A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH2 foils have been irradiated with a laser intensity of ∼10 16 W/cm 2 . The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2,000 km/s, an electron temperature of ∼110 eV, an ion temperature of ∼30 eV, and a density of ∼10 18 cm −3 were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/ωpi ∼0.16 mm, the interaction length, int, of ∼8 mm, and the Coulomb mean free path, λ mf p ∼27 mm. With c/ωpi int < λ mf p we are in a regime where collisionless shock formation is possible.

The parameters affecting strength development in compacted cylinders of 3Ca0. SiO, and P-2CaO. S O , mortars exposed to CO, were investigated. Strength increased with time up to 81 min, the duration of the longest detailed study. The P-2CaO. SiO, develops strength more slowly initially, but both silicates achieved compressive strengths of 7,000 to 10,000 psi. The rate of increase in strength depends on both the amount of water used in molding the compact and the amount of water present in the CO,. Increasing CO, pressures from 1 to 2 atm increased the rate of reaction, but a further increase to 4 atm had little additional effect. Carbonation occurs mainly in the outer portions of the cylindrical compacts. The initial reaction on exposure to CO, appears to be accelerated hydration of the silicates to a CaO-Si0,-H,O-like gel and calcite. The gel has a stoichiometry similar to that found in conventional hydration. Further reaction results in progressive carbonation of the gel, which decreases its lime content. The reaction products appear to be intimately dispersed in the microstructure.

The kinetic theory of ion-acoustic waves in multi-ion-species plasmas is discussed. Particular application is made to hydrocarbon (CH) plasmas, which are widely used in laser–plasma experiments. The mode frequencies and Landau damping of the two, dominant, ion-acoustic modes in CH plasmas are calculated by numerical solution of the kinetic dispersion relation. In addition, some useful results are obtained analytically from expansions of the kinetic dispersion relation and from fluid models. However, these results disagree with the numerical results in domains of particular practical interest. When ion temperatures exceed two-tenths of the electron temperature, the least damped mode is the one with the smaller phase velocity, and this mode is then found to dominate the ponderomotive response of the CH plasma.

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A desire to interpret recent experiments on filamentation with and without beam-smoothing techniques led to the development of a three-dimensional fluid model that includes the effects of nonlocal electron transport and kinetic ion damping of the acoustic waves. The damping of the electron-temperature perturbations that drive thermal filamentation by nonlocal electron conduction, valid in the diffusive limit, is supplemented in the present model by electron Landau damping in the collisionless limit when the wavelength of the perturbation is much less than the electron–ion scattering mean-free path. In this collisionless limit, Landau damping of the ‘‘temperature’’ fluctuations makes ponderomotive forces universally more important than thermal forces. Simulations in plasmas of current interest illustrate the relative importance of thermal and ponderomotive forces for strongly modulated laser beams. Although thermal forces may initiate filamentation, the most intense filaments are associated with ponderomotive forces. The present simulations of filamentation model well the density perturbations observed in experiments [Young et al., Phys. Rev. Lett. 61, 2336 (1988)]. In addition, a simple criterion is obtained analytically and supported by simulations for stabilization of filamentation by laser beam-smoothing techniques such as induced spatial incoherence and random phase plates [Eq. (1)].