Interfacial instability in dense neutral gas confinement of laser-produced plasmas.

摘要

There has been increasing interest in physical properties of dense plasmas (n_{electron18/cm3) to understand nonlinear processes. Strong ionization is readily achieved in laser-produced-plasmas but the fast plasma expansion in vacuum is an obstacle to achieving higher density. The higher densities could be realized if the plasma expansion is suppressed in a dense gas.; In this dissertation, the plasma evolution in a dense neutral gas has been investigated. The plasma was produced by a high power Nd:glass laser pulse incident on a metallic target (aluminum or copper) in a high pressure chamber containing helium, argon, or their admixture. The laser pulse operating at 1.06$m$m has 40nsec-pulse-width and 2.5Joule-total-energy. The laser beam was focused to a size of 4mm in diameter at the target. The plasma density and evolution were observed by means of streak photography and incident laser attenuation by the plasma. The shock speed driven by the plasma was also monitored.; At high neutral gas densities above a threshold, an instability was observed, however. When the absorption measurement was repeated under identical conditions, the fluctuation in the transmitted intensity sharply increased with the neutral-gas-mass-density exceeded the threshold, indicating a Rayleigh-Taylor interfacial instability.; We have devised an advanced streak photography for more quantitative analysis. The plasma was imaged from two orthogonal side-directions at 0.51mm from the target surface. An early time front-view photograph was simultaneously taken. A generalized Abel inversion algorithm was devised for reconstruction of the plasma cross-sectional profile from the above measurements without invoking axial symmetry.; A 2-D fast Fourier transformation was then performed on the reconstructed cross-sectional profiles. The growth rates of the instability have been determined from the fluctuation of the power amplitudes.; In addition, from the time-resolved laser absorption measurement and the target mass loss per laser shot, we were able to estimate the numerical values of Atwood number. This has provided a quantitative criterion for the Rayleigh-Taylor instability onset.; The strong interfacial instability compromises the neutral gas confinement. The strength of the instability can be minimized, however, if Atwood number is controlled by judicious choice of target materials and the neutral gas. Specific suggestions for possible improvements are offered.}