On the Role of Ionization Physics in Intense Laser-Plasma Interactions

Ionization is critical in the formation and evolution of plasma dynamics; collisional ionization, in particular, is an often overlooked source of electrons when dealing with laser-plasma interactions. It, however, plays a crucial role in understanding the complex plasma kinetics, ranging from cold and sparse astrophysical settings to hot and dense fusion systems. This dissertation presents a new, deterministic algorithm that adds collisional ionization physics to particle-in-cell (PIC) codes. This algorithm offers improved accuracy, achieving up to two orders of magnitude decrease in the error of the ionization rate calculations versus the alternatives, scales linearly in execution time with the number of macro-particles per cell, and was rigorously tested for physical correctness. The first simulation study we present using this new algorithm examines a method of measuring the collisional relaxation time of a plasma with picosecond resolution through the evolution of short-pulse laser-generated plasma. We describe the evolution of a two-temperature plasma, created due to above-threshold ionization, that expands from ∼1 μm to a radius of ≈50 μm and is sustained due to the balancing currents of these “hot”, 500 eV, and “cold”, 100 eV, electrons. Our second study offers a model and supporting simulations for the stable generation of low divergence (≤20 mrad) proton beams from a novel liquid sheet target. Through the generation of a cold electron plasma (≲100 eV) via proton-impact ionization of a background water vapor, these proton beams drive a single filament Weibel instability, which causes the rapid growth of an azimuthal magnetic field that focuses these protons over long distances (cm scale). These studies provide a novel look at laser-plasma interactions that explore the dynamics of collisional ionization and its interplay with the plasma kinetics and are in good agreement with experimental data. Finally, our algorithm offers an alternative means of simulating collisional ionization inside a PIC framework that could easily be expanded, along with its described benefits, to include any other ionization or recombination scheme a user may desire.

Recommended citation: S. DiIorio, On the Role of Ionization Physics in Intense Laser-Plasma Interactions, Ph.D. dissertation, The University of Michigan, Ann Arbor, 2024.
Download Paper