PhD thesis by Zahra Chitgar
Optical Control of Laser-driven X-ray and XUV Radiation Sources
Zahra Chitgar
RWTH Aachen, xii, 111 p. (2021) [10.18154/RWTH-2021-10211]
In the framework of this thesis, a new methodology has been introduced to study the properties of radiation emitted by relativistic electrons driven by high-intensity lasers. Unlike the conventional ’far-field’ calculators of EUV and X-ray radiation emitted from laser-accelerated particles, the new method is designed to reveal near-field spatio-temporal information such as phase coherence and polarization. In order to establish the utility of this approach, two important classes of laser-based radiation sources, betatron radiation and high harmonic generation, are studied with the aim of enhancing their characteristics via optical control. Although primarily theoretical, this work has been conducted in close collaboration with experimental groups at PGI-6 in FZ Jülich and Institute of Plasma Physics in Prague.
The first part of this work is devoted to a novel form of laser wakefield electron acceleration in a nonlinear regime, which enhances the electron injection into an ion cavity by using two co-propagating laser pulses of the same duration and focal spot size. It is shown that electron injection in the double-pulse scheme occurs for 50% lower laser intensities compared to the standard single-pulse scheme. This lowered injection threshold is accompanied by higher injected charge and final energy. As a result, the quality of betatron radiation from electron oscillations within the field of a laser-generated cavity is also improved. Preliminary experimental results at IPP Prague using the PALS laser facility demonstrate the feasibility of this tandem-pulse scheme in terms of optical amplification and jitter stability. This injection mechanism is particularly advantageous for the new class of kHz laser facilities with terawatt peak power.
As a first step towards a near-field radiation model, a one-dimensional fluid model is formulated and put to use to study coherent harmonic generation arising from collective oscillation of relativistic electrons within the electromagnetic field of a laser. Of particular interest in this context is a special optical arrangement yielding circularly polarized harmonics. Here, two circularly polarized laser pulses (counter- or co polarized) with different wavelengths (400 + 800 nm) are combined in order to generate circularly polarized harmonics in a fully ionized plasma medium. General rules for helicity and selectivity of each mode are derived as well as a formula for the power of each mode using an analytical model. These results are verified numerically using both the fluid model and particle-in-cell simulation and have stimulated experimental studies of circularly polarized harmonic generation, being prepared at PGI-6, at FZ Jülich using the kHz JuSPARC-VEGA laser facility.
Finally, a more general 2-dimensional model is presented for use together with the EPOCH particle-in-cell code, which provides spatio-temporal information on the electrons trapped in the relativistically driven wakefield cavity. This model is verified via low energy O(100 eV) Thomson scattering radiation showing qualitative agreement with a standard far-field radiation postprocessor. The prospects for calculation of higher energy O(100 keV) betatron emission at sub-nanometer wavelengths with a future parallelized version of the model is discussed.