Ion emission properties of tin plasmas generated by 2 µm-wavelength laser pulses

Using radiation-hydrodynamic simulations, we investigate the properties of ion emission from a plasma generated by irradiating tin droplets with λ_laser = 2 μm-wavelength laser light. Two cases are considered: first a ‘short pulse’ (27 ns-long) case with laser intensity I_laser ∼ 10^11 W cm^−2 that can be readily benchmarked with present experimental systems. Then, a ‘long pulse’ case is studied, in which the droplet is irradiated until it is fully vaporized, which takes ∼ 150 ns. The kinetic energy-resolved ion spectra in both cases feature a high-energy peak on the order of keVs. At lower kinetic energies the spectra are substantially different: the short pulse spectrum exhibits much higher ion numbers due to plasma cooling at the end of the pulse, which is not present in the long pulse case. The following quantities of interest are analyzed: angle-dependence of the peak kinetic energy, total kinetic energy, and total ion number, as well as intensity-dependence of the kinetic energy peak. To provide a measure of non-fluid behavior of the expanding plasma, we calculate the local Knudsen number; though the obtained values are significant, the kinetic energy data are found to be reliable, and can be extrapolated to larger distances.

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