Laser-Matter Interactions at Extreme Irradiance: X-ray Generation and Relativistic Channelling

Sammanfattning: This thesis summarizes experimental work in which the powerful Ti:sapphire lasers at the High Power Laser Facility at the Lund Laser Centre have been employed to investigate laser-produced X-rays and relativistic channelling. These lasers produce short pulses, of the order of 50 fs, with energies ranging from 1 mJ to 1 J. Thus, the peak powers reach several terawatts and with the most powerful laser system the peak irradiance exceeds 10^19 W/cm^2. Furthermore, X-rays generated at a synchrotron radiation facility have been used in conjunction with a short-pulse laser to study temporal oscillations on a picosecond timescale in a crystalline material. In the continuation of a research project, pursued at the High Power Laser Facility since 1992, X-ray generation up to several hundred keV has been investigated by focusing the laser pulses onto solid metal targets. One of the motivations for these experiments is the potential use of laser-produced X-rays in medical radiography. In another experiment the temporal features of X-rays of a few keV was investigated. In this case, the X-rays were generated by focusing laser pulses into clustered argon gas jets. Various detection techniques have been used to measure the X-ray spectrum in the different experiments, including energy-resolved photon counting with a charge-coupled device (CCD) and X-ray diffraction on crystals in combination with a CCD. Germanium detectors have also been employed in order to investigate the X-ray spectrum above a few keV more accurately. The laser-produced X-rays show extraordinary properties compared for example with those generated by the common X-ray tube. Very short pulse duration and a small source size are two important factors that differentiate the laser-based X-ray source from others. In particular, when focusing onto solid targets, a compact, high-repetition-rate laser system has been employed to evaluate the use of laser-produced X-rays (above 10~keV) in diagnostic medicine by recording radiographs using image plates. Since 1999, a research project at the High Power Laser Facility concentrates on a new field of research: Relativistic channelling. This involves meticulous control over the laser system that has been constructed to this end and also, comparing with other laser systems in Northern Europe, unprecedented levels of laser irradiance. For the first time in this part of Europe, relativistic channelling has been achieved by focusing laser pulses in a gas jet. The plasmas generated by the laser pulses have been diagnosed through Raman- and Thomson-scattering of the laser pulse. The extension of the generated relativistic channels has been measured and their dependence on laser pulse parameters and plasma density has been investigated. A typical channel length is 0.5 mm, and in such a channel, electrons are accelerated to several MeV, implying extreme accelerating electric fields, thousand times stronger than the maximum field strengths achieved in today's conventional accelerators. The number and energy of the electrons that are accelerated in the relativistic channels have been measured. This thesis presents more than the experimental work described above. The various laser-matter interactions encountered in the experiments are also discussed from a theoretical point of view. In particular, theory on laser-plasma interactions is introduced to allow better understanding of the experimental work.