S Tripathi

X-ray back-lighter characterization for iron opacity measurements using laser-produced aluminium K-alpha emission

Aluminium Kα emission (1.5 keV) produced by an 8 J, 500 ps, Nd:glass laser incident at 45° onto a layered target of 0.8 µm thick aluminium (front side) and 1 µm thick iron (backside) has been used to probe the opacity of iron plasma. Source broadened spectroscopy and continuum emission analysis show that whole beam self-focusing within the aluminium plasma results in a two-temperature spatial distribution. Thermal conduction from the laser-irradiated aluminium into the iron layer, enhanced by the whole beam self-focusing, results in a temperature of ~10–150 eV in the iron layer. The iron opacity at a photon energy of 1.5 keV is shown to be strongly modified from cold values in agreement with IMP code opacities. Results presented here represent a feasibility study to seed future work using table-top laser systems for plasma opacity experiments.

Efficient plasma production by intense laser irradiation of low density foam targets

Experimental investigations conducted on low density structured materials, such as foams have been presented in this paper. These low density foam targets having a density greater than the critical density of the laser produced plasma (ρcr≈3 mg⋅cm−3 at laser wavelength 1.06 μm) have been envisaged to have enhanced laser absorption. Experiments were done with an indigenously developed, focused 15 Joule/500 ps Nd: Glass laser at λ = 1064 nm. The focused laser intensity on the target was in the range of I≈1013−2×1014 W/cm2. Laser absorption was determined by energy balance experiments. Laser energy absorption was observed to be higher than 85%. In another set of experiments, low density carbon foam targets of density 150 mg/cc were compared with the solid carbon targets. The x‐ray emission in the soft x‐ray region was observed to increase in foam target by about 1.8 times and 2.3 times in carbon foam and Pt doped foam as compared to solid carbon. Further, investigations were also carried out to measure the e...

Shock pressure measurements in Polyvinyl alcohol (PVA) films using multi-frame optical shadowgraphy

The knowledge of the equation of state (EOS) of materials at high pressures in excess of 10 Mbar is important in several branches of physics including astrophysics and inertial confinement fusion. It is possible to access this high pressure regime in the laboratory using shock waves launched by the interaction of a high power laser with a solid target. To study laser driven shock waves in plastic (Polyvinyl alcohol) (C2H4O)n targets, a multiframe optical shadowgraphy technique has been developed, with spatial and temporal resolution of 12 μm and 500 ps respectively. The experiments were performed using the 1064 nm 20 J /500 ps Nd: Glass laser at BARC. The focused laser intensity on target was varied between 6 × 1013 W/cm2 and 2.7 × 1014 W/cm2. The experimental data have been compared with the results of previous experimental and theoretical studies. The results are also found to be in agreement with SESAME data. The maximum pressure attained in the experiments was 30 Mbar, achieved with a laser intensity of 2.7 × 1014 W/cm2.

X-ray emission from Au-Sm alloy target irradiated with high power sub nanosecond laser

The hohlraum cavity is generally made out of gold (Au) and recently it has been shown that laser-target coupling efficiency can be increased by using cocktail or mixed targets such as gold-samarium (Au-Sm). We report here results of experiments performed on Au-Sm alloy in various spectral regions for various compositions. In these experiments, a 12 Joule/500ps Nd:glass laser has been used. It is observed that the soft x-ray emission in the spectral region 0.7 -1.56 keV is enhanced by about 40-50% by using a composition of Au:Sm:: 3:7 as compared to pure Au .However, in case of hard x-ray emission (3.2 -5 keV), there is a reduction in x-ray emission from Au-Sm target as compared to pure gold. This behaviour, that enhancement occurs in soft x-rays in the case of mixed Au-Sm targets is desirable in the ICF scheme.

Optimization of x-ray line emission from copper plasma with laser focal spot

Laser produced x-ray are having variety of applications. X-ray emission from copper plasma produced by a sub-nanosecond Nd: glass laser have been studied as a function of distance of the target from the best focus position. Optimization of soft (0.7 – 1.56 keV) and hard x-ray (3- 5 keV) emission as a function of the laser focal position has been undertaken. The x-ray line emission in the spectral range of 7.7 A – 9.6 A range with respect to the laser focal position is important for applications and has been separately measured. It is observed that the maximum soft x-ray emission is away on both side of the best focus while hard x-ray shows peak close to best focus position. The line emission intensity variation with respect to focal position also exhibits the double hump variation with varying focal position. This indicates that the line emission is a strong function of plasma volume rather than laser intensity