Diagnostics and applications of laser-produced plasmas

Abstract

X-ray spectroscopy is one of the most important diagnostics of laser-produced plasmas, finding application in diverse areas such as laser fusion, x-ray lasers, and novel experiments using short-pulse lasers to probe chemical and biological phenomena on the femtosecond timescale. Depending on the aims of these experiments, either high resolution spectra combined with either spatial or time resolution, or monochromatic x-ray images can be obtained. Arrays with up to 10 toroidally bent crystals have been developed to study complex implosion processes in laser fusion experiments at the GEKKO XII laser facility. Another specially designed x-ray spectrometer was also used here in x-ray diagnosis of 4f → 3d transitions in Nickel-like transitions of elements with atomic numbers between 70 and 74. The dependence of this x-ray emission on laser energy, spot size, and target material provides information about ionization degree, electron temperature and density - important parameters for the population inversion of a Ni-like X-ray laser in the water window. These x-ray spectrometers have been designed using ray-tracing and Bragg reflection codes for 1D or 2D bent crystals or combinations thereof. In the preparation process, extreme care has been taken over crystal perfection, selection of optimum reflections, precision bending, measurement of imaging and reflection properties. X-ray topographic cameras and diffractometers were used to check the relevant properties of the analyzer crystals. High-intensity femtosecond lasers provide an inexpensive and powerful source of ultra-short x-ray pulses when focused onto solid targets. Information on the production efficiency, energy distribution and transport of hot electrons is necessary to maximize x-ray output in desired K-shell emission lines or continuum ranges so that peak brilliances close to those of third generation synchrotron radiation sources may be feasible. Combining these new sources with bent-crystal optics has enabled medical imaging as well as real-time diffraction experiments to be demonstrated on sub-picosecond time scales.