Nonlinear laser–matter interaction processes in long‐scale‐length plasmas

This paper reports on nonlinear laser–plasma interaction experiments using long‐scale‐length plasmas produced by the 24‐beam OMEGA laser system operating at 351 nm. The experiments were carried out with distributed phase plates (DPP’s) in all beams and with and without smoothing by spectral dispersion (SSD). Most of the beams were used to create a large preformed plasma, which had gradient scale lengths of ≤800 μm at electron densities below a quarter of the critical density nc and temperatures in excess of 1 keV. One of the beams, the ‘‘interaction beam,’’ was timed independently and tightly focused to intensities ∼1015 W/cm2. All beams had pulse durations of ∼0.6 nsec. The interaction processes studied were mainly Raman scattering and the two‐plasmon decay (TPD) instability as evidenced by its characteristic 3/2‐harmonic emission. Details of the Raman and 3/2‐harmonic spectra are presented. Evidence was found for the TPD instability close to its Landau cutoff density at ∼ 0.2nc. Raman emission was narro...

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