N. K. Gupta

Dependence of soft x-ray conversion on atomic composition in laser produced plasma of gold-copper mix-Z targets

A comparative experimental study of soft x-ray emission from laser-irradiated Au–Cu mix-Z targets of different atomic compositions has been performed. Plasma was produced from planar targets using second-harmonic laser pulses from an Nd:glass laser at a focused intensity of ∼1013 W cm−2. Radiation intensity in the spectral region ∼15–150 A and integrated x-ray yield for mix-Z target were observed to be higher than those for individual elements. The maximum conversion occurred for an atomic composition of Au 0.43–Cu 0.57. These observations are consistent with the variation of Rosseland mean opacity with atomic composition calculated using a screened hydrogenic average atom model.

Effect of initial plasma density on laser induced ion acceleration

The effect of initial plasma density on the energetics of the laser accelerated ions is studied using one dimensional particle in cell simulations. It is observed that the initial plasma density plays an important role in the generation of high energy particles. In the case of a spatially constant initial density, there exists an optimum value for the maximum ion acceleration. Similarly for the case of a density ramp, an optimum value of ramp length exists for the maximum ion acceleration. At a laser intensity of 5×1020 W/cm2, a maximum energy of about 1 GeV is seen with an optimum initial density ramp.

Molecular dynamic simulation for laser–cluster interaction

A three dimensional relativistic molecular dynamic model for studying the laser interaction with atomic clusters is presented. The model is used to simulate the interaction dynamics of deuterium, argon, and xenon clusters when irradiated by the short and high intensity laser pulses. The interaction of 82 A argon cluster by 100 fs, 806 nm laser pulse with the peak intensity of 8 × 1015 W/cm2 is studied and compared with the experimental results. The maximum ion energy in this case is found to be about 200 keV. Ion energies along and perpendicular to laser polarization direction is calculated and asymmetry along laser polarization direction is detected which is further explained on the basis of charge flipping model. The effect of cluster density on the energetics of the laser–cluster interaction is also being studied, which provides a qualitative understanding of the presence of optimum cluster size for maximum ion energies.

Generation of ultraintense proton beams by multi-ps circularly polarized laser pulses for fast ignition-related applications

A scheme of generation of ultraintense proton beams relevant for proton fast ignition (PFI) which employs multi-ps, circularly polarized laser pulse irradiating a thick (≥ 10 μm) H-rich target is p ...

Role of radial nonuniformities in the interaction of an intense laser with atomic clusters

A model for the interaction of an intense laser with atomic clusters is presented. The model takes into account the spatial nonuniformities of the cluster as it evolves in time. The cluster is treated as a stratified sphere having an arbitrary number of layers. Electric and magnetic fields are obtained by solving the vector Helmholtz equation coupled with one-dimensional Lagrangian hydrodynamics. Results are compared with the uniform density nanoplasma model. Enhancement in the amount of energy absorbed is seen over the uniform density model. In some cases the absorbed energy increases by as much as a factor of 40.

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.

Laser plasma interaction in copper nano-particle targets

AbstractIn this paper, we present the results of studies on ion emission characteristics of a laser plasma produced from a copper nano-particle layer of 1–3 mm thickness coated over polished surface of a solid copper target. Laser intensityof 10 13 –10 14 W/cm 2 was produced on the targets by a 2 J Nd:glass laser having a variable pulse duration of 300–800 ps. Nano-particle size wasin the range of 15–25 nm. Ion emission from the nano-particle plasma was compared with plasma generated from apolished copper target. Ion emission from the nano-structured target was observed to depend on the polarization of theincident laser beam. This effect was stronger for a shorter laser pulse. X-ray emission was measured in the soft andhard X-ray region (0.7 to 8 keV) using various X-ray filters. A nano-particle coated target is found to yield a largerflux as well as velocity of ions as compared to polished target when the laser polarization is parallel to the planecontaining target normal and detector axis. However, no X-ray enhancement has been observed in the wavelengthrange 1.5 to 20 A˚ .Keywords: Ion emission; Laser-plasma interaction; Laser-plasma; Nano-particles

X-ray and ion measurements in laser produced plasma from gold-copper alloy targets

Enhancement in x-ray emission from laser plasmas produced from an alloy of gold and copper (Au 0.43+Cu 0.57 atomic composition) has been observed in a narrow spectral region of 1.5–3.9A. X-ray emission is observed to be as high as six to nine times as compared to pure copper and 1.5 to 1.8 times as compared to pure gold targets. In these experiments, a single pulse from a Nd:glass laser (λ=1.06μm) having an energy up to 2J and 500ps pulse duration was used to obtain a focused intensity of 1013–1014W∕cm2 on the targets. The results are explained on the basis of enhanced group Rosseland opacity (Rosseland mean over the spectral range under consideration) for the Au–Cu alloy target as compared to either of the pure targets.

Formation of a supersonic laser-driven plasma jet in a cylindrical channel

A simple method of supersonic plasma jet production where the jet is formed in a cylindrical channel guiding the plasma generated from a laser-irradiated thin foil target is proposed and examined. High-Mach number (≥10) plasma jets of parameters relevant to laboratory astrophysics applications using 120 J, 1.315 μm, and 0.3 ns laser pulse for a thin CH foil irradiation are demonstrated. The method seems to be flexible in the production of jets of various compositions and hydrodynamic parameters and does not require high-energy lasers for the jet generation.

Molecular dynamic studies on anisotropic explosion of laser irradiated Xe cluster

A three dimensional molecular dynamic model is used to investigate the dynamics of Xe clusters of various radii irradiated by laser of moderate intensities (∼1014−1016W/cm2). The FWHM pulse duration of the laser is varied from few laser cycles to hundreds of femtosecond. For cluster of radius 50 A irradiated by a laser of 170 fs pulse duration, it is observed that ion yield is more along the direction of laser polarization than perpendicular to it. This trend reverses (more ions are emitted along the direction perpendicular to laser polarization than parallel to it) when laser pulses of few cycles are used. This reversal of anisotropy is explained on the basis of spatial shielding of ions due to the oscillating inner electron cloud along direction of laser electric field. The nature of anisotropy remains same with variations in laser intensity and cluster size.