High harmonic generation and ion acceleration with high-intensity laser pulses

This thesis reports on research performed on high harmonic generation and ion acceleration, processes that are based on the interaction of gaseous and solid media with high-intensity laser pulses. High harmonic generation leads to the production of beams of ultrashort, laser-like radiation in the wavelength range reaching from the extreme ultraviolet (XUV) to soft X-rays. Ion acceleration generates highly energetic protons and ions in the form of highly directional and pulsed beams with an ultrashort duration. The scope of this thesis is to present novel investigations and methods towards an enhancement of the flux and maximum energy of the accelerated ions and the harmonic radiation via an improved understanding of the underlying physics and, to provide a more complete identification of the actual requirements of the laser and the target for optimum output.\ud The first part of this work concerns the construction and testing of a setup to generate high harmonics in gaseous media. The setup employs a capillary waveguide in which we implemented a novel manner of differential pumping. We performed an experiment on an enhancement technique, called Harmonic Excitation, and demonstrated for the first time that this technique can be applied in a capillary waveguide.\ud The second part of the research concerns ion acceleration and high-order harmonic generation from solid-state targets using high-intensity laser pulses with ultra-high contrast. We investigated the coherence properties of high-order harmonic generation via coherent wake emission. Special attention was given to the preparation and investigation of extremely thin foils, i.e. freestanding nanofoils, to enter a very promising, but largely unexplored, regime. We experimentally investigated ion acceleration in this, so-called, transparent regime for the first time, and compared the results with predictions from improved numerical calculations. We also investigated for the first time high-order harmonic generation in the transparent regime, experimentally and via numerical modeling

Feasibility of Intestinal MR Elastography in Inflammatory Bowel Disease

Background: While MR enterography allows detection of inflammatory bowel disease (IBD), the findings continue to be of limited use in guiding treatment-medication vs. surgery. Purpose: To test the feasibility of MR elastography of the gut in healthy volunteers and IBD patients. Study type: Prospective pilot. Population: Forty subjects (healthy volunteers: n = 20, 37 ± 14 years, 10 women; IBD patients: n = 20 (ulcerative colitis n = 9, Crohn's disease n = 11), 41 ± 15 years, 11 women). Field strength/sequence: Multifrequency MR elastography using a single-shot spin-echo echo planar imaging sequence at 1.5 T with drive frequencies of 40, 50, 60, and 70 Hz. Assessment: Maps of shear-wave speed (SWS, in m/s) and loss angle (φ, in rad), representing stiffness and solid-fluid behavior, respectively, were generated using tomoelastography data processing. Histopathological analysis of surgical specimens was used as reference standard in patients. Statistical tests: Unpaired t-test, one-way analysis of variance followed by Tukey post hoc analysis, Pearson's correlation coefficient and area under the receiver operating characteristic curve (AUC) with 95%-confidence interval (CI). Significance level of 5%. Results: MR elastography was feasible in all 40 subjects (100% technical success rate). SWS and φ were significantly increased in IBD by 21% and 20% (IBD: 1.45 ± 0.14 m/s and 0.78 ± 0.12 rad; healthy volunteers: 1.20 ± 0.14 m/s and 0.65 ± 0.06 rad), whereas no significant differences were found between ulcerative colitis and Crohn's disease (P = 0.74 and 0.90, respectively). In a preliminary assessment, a high diagnostic accuracy in detecting IBD was suggested by an AUC of 0.90 (CI: 0.81-0.96) for SWS and 0.84 (CI: 0.71-0.95) for φ. Data conclusion: In this pilot study, our results demonstrated the feasibility of MR elastography of the gut and showed an excellent diagnostic performance in predicting IBD. Evidence level: 1 TECHNICAL EFFICACY: Stage 1

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements