It is shown that the Landau-Spitzer theory for temperature relaxation between electrons and ions, which was originally derived for ideal plasmas, is in fact more general. A relaxation formula is derived, for arbitrary ion-ion coupling that follows from elementary considerations combined with the fluctuation-dissipation theorem and the f-sum rule. The conditions for the validity of this theory are weak electron-ion coupling and that the spectrum of fluctuations of the ions lies at energies far below the resonances of the electrons spectrum. It is found that the rate of energy relaxation is not sensitive to the details of the ion-excitation spectrum. For classical electrons the formula reduces to the Landau-Spitzer form with minor modifications.

It is found that when a planar shock is imparted to a system of a foam of low mean density filled with fluid (fiber density>ambient fluid density), an undercompressed quasisteady post-shock state is established. In this state, pressure and density are lower than the values predicted by Hugoniot relations for a uniform fluid with the same average fluid-foam density. It is shown that this undercompression phenomenon is due to residual correlations of fluctuations left after the saturation of the initially rapid mixing of fiber material with the background fluid. Generalized Hugoniot relations are derived for quasiplanar shocks in nonuniform systems, with foam as an example. The behavior of fluctuations in such systems is studied. Formulas for the level of fluctuations induced by a shock as it propagates through foam and for the relaxation time of the fluctuations, are derived.

This paper analyzes the requirements of a photoionization-front experiment that could be driven in the laboratory, using thermal sources to produce the necessary flux of ionizing photons. It reports several associated conclusions. Such experiments will need to employ the largest available facilities, capable of delivering many kJ to MJ of energy to an X-ray source. They will use this source to irradiate a volume of neutral gas, likely of N, on a scale of a few mm to a few cm, increasing with source energy. For a gas pressure of several to ten atmospheres at room temperature, and a source temperature near 100 eV, one will be able to drive a photoionization front through a system of tens to hundreds of photon mean free paths. The front should make the familiar transition from the so-called R-Type to D-Type as the radiation flux diminishes with distance. The N is likely to reach the He-like state. Preheating from the energetic photons appears unlikely to become large enough to alter the essential dynamics of the front beyond some layer near the surface. For well-chosen experimental conditions, competing energy transport mechanisms are small.

a b s t r a c tA new method, 'Configurationally-Resolved-Super-Transition-Arrays', for calculation of the spectral absorption coefficient in hot plasmas is presented. In the new method, the spectrum of each SuperTransition-Array is evaluated as the Fourier transform of a single Complex Pseudo Partition Function, which represents the exact analytical sum of the contributions of all constituting unresolved transition arrays sharing the same set of one-electron solutions. Thus, in the new method, the spectrum of each Super-Transition-Array is resolved down to the level of the (unresolved) transition arrays. It is shown that the corresponding spectrum, evaluated by the traditional Super-Transition-Arrays (STA) method [14], is just the coarse-grained Gaussian approximation of the Configurationally-Resolved-Super-TransitionArray. A new computer program is presented, capable of evaluating the absorption coefficient by both the new configurationally resolved and the traditional Gaussian Super-Transition-Arrays methods. A numerical example of gold at temperature 1 keV and density 0.5 gr/cm 3 , is presented, demonstrating the simplicity, efficiency and accuracy of the new method.

It is shown that the contributions of all partially resolved transition arrays (PRTAs) to the spectrum of a supertransition array (STA) may be summed by an efficient analytical method. The method is similar to the configurationally resolved super transition array method [G. Hazak and Y. Kurzweil, High Energy Density Phys. 8, 290 (2012)1574-181810.1016/j.hedp.2012.05.001] and avoids the Gaussianity assumption of the partially resolved super transition arrays method [B. G. Wilson, C. A. Iglesias, and M. H. Chen High Energy Density Phys. 14, 67 (2015)1574-181810.1016/j.hedp.2015.02.007], thus yielding an STA spectrum which is resolved down to the PRTA level.