LASER HEATING OF ALUMINUM EXPLODING WIRE PLASMAS

The major objectives of this research project were to study the x-ray emissions and dynamical behavior of exploded aluminum wire plasmas when subjected to intense CO(,2) laser radiation. The laser system is based on a Lumonics 601 TEA laser. It was optimized to produce a diffraction limited spot, diameter of 125 microns, yielding a focused intensity of 10('12) W/cm('2). The principal x-ray diagnostics consisted of a two-foil temperature measurement using filtered photomultiplier detectors and an x-ray pinhole camera. These measurements were augmented by a variety of electrical and optical diagnostics to determine the time evolution of the effects of laser irradiation. Interaction experiments were performed with a radial, one-sided illumination at the plasma midplane.; The evolution of the exploding wire can be divided into three stages: (1)Ohmic heating of the wire until a plasma forms, (2)azimuthally symmetric plasma, (3)plasma disruption by instabilities. The second stage was used as the target for laser interaction experiments. The plasma consisted of two regions: (1)a low density, high temperature sheath (kT(TURN) 30 eV and N(,i)(TURN) 10('16-17)/cm('3)) and (2) a high density, low temperature interior or core (kT(TURN) 5 eV and N(,i)(TURN) 10('18)/cm('3)).; There is negligible laser absorption in the sheath, but in the core it is very significant. The absence of photons with E > 3 keV, no indication of saturation in the x-ray signals (plasma heating), and the lack of correlation between the incident and reflected laser power seem to indicate that nonlinear process such as stimulated Brillouin scattering, stimulated Raman scattering, nonlinear inverse bremsstrahlung, and resonant absorption are not significant factors in the laser interaction.; Conduction of laser energy away from the focal volume limits laser heating. Thermal conduction, hydrodynamics, and ionization have been identified as potentially significant processes. No direct interpretation of two-foil temperature data could be performed assuming a one temperature, homogenous plasma emission model. Either temperature gradients or line and recombination radiation contribute to the failure of this model.