Collisional effects on electrostatic shock waves and heating in laser-generated plasmas

Licentiate thesis, 2020

Electrostatic shock waves are associated with an electrostatic field structure propagating at supersonic speed through laboratory or astrophysical plasmas. Shock ion acceleration schemes, based on the strong electrostatic field in the shock structure, show promising potential due to the narrow energy spread of accelerated ions – which can be applied in plasma diagnostics, the generation of warm dense matter or medical purposes. The use of high-intensity laser pulses to generate shocks in the laboratory commonly result in plasmas which are weakly collisional; thus collisions are usually neglected in the corresponding theoretical, kinetic studies. By contrast, this thesis considers the effects of collisions on the structure and dynamics of electrostatic shocks as well as laser absorption and subsequent plasma heating.

First, the structure of electrostatic shocks is considered in weakly collisional plasmas, via a semi-analytical model. Collisions are found to cumulatively affect the shock structure on longer time scales, despite the low collisionality. Then, the impact of collisions on laser-driven plasmas is analyzed via numerical, particle-in-cell, simulations. The importance of collisions is heightened in plasmas comprising highly charged ions at solid density. Collisional inverse Bremsstrahlung heating is found to be able to generate well-thermalized electrons at energy densities relevant for warm- and hot-dense-matter applications. The strong electron heating also creates favorable conditions for electrostatic shocks. Collisions between shock-accelerated and upstream ions are found to increase the fraction of accelerated ions, thus bootstrapping the shock ion acceleration. Lastly, collisional ion heating is studied in connection to the shock. Different modeling approaches available to treat the highly collisional, solid density plasmas may predict qualitatively different shock dynamics, providing an opportunity for experimental model validation.