We report results from the analysis of the shock wave formed following the creation of a laser-produced plasma in a gaseous atmosphere, using both interferometry and shadowgraphy. A Nomarski polarization interferometer and a focused-type shadowgraphy setup were utilized to track the evolution of the shock wave with high spatial and temporal resolution for a variety of incident laser energies and ambient gas pressures. It was found that the visibility of the shock wave was high for both techniques at high gas pressure (100 mbar) and incident laser energies (400 mJ). The velocity of the shock wave in these regimes was of the order of several km s −1. At lower pressures (≈ 1-10 mbar) and incident laser energies (≈ 100-200 mJ), the visibility of the shock wave decreased dramatically and, in some cases, disappeared completely from the shadowgrams. In contrast, the shock wave remained visible in the interferograms, manifesting itself as a blurring of the fringes. The shock wave visibility was improved further by simply differentiating the interferograms to enhance the fringe boundaries. Shock velocities, exceeding 100 km s −1 , were detected at low background gas pressures where the enhanced shock wave visibility was provided by the interferometer.
Ion signals from laser produced plasmas (LPPs) generated inside aluminum rectangular cavities at a fixed depth d = 2 mm and varying width, x = 1.0, 1.6, and 2.75 mm were obtained by spatially varying the position of a negatively biased Langmuir probe. Damped oscillatory features superimposed on Maxwellian distributed ion signals were observed. Depending on the distance of the probe from the target surface, three to twelve fold enhancements in peak ion density were observed via confinement of the LPP, generated within rectangular cavities of varying width which constrained the plasma plume to near one dimensional expansion in the vertical plane. The effects of lateral spatial confinement on the expansion velocity of the LPP plume front, the temperature, density and expansion velocity of ions, enhancement of ion flux, and ion energy distribution were recorded. The periodic behavior of ion signals was analyzed and found to be related to the electron plasma frequency and electron-ion collision frequency. The effects of confinement and enhancement of various ion parameters and expansion velocities of the LPP ion plume are explained on the basis of shock wave theory.
Measurements of the total ion emission from a pair of colliding laser-produced aluminium plasmas were obtained with the aid of a Faraday cup detector. The energy profile width at half height of the kinetic energy distribution for ions emitted normal to the target was found to be 30% narrower for colliding plasmas compared to a single plasma. Similar to ion emission from single plumes, the mean ion kinetic energy is observed to increase with the energy of the incident laser pulse. However, the width of the ion energy distribution increases at a significantly slower rate than in the single plume case.
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