In the two years since the last Gordon Conference, we have shifted our interest in laser-plasma interactions to the long pulse (1 ns) regime on the Argus facility. A variable Z disk series (at 3 x 10/sup 14/ W/cm/sup 2/) has been carried out and the Z dependence of line emission, backscatter, /sub h/ (hot electron temperature) and transport inhibition will be described. A variable thickness Au disk series that studied preheat and shocks will be analyzed. A simple theory for why T/sub h/ scales as Z/sup 1/4/ is presented, and a description of an electron transport experiment through layered disk targets is given.

When photon energies are well below 1 MeV the only significant contribution to elastic (coherent) photon-atom scattering comes from Rayleigh scattering, the elastic scattering of photons from bound atomic electrons. This report discusses the Rayleigh scattering cross sections for atoms and ions of low nuclear charge, particularly for photon energies in the vicinity of the threshold for photoionization from the K-shell. Just below this threshold energy there is a sequence of resonances in the elastic scattering amplitude. Each resonance occurs at an energy corresponding to the excitation of a K-shell electron to a higher unfilled shell. For a multi-electron atom the total cross section can go through a near zero minimum just below the resonance region due to interference between K and L amplitudes. The resonance region expands with increasing ionization, on the low side as more interior shells become unfilled and accessible, and, on the high side as the ionization threshold increases. Above the ionization threshold, in an isonuclear sequence the K-shell amplitudes share a common curve differing only in the position of the threshold. When the K-shell is opened the amplitude departs from this common curve. Above, but near, threshold the imaginary part of the K-shell amplitude is …

The mathematical formulations of the wave propagation problem in a linear viscoelastic solid are reviewed from the point of view of constitutive equations and the theory of linear physical systems. Various general results from the theory of propagating singular surfaces and from the mathematical theory of hyperbolic equations are applied to the analysis of the wave propagation process. The impulse responses of three viscoelastic media are analyzed by use of asymptotic methods. The three material models are the standard linear solid, the standard linear solid with a continuous spectrum of relaxation times, and the power law solid. The standard linear solid with a continuous spectrum of relaxation times and the power law solid have a nearly constant quality factor, Q, over the seismic frequency band. The impulse responses of these two viscoelastic solids are compared. The results show significant and discernible features in the wave profile. It is concluded that differentiation of the models can be made by comparing wave shapes and that a complete knowledge of Q over the entire frequency range is required to determine the wave propagation problem when initiated by an impulsive process. 11 figures, 1 table.