Dynamic stabilization of incompressible and immiscible newtonian fluids is studied by means of an approximate analytical model that considers the vertical vibration of the interface between the fluids. The force driving the vibration is modeled by periodic sequences of Dirac deltas. The model shows the roles played by surface tension and viscosity in determining the stability boundaries and the relevant similarity parameters are found. The results are compared with previous theoretical and experimental studies that used a sinusoidal vibration and they are found to present the same qualitative features provided a symmetric sequence of Dirac deltas is considered. Instead, important differences are observed when an asymmetric driving is used.

Dynamic stabilization of Rayleigh-Taylor instability in an ablation front is studied by considering a modulation in the acceleration that consists of sequences of Dirac deltas. This allows obtaining explicit analytical expressions for the instability growth rate as well as for the boundaries of the stability region. As a general rule, it is found that it is possible to stabilize all wave numbers above a certain minimum value k m , but the requirements in the modulation amplitude and frequency become more exigent with smaller k m . The essential role of compressibility is phenomenologically addressed in order to find the constraint it imposes on the stability region. The results for some different wave forms of the acceleration modulation are also presented.

An analysis of dynamic stabilization of Rayleigh-Taylor instability in an ablation front is performed by considering a general square wave for modulating the vertical acceleration of the front. Such a kind of modulation allows for clarifying the role of thermal conduction in the mechanism of dynamic stabilization. In addition, the study of the effect of different modulations by varying the duration and amplitude of the square wave in each half-period provides insight on the optimum performance of dynamic stabilization.

Explosion of a metallic wire due to a large electrical 2 current can be used for studying metallic states difficult to 3 reach with other methods. Due to experimental constraints, direct 4 measurement of the voltage drop across the wire is impractical, 5 although many characteristics of the metal state in the wire can 6 be derived from these waveforms. Usually, the transformation of 7 the electrical signals is made with the assumption of a lumped 8 model for all the elements of the circuit, including the wire. We 9 discuss the validity of a lumped model, and we show that due 10 to the variation in time of the current density distribution on 11 the wire, this model will not provide accurate values for the 12 wire resistivity. Wire resistivity inaccuracies are specially clear 13 in gas and plasma states, due to the diffusion and movement of 14 the current that produce a large variation of the magnetic flux 15 inside the wire. 16 In order to obtain more precise results in the resistivity of the 17 wire metal, regardless of its state, a better approach is the use 18 of the Faraday's law of induction on a path along the border of 19 the wire. Our experiments of exploding wires in atmospheric air 20 present the advantage of the clear electrical boundary between 21 the expanding wire and the surrounding air, where no current 22 circulates. As the state of the wire boundary layer changes form 23 solid to plasma, it is possible to estimate the resistivity of the 24 metal in those states in a more precise way. 25 Index Terms-Circuit analysis, metals, atmospheric-pressure 26 plasmas, exploding wire, resistivity. 27 I. INTRODUCTION 28 W HEN a large electrical current passes through a metal-29 lic wire of the proper dimensions, typically 100 µm of 30 diameter and centimeters length, the metallic wire is heated 31 rapidly by Joule effect, becoming liquid, then gas, to later 32 be transformed in plasma. This system is called exploding 33 wire, and it is well known to science since a long time. 34 It had been used in multiple endeavors, because the rich 35 phenomena that can be accessed with it. Broad examples of 36 the use of exploding wire are the general use as generator 37 mechanism for blast waves [1] or the better understanding 38 of the fuse dynamics through experiments like in the work 39 of Vermij [2]. Exploding wire systems can also be used 40 for important industrial or military applications, like in the 41 preparation of metallic nano-powders reviewed by Kotov et 42 al. [3], or the study of the mitigation of blast waves by foam, 43 through the use of a surrogate setup, as in the recent work of 44 Liverts et al. [4].

The chlorine He a radiation of polyvinyl chloride~PVC! was investigated with respect to X-ray scattering experiments on dense plasmas. The X-ray source was a laser-produced plasma that was observed with a highly reflective highly oriented pyrolytic graphite~HOPG! crystal spectrometer as it is used in current x-ray scattering experiments on dense plasmas. The underlying dielectronic satellites of He a cannot be resolved, therefore the plasma was observed at the same time with a focusing spectrometer with spatial resolution. To reconstruct the spectrum a simple model to calculate the spectral line emission based on dielectronic recombination and inner shell excitation of helium-and lithium-like ions was used. The analysis shows that chlorine dielectronic satellite emission is intense compared to He a in laser-produced chlorine plasmas with a temperature of 300 eV in this wavelength range of Dl ϭ 0.07 Å~DE ϭ 43 eV!. The method proposed in this paper allows deducing experimentally the role of the underlying dielectronic satellites in the scatter spectrum measured with a HOPG crystal spectrometer. It is shown that the dielectronic satellites can be neglected when the scattering is measured with low spectral resolution in the non-collective regime. They are of major importance in the collective scatter regime where a high spectral resolution is necessary.