Interaction of intense laser pulses with noble gas clusters and droplets

The time resolved dynamics of high intensity (1014 − 1017 W/cm2) laser interactions with noble gas (argon and krypton) nanometer-sized clusters and micron-sized droplets were experimentally investigated. For these investigations, a cryogenically cooled high-pressure pulsed gas jet, capable of stable operation in both cluster and droplet regimes, was specially constructed. The valve operation in the droplet mode, a regime not extensively studied in the literature, was characterized. Measurements of the total mass flow, the droplet size distributions, and droplet flow speeds as well as the temporal behavior of the valve were made and the results explained within the context of the phase diagrams for these gases and the dynamic operation of the valve.; Measurements were presented of x-ray and extreme ultraviolet (EUV) emission from both clusters and droplets irradiated with constant energy, variable width laser pulses ranging from hundred femtoseconds to several nanoseconds. Resonant coupling was observed for both targets which was understood in terms of two time scales natural to the plasma: a short time scale for optimal resonant absorption at the critical density layer in the expanding plasma, and a longer time scale for the plasma to drop below critical density.; Detailed simulations of the intense laser interaction with clusters, using a hydrodynamic model developed by our group, reveal a nonuniform expansion of the heated material which results in a long-time resonance of the laser field at the critical density plasma layer. Calculations simulating the variable pulse width measurements show resonant timescales in agreement with the experimental data.; The temporal character of the laser-droplet interaction was further investigated using a series of pump-probe experiments monitoring the delay-dependent x-ray and EUV emission, and by imaging frequency-doubled probe light scattered from the interaction region. Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective plasma lifetime was found to extend from a few hundred picoseconds to as long as several nanoseconds in agreement with expected plasma expansion, EUV excitation and recombination emission time scales.