also causing rapid thermalization of the target electrons.

Finally, two diagnostic methods for the electron density utilizing attosecond extreme-ultraviolet pulses, are presented. The first method is based on the dispersion of a probe pulse, which can be used to infer information about the peak density and line-integrated density of the probed plasma. The second method is based on stimulated Raman scattering, which uses two pulses, and can give a localized reading of the electron density in the interaction regions where the two pulses meet.