Optical diagnosis of dense plasma evolution during irradiation by ultra-intense laser pulses

Intense laser-solid interactions have been the focus of much research for a number of decades, and many attractive properties have been demonstrated, such as the ability to produce beams of highly energetic particles, as well as intense X-ray, γ-ray, and THz radiation. This thesis reports on investigations into ultra-intense (10¹⁹ - 10²¹ Wcm⁻²) laser-solid interactions, using optical diagnostic techniques applied at state-of-the-art high power laser facilities. Experiments and numerical modelling are undertaken to introduce new methods of optically probing the properties of dense plasmas and their evolution during irradiation by intense laser light. The results reported provide new insight into the physical processes occurring, and the methods that are developed and demonstrated in this thesis have the potential to be employed on future experiments to characterise laser-plasma interactions. The new results introduced in this thesis are presented in three parts. In the first, experimental measurements of the specularly reflected light from the front surface of microstructured targets are reported. Based on these measurements and ray-tracing modelling, an analytical model is developed that utilises microstructured targets, and measurements of the spacing between the intensity maxima in the reflected light, to determine the laser focal spot size and the electron temperature within the region of the laser focal spot. These properties are difficult to measure by other means, and so the method proposed could be employed on future experiments to provide an enhancement to the research being undertaken. The second study is an investigation of stripe patterns measured in the profile of the laser light transmitted through expanding ultrathin foil targets, as they undergo relativistic self-induced transparency. Analysis of these results, along with numerical modelling, demonstrate that the size, ellipticity, and angular orientation of the plasma aperture directly influences the spatial-intensity distribution of these stripes. Previous studies have shown the potential for directly controlling the distribution of the electron beams produced during laser-plasma interactions by altering the spatial properties of the relativistic plasma aperture. The results presented here form part of an important study into how relativistic self-induced transparency can be utilised to enhance laserdriven ion acceleration. In the final study, measurements of the spectra of laser light back-reflected from thin foil targets are analysed. Observed shifting and broadening of the spectra, relative to the input laser spectrum, is discussed. It is found that the interaction between the outwards expansion due to thermal pressure and the inwards laser hole-boring radiation pressure can be probed by spectral measurements. The force which is dominant over the timescale of the interaction can be determined. From this, a diagnostic technique is presented that could be employed on future experiments to probe the direction of motion of the plasma critical surface during irradiation. In addition, this technique provides a method of estimating the reflectivity of the plasma, through measuring the degree of self-phase modulation occurring in the spectra. These three studies provide new insight into the complex dynamics of relativistically intense laser-plasma interactions, and demonstrate optical techniques that can be utilised in future experimental investigations to diagnose various properties of the motion and evolution of the dense plasma.Intense laser-solid interactions have been the focus of much research for a number of decades, and many attractive properties have been demonstrated, such as the ability to produce beams of highly energetic particles, as well as intense X-ray, γ-ray, and THz radiation. This thesis reports on investigations into ultra-intense (10¹⁹ - 10²¹ Wcm⁻²) laser-solid interactions, using optical diagnostic techniques applied at state-of-the-art high power laser facilities. Experiments and numerical modelling are undertaken to introduce new methods of optically probing the properties of dense plasmas and their evolution during irradiation by intense laser light. The results reported provide new insight into the physical processes occurring, and the methods that are developed and demonstrated in this thesis have the potential to be employed on future experiments to characterise laser-plasma interactions. The new results introduced in this thesis are presented in three parts. In the first, experimental measurements of the specularly reflected light from the front surface of microstructured targets are reported. Based on these measurements and ray-tracing modelling, an analytical model is developed that utilises microstructured targets, and measurements of the spacing between the intensity maxima in the reflected light, to determine the laser focal spot size and the electron temperature within the region of the laser focal spot. These properties are difficult to measure by other means, and so the method proposed could be employed on future experiments to provide an enhancement to the research being undertaken. The second study is an investigation of stripe patterns measured in the profile of the laser light transmitted through expanding ultrathin foil targets, as they undergo relativistic self-induced transparency. Analysis of these results, along with numerical modelling, demonstrate that the size, ellipticity, and angular orientation of the plasma aperture directly influences the spatial-intensity distribution of these stripes. Previous studies have shown the potential for directly controlling the distribution of the electron beams produced during laser-plasma interactions by altering the spatial properties of the relativistic plasma aperture. The results presented here form part of an important study into how relativistic self-induced transparency can be utilised to enhance laserdriven ion acceleration. In the final study, measurements of the spectra of laser light back-reflected from thin foil targets are analysed. Observed shifting and broadening of the spectra, relative to the input laser spectrum, is discussed. It is found that the interaction between the outwards expansion due to thermal pressure and the inwards laser hole-boring radiation pressure can be probed by spectral measurements. The force which is dominant over the timescale of the interaction can be determined. From this, a diagnostic technique is presented that could be employed on future experiments to probe the direction of motion of the plasma critical surface during irradiation. In addition, this technique provides a method of estimating the reflectivity of the plasma, through measuring the degree of self-phase modulation occurring in the spectra. These three studies provide new insight into the complex dynamics of relativistically intense laser-plasma interactions, and demonstrate optical techniques that can be utilised in future experimental investigations to diagnose various properties of the motion and evolution of the dense plasma