Wigen Nazarov

Observation of ionization fronts in low density foam targets

Ionization fronts have been observed in low density chlorinated foam targets and low density foams confined in gold tubes using time resolved K-shell absorption spectroscopy. The front was driven by an intense pulse of soft x-rays produced by high power laser irradiation. The density and temperature profiles inferred from the radiographs provided detailed measurement of the conditions. The experimental data were compared to radiation hydrodynamics simulations and reasonable agreement was obtained.

Equation of state data experiments for plastic foams using smoothed laser beams

The importance of foams in laser produced plasmas has been recently pointed out in both Inertial Confinement Fusion (ICF) and astrophysics laboratory dedicated experiments. In this paper, Equation of State (EOS) data measurements of plastic porous materials have been experimentally determined using ns laser pulses smoothed with Phase Zone Plates (PZP). Foams of density in the range 20–400 mg/cm3 and thickness about 20 μm were used. A new original scheme for the targets has been designed which allowed, for the first time with laser, EOS data to be obtained for pressures ranging from 0.1 to 2.5 Mbars. Results are discussed and compared with available models.

In situ production of very low density microporous polymeric foams

Small, open‐ended, parylene microcylinders of 400 μm diameter, 700 μm length, and 10 μm wall thickness were filled with a solution of a polyfunctional monomer of low concentration. The solution was polymerised in situ with ultraviolet light to produce a gel. Precipitation of these gels in a nonsolvent and subsequent drying by means of a critical point drying apparatus produced microcylinders filled with a low density foam. Radiographic examination of the foam gave a density of the order of 2 mg cm−3, and scanning electron microscopy micrographs showed cell sizes of the order of 1 μm. No shrinkage of the foam was observed with cylinders of the dimensions given, but with larger cylinders of diameter and length of the order of 1000 μm, some axial shrinkage was observed which resulted in cylinders with slightly concave ends (of the order of 20 μm).

Recent advances in laser–plasma experiments using foams

The paper discusses recent advances in the use of foams in laser-plasma experiments, concerning in particular: (1) the use of foam in order to get an efficient smoothing of laser energy deposition, (2) the problem of hydrodynamics of layered foam-payload targets, (3) the use of foam for shock pressure amplification in equation-of-state experiments, (4) the study of the equation of state of foams in the Megabar regime, and (5) the use of foams for astrophysics relevant experiments, here in particular shock acceleration experiments.

Laser driven shock wave acceleration experiments using plastic foams

In this letter, we present an experimental study of shock propagation in a decreasing density gradient which is a common feature in many astrophysical objects. An original scheme for the targets has been designed to investigate the case of a discrete density step. Using foams in a wide density range (20–400 mg/cm−3), we obtained accurate data on shock acceleration at the solid-foam interface. Results are discussed and compared with models.

Dynamics of laser produced shocks in foam–solid targets

The influence of foams on laser shocks was studied with ns laser pulses smoothed with phase zone plates and focused onto layered foam–aluminum targets. Foams of 5–200u2009mg/cm3 density and 60 μm thickness were used. A strong pressure increase was measured with the foam in comparison to focusing the beam directly onto aluminum due to impedance mismatch at the aluminum–foam interface. Below a particular density, the measured pressure decreased as a result of hydrodynamics effects. Results are compared with computer simulations.

Propagation issues and energetic particle production in laser-plasma interactions at intensities exceeding 1019 W/cm2

A series of experiments recently carried out at the Rutherford Appleton Laboratory investigated various aspects of the laser‐plasma interaction in the relativistic intensity regime. The propagation of laser pulses through preformed plasmas was studied at intensities exceeding 10 19 W0cm 2 . The transmission of laser energy through long-scale underdense plasmas showed to be inefficient unless a plasma channel is preformed ahead of the main laser pulse. The study of the interaction with overdense plasmas yielded indication of collimated energy transport through the plasma. The production of fast particles during the interaction with solid density targets was also investigated. The measurements revealed the presence of a small-sized directional source of multi-megaelectron volt protons, which was not observed when a plasma was preformed at the back of the solid target. The properties of the source are promising in view of its use in radiographic imaging of dense matter, and preliminary tests were carried out.

Shock velocity and temperature measurements of plastic foams compressed by smoothed laser beams

The importance of foams in laser produced plasmas has been recently pointed out in both inertial confinement fusion (ICF) and astrophysics laboratory dedicated experiments. In this paper, measurements of shock velocity using velocity interferometer for any reflector and of temperature in porous materials are presented. Reflectivity from the rear side of the target was measured using a probe beam and only foams of density higher than 800mg∕cm3 gave valuable data. Results are discussed and compared with available models.

EOS Data Experiments for Plastic Foams Using Smoothed Laser Beams

The importance of foams on laser-produced plasmas has been recently pointed out. In this paper EOS measurements of plastic porous materials have been experimentally determined using ns laser pulses smoothed with phase zone plates. Foams of density in the range 50-200 mg cm-3 and thickness about 20 microns were used. An original scheme for the targets has been designed which allowed us to obtain, for the first time, EOS data for pressures between 0.2-0.8 Mbar. Results are discussed and compared with EOS models.

Shock impedance matching experiments in foam - solid targets: implications for `foam-buffered ICF'

The influence of foams on laser-produced shocks has been studied experimentally using sub-ns laser pulses smoothed with phase zone plates and focused on layered foam - aluminium targets. A strong pressure increase was measured when the foam was present in comparison with that obtained by focusing the beam directly onto the aluminium target, due to impedance mismatch at the aluminium - foam interface. Results are compared with computer simulations. The impact of these measurements on the possible use of `foam-buffered targets for direct-drive inertial confinement fusion is briefly discussed.