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Short wavelength laser gain studies in plasmas produced by a small ND:glass slab laser

A high-power, high-repetition-rate Nd:glass laser system was developed and used to study a collisionally pumped x-ray laser scheme near 200 A. The laser system used a commercial oscillator injecting 60-psec modelocked pulses at 1053 nm into a Nd:glass zig-zag slab amplifier which was developed for this project. The amplifier was capable of producing 2-J, near-diffraction-limited pulses at repetition rates of up to 0.3 Hz. Single-pass gains of up to 7.5 were measured with storage efficiencies of 3-4%, and the gain profile was flat to within 10%. Use of air cooling and subhertz repetition rates alleviated thermal distortion, stringent coolant channel sealing requirements, and optical surface degradation. An optical system was designed which incorporated flattop rectangular beams and relay imaging to generate 15-pm-wide, uniform line foci on the target. The focused laser radiation was used to generate linear plasma columns suitable for amplified-spontaneous-emission laser output near 200 A. An electron collisionally pumped scheme in Ni-like Nb, analogous to previously demonstrated Ni-like schemes at higher Z, was investigated experimentally. A solid Nb target was irradiated with modelocked trains of pump pulses; a gain at 204 A of about 5 cm 1 in a length of 3 mm at pump energies of 1.1 J/pulse was inferred from experiments in which the line focus length was varied. Comparable amounts of gain were seen following the second and third pump pulses, suggesting that an x-ray cavity might be feasible using this pumping scheme. Supporting evidence for gain was observed in the angular and temporal dependence of the emission. The measured gain was low compared to predictions, and the pump laser power appeared therefore to have been insufficient to achieve a high gain-length product. Theoretical work supporting the experimental efforts included calculation of ionization and recombination rate coefficients, development of a hydrodynamics model for the plasma cooling phase between pulses, and study of the extension of nonlinear four-wave-mixing and tripling processes to the soft x-ray regime. Thesis supervisor: Associate Professor Peter L. Hagelstein