Two low energy (1.6 and 8 kJ) portable electrically exploding foil accelerators are developed for moderately high pressure shock studies at small laboratory scale. Projectile velocities up to 4.0 km/s have been measured on Kapton flyers of thickness 125 microm and diameter 8 mm, using an in-house developed Fabry-Perot velocimeter. An asymmetric tilt of typically few milliradians has been measured in flyers using fiber optic technique. High pressure impact experiments have been carried out on tantalum, and aluminum targets up to pressures of 27 and 18 GPa, respectively. Peak particle velocities at the target-glass interface as measured by Fabry-Perot velocimeter have been found in good agreement with the reported equation of state data. A one-dimensional hydrodynamic code based on realistic models of equation of state and electrical resistivity has been developed to numerically simulate the flyer velocity profiles. The developed numerical scheme is validated against experimental and simulation data reported in literature on such systems. Numerically computed flyer velocity profiles and final flyer velocities have been found in close agreement with the previously reported experimental results with a significant improvement over reported magnetohydrodynamic simulations. Numerical modeling of low energy systems reported here predicts flyer velocity profiles higher than experimental values, indicating possibility of further improvement to achieve higher shock pressures.

Shock experiments and numerical simulations on low energy portable electrically exploding foil accelerators.

Study of laser-driven shock wave propagation in Plexiglas targets

A three-term equation of state is used for the simulation of hydrodynamic shock phenomena. The reliability of it is tested by comparing the isotherms and shock Hugoniots obtained from it with available experimental data. This is illustrated for three typical materials Al, Pt and Au, which are of considerable interest for inertially confined thermonuclear fusion studies. We observe that reliable equation of state (EOS) data can be generated for nonmetallic substances like CH 2 . The EOS is used in the simulations of laser-driven shocks in double-layered target aluminum on gold (Al-Au) using hydrodynamic code employing a simplified laser-absorption model. The output from it is compared with the recent laser-driven shock experiments of Koenig et al.

Equation of State in Laser Shock Simulations

Laser-driven shock wave propagation in a transparent material such as Plexiglas coated with a thin overlayer of gold is studied using the technique of high speed optical shadowgraphy. A Nd: glass laser was focussed to produce intensities in the range of 10′2-10′4W/cm2 on the target, within an irradiation spot diameter of 160 pm, optical shadowgrams were recorded bya second harmonic (0.53 pm wavelength) pulse. Shock pressures and scaling of pressure with laser intensity was studied. Shock pressures in gold-coated Plexiglas target was observed to be considerably higher compared to those in uncoated targets. This enhancement of shock pressure has been explained on the basis of contribution of an X-ray driven ablative heat wave in the gold plasma. Shock pressure values show a close agreement with those obtained from a one-dimensional Langrangian hydrodynamic simulation. Shadowgrams of shock fronts produced by non-uniform spatial laser beam irradiation profiles have shown complete smoothing when a gol...

Laser driven shock wave studies in gold coated Plexiglas targets

Single copper wires have been electrically exploded at current densities of 107–108 A/cm2 using a pulse power setup consisting of a capacitor bank of $2.0~\mu \text{F}$ with discharge time period of $4.4~\mu \text{s}$ . A reproducible distinct kink or slope change is observed in measured postburst voltage fall across the wire under certain specific conditions of experimental parameters. To get an insight of possible mechanisms responsible for this, a 1-D numerical simulation has been utilized. The voltage profiles predicted by code appear to match well in shape and features with the experimentally recorded signals. The outcomes of numerical simulations indicate the formation of a density front inside the wire. Experimental analysis, carried out to explore the dynamics, predicts that this feature appears beyond the certain value of specific power. Specific power is found to be a more significant parameter than specific energy to obtain condition of such transient changes. For experiments reported here, the threshold value of specific power is found to be around 80 J/g/ns for copper. This work provides an insight into mechanisms involved in the explosion process of exploding wires.

Experimental and Numerical Analysis of Postburst Dynamics in Electrically Exploding Single Copper Wire

The effect of impedance mismatch on shock‐pressure enhancement in laser‐irradiated layered targets has been investigated by numerical simulation of the entire time history of shock generation. The model is a simple, one‐dimensional, single‐temperature Lagrangian hydrodynamic code incorporating a simplified laser‐absorption model and an equation of state suitable to describe the thermodynamic state of matter in a wide range of density and temperature. Effects of change in layer materials and other experimental parameters on the attainable shock pressure and compression have been studied. From an exhaustive study of such targets, it is found that it is possible to enhance the attainable values of compression (ρ/ρ0) and shock pressure with a given laser pulse by suitable optimization of the target. A pressure of more than 60 Mbar can be generated with a 1014 W/cm2, 300‐ps rise‐time laser pulse in such targets. In order to generalize the applicability of the results, within existing experimental uncertainties of measurements, and facilitate a suitable target design for any laser system, simple scaling laws are obtained in terms of parameters like intensity and rise time of the laser pulse.

T. C. Kaushik

Papers

Numerical study of the impedance mismatch effect in laser‐irradiated layered targets