Synthetic radiation diagnostics as a pathway for studying plasma dynamics from advanced accelerators to astrophysical observations
Creators: name: Pausch, Richard
Publisher: name: Zenodo
Year: 2019

description: In this thesis, two novel diagnostic techniques for the identification of plasma dynamics and the quantification of essential parameters of the dynamics by means of electromagnetic plasma-radiation are presented. Based on particle-in-cell simulations, both the radiation signatures of micrometer-sized laser plasma accelerators and light-year-sized plasma jets are simulated with the same highly parallel radiation simulation framework, in-situ to the plasma simulation. The basics and limits of classical radiation calculation, as well as the theoretical and technical foundation of modern plasma simulation using the particle-in-cell method, are briefly introduced. The combination of previously independent methods in an in-situ analysis code as well as its validation and extension with newly developed algorithms for the simultaneous quantitative prediction of both coherent and incoherent radiation and the prevention of numerical artifacts is outlined in the initial chapters. For laser wakefield acceleration, a hitherto unknown off-axis beam signature is observed, which can be used to identify the so-called blowout regime during laser defocusing. Since significant radiation is emitted only after the minimum spot size is reached, this signature is ideally suited to determine the laser focus position itself in the plasma to below 100 μm and thus to quantify the influence of relativistic self-focusing. A simple semi-analytical scattering model was developed to explain the blowout radiation signature. The spectral signature predicted by the model is verified using both a large-scale explorative simulation and a simulation parameter study, based on an experiment conducted at the HZDR. Identified by the simulations, a temporal asymmetry in the scattered laser light, which cannot be described by state of the art quasi-static models of the blowout regime, makes it possible to determine the focus position precisely by using this radiation signature. For the so-called Kelvin-Helmholtz instability, a polarization signature is identified that allows both identifying the linear phase of the instability and quantifying its most important parameter, the growth rate. This plasma instability is suspected to occur in the shear region between plasma jets of active galactic nuclei or supernova remnants and the surrounding plasma and causes strong magnetic fields to grow along the shear surface. The measurement of the growth rate of these fields allows deducing the internal structure and dynamics of these jets and gaining an insight into previously inaccessible regions. A microscopic model of the electron dynamics was developed which describes the main radiation properties. With an unprecedentedly large and accurate simulation of the relativistic Kelvin-Helmholtz instability, the microscopic model was validated. The discovered polarization signature can be clearly identified even under arbitrary Lorentz transformations for observers on Earth and poses thus an ideal method for astronomical observations. These very different physical scenarios clearly exemplify the possibilities of synthetic radiation diagnostics and represent the first step towards future explorative studies of plasmas and their radiation in other scenarios using simulations.