Development, characterization and application of a high average power capillary discharge soft x-ray laser

Abstract

A compact high repetition rate, high average power capillary discharge laser operating at a wavelength of 46.9 nm that is of the size of many widely utilized visible and UV lasers has been developed and characterized. Two significant differences from previously developed capillary discharge soft x-rays lasers, the use of ceramic capillaries rather than poly-acetal and significantly longer plasma columns of up to 36 cm, allowed for the generation of greatly increased output pulse energies and average power. Lasing at a repetition rate of 4 Hz and an average laser pulse energy of 0.88 mJ has been obtained, which amounts to an average power of 3.5 mW. Lasing at repetition rates as high as 10 Hz was also achieved, but with lower output pulse energy. In this work the beam energy and divergence were measured as a function of capillary length, and the temporal evolution of the laser pulse was also studied. The combined high pulse energy and high repetition rate of this laser make it unique in the field of soft x-ray lasers to date. This laser has been used to perform angular dependent reflectivity measurements to determine optical constants of materials at 46.9 nm, which are in good agreement with those previously measured, or in some cases represent the first recorded values at this wavelength. This experiment constitutes the first application of a table-top soft x-ray laser to the field of material characterization. In a separate experiment, the output beam was polarized using two multi-layer coated mirrors that were configured for optimum reflectivity at 45 degrees. The resulting 96% polarized beam was then used to characterize the efficiency of a diffraction grating. Finally, in a third experiment, the beam was focused using a spherical multi-layer coated mirror to a spot size where the majority of the energy was confined to a 2 μm diameter. The peak intensity was estimated to be 1x1011 W/cm2. Through ray tracing computations, the focused spot size was determined to be dominated by spherical aberration. This focused beam reached intensities that were sufficient to induce ablation on brass and stainless steel targets realizing the first demonstration of material ablation with a coherent soft x-ray beam.