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In this paper, the interaction of high power laser with low density polymer foam with density as low as 2 mg/cm 3 , 4 mg/cm 3 , 20 mg/cm 3 30 mg/cm 3 and 50 mg/cm 3 targets are investigated and compared with solid polymer targets. An understanding of such interaction is important from fusion research point of view. Low density foam coating of fusion capsule has been proposed in order to smooth in direct drive scheme and also it is being used as efficient x-ray converter in indirect drive scheme. It is observed that about 75–80% of the laser energy is absorbed in the subcritical (with density 3 ) foam targets and the soft x-ray yield in this case is almost two times that measured in the over dense (supper critical) targets. The optical shadowgraphy of the targets shows that the laser coupling in low density foam is associated with a supersonic heat wave while, with increasing density this phenomenon is replaced by subsonic absorption and shock formation. In the case of a 50 mg/cm 3 foams the foil velocity reduced by 35% (i.e. 5×10 6 cm/s), which further reduced to 3.8×10 6 cm/s in case solid polymer targets.

The properties of microheterogeneous plasma produced by irradiation of a polymer aerogel target with an intense (1014 W/cm3) short (0.5 ps) 1.064-μm laser pulse were studied. It is found that, even at plasma densities exceeding the critical density, a small fraction of the incident laser radiation penetrates through the plasma in which the processes of density and temperature equalization still take place. The intensification (as compared to plasmas produced from denser foams and solid films) of transport processes in such plasma along and across the laser beam can be caused by the initial microheterogeneity of the solid target. The replacement of a small (10% by mass) part of the polymer with copper nanoparticles leads to a nearly twofold increase in the intensity of the plasma X-ray emission.

Yb3+-doped fluorophosphate glasses were prepared by melt-quenching technique and characterized their spectroscopic properties to assess the laser performance parameters. The magnitude of absorption (emission) cross-sections at 975 nm for all the studied Yb3+-doped glasses is found to be in the range of 0.29–1.50 × 10−20 (0.59–1.99 × 10−20 cm2) which is much higher than those of commercial Kigre QX/Yb: 1.06 × 10−20 (0.5 × 10−20 cm2) laser glass. The luminescence lifetimes of 2F5/2 level decrease (1.15–0.45 ms) with increase in Yb2O3 concentration (0.1–4.0 mol%). Effect of OH− content on luminescence properties of Yb3+ ions has also been investigated. The effect of radiative trapping has been discussed by using McCumber (McC) and Fuchtbauer–Ladenburge (F–L) methods. The product of experimental lifetimes and emission cross-sections for 0.1 mol% Yb2O3-doped glass is found to be 2.28 × 10−20 cm2 ms which indicates that the higher energy storage and extraction capability could be possible. The detailed spectroscopic results suggest that the studied glasses can be considered for high-power and ultrashort pulse laser applications.

The main aim of the study of thin target foil–laser interaction experiments is to understand the physics of hydrodynamics of the foil acceleration, which is highly relevant to inertial confinement fusion (ICF). This paper discusses a simple, inexpensive multiframe optical shadowgraphy diagnostics developed for dynamic imaging of high velocity laser-accelerated target foils of different thicknesses. The diagnostic has a spatial and temporal resolution of 12 μm and 500 ps respectively in the measurements. The target velocity is in the range of 106–107 cm/s. Hydrodynamic efficiency of such targets was measured by energy balance experiments together with the measurement of kinetic energy of the laser-driven targets. Effect of target foil thickness on the hydrodynamics of aluminum foils was studied for determining the optimum conditions for obtaining a directed kinetic energy transfer of the accelerated foil. The diagnostics has also been successfully used to study ablatively accelerated targets of other novel materials.

The concentration dependent Yb 3+ : phosphate (P2O5 + K2O + MgO + Al2O3 + Yb2O3) glasses have been prepared and characterized their fluorescence and laser properties. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. The refractive index, absorption and emission cross-sections and fluorescence lifetimes varied with Yb3+ ion concentration. The higher emission cross-section was found to be 1.01 × 10-20 cm2 at the extraction wavelength of 1005 nm. The fluorescence lifetime of 2F5/2 level decreases from 1.04 ms to 0.28 ms with increase of Yb2O3 concentration from 0.05 to 6.0 mol %. The gain cross-section spectra can be obtained from the measured absorption and emission cross-sections with different population levels. The values of emission cross-section, fluorescence lifetime, minimum pulse duration, pump power, extraction efficiency and gain coefficients suggest that these glasses can be used as a laser gain media for the generation of ultrashort pulse and high power laser applications.

X-ray emission from copper plasma produced by a sub-nanosecond Nd:glass laser was studied as a function of distance of the target from the focus position. Optimization of soft (0.7–1.56 keV) and hard (3.2–5 keV) X-ray emissions as a function of the laser focal position was studied. In addition, a thallium acid phthalate (TAP) crystal spectrometer with spectral resolution of 30 mA was also developed to study variation in X-ray line emission in the spectral range of 1.291–1.610 keV (7.7–9.6 A) as a function of laser focal position. It is observed that the maximum soft X-ray emission is on either sides of the focus, indicating a dependence on plasma volume, whereas hard X-ray emission shows a single peak close to the ‘best focus’ position. The line X-ray emission intensity with respect to laser focal position also shows a double hump structure as in the case of soft X-ray emission. This indicates that the line emission is also a function of plasma volume. Scaling of X-ray yield with laser intensity has also been determined. Ion emission was also studied as a function of focal position variation. It is observed to match well with the trend shown by X-ray emission.

Ytterbium-doped phosphate glasses have been prepared and studied their spectroscopic properties through absorption, emission and Fourier transform infrared (FTIR) spectral studies and time-resolved luminescence decay curves. The absorption cross-section has been found to vary with the variation of Yb 2 O 3 concentration. The results of the FTIR spectra show that the OH − content is increasing with increase of the Yb 2 O 3 concentration in these glasses. The decay curves of the 2 F 5/2 level of Yb 3+ ions exhibit a single exponential nature for all the concentrations. The lifetimes of the 2 F 5/2 level of Yb 3+ ions decreases from 1.04 to 0.27 ms when the Yb 2 O 3 concentration is increased from 0.1 to 6.0 mol%. The quenching of lifetimes has been found to vary directly with the inter-ionic distance between the Yb 3+ ions. The concentration quenching of the lifetime has been analyzed using different energy transfer processes and no evidence of cooperative luminescence of Yb 3+ ions has been found in these glasses, which reveals that the present glasses are useful for photonic device applications. The laser performance properties have also been evaluated for these glasses and compared with those of other reported Yb 3+ -doped glass systems.

Experiments were performed with a 15 J/500 ps Nd:glass laser (λ = 1064 nm) focussed to an intensity >1014 W/cm2. X-ray emissions from carbon foam and 5% Pt-doped carbon foam of density 150–300 mg/cc were compared with that of the solid carbon targets. The thickness of the carbon foam was 15 µm on a graphite substrate. X-ray emission was measured using semiconductor X-ray diodes covered with various filters having transmissions in different X-ray spectral ranges. It covered X-ray spectrum of 0.8–8.5 keV range. The X-ray emission in the soft X-ray region was observed to increase to about 1.8 times and 2.3 times in carbon foam and Pt-doped foam, respectively with respect to solid carbon. In hard X-rays, there was no measurable difference amongst the carbon foam, Pt-doped carbon foam and solid carbon. Scanning electron microscope (SEM) analysis demonstrates that foam targets smoothens the crater formed by the laser irradiation.

X-ray and ion emission from gold plasma produced by a sub-nanosecond Nd:glass laser has been studies as a function of distance of the target from the best focus position. Thermal ion (kinetic energy 0.7 keV) measurements decrease as the target is moved closer to the best focus position in spite of an increase in laser intensity. We observe simultaneously a strong correlation between the onset of this drop in the flux of soft X-ray and the growth of harder X-ray (photon energy 3–5 keV), alongside a growth in fast ion (energy >67 keV) numbers. This is indicative of the onset of non-linear processes at the higher irradiances (∼1014 W/cm2) associated with the best focus position. Our results show that when using laser plasmas as X-ray or ion sources, X-ray and ion emission in a desired spectral range can be optimized by adjusting the focusing on the target.

Measurement of X-ray flux from laser-produced plasmas has always been important for several applications of the laser-plasma X-ray source. A CaSO 4 :Dy thermo-luminescent dosimeter (TLD) in powder form has been used in experiments reported in this paper to measure absolute X-ray yield from a gold laser plasma in a spectral region of 0.7–10 keV. The results are compared with indigenously developed X-ray vacuum photodiodes (XVPD). The TLD is a simple and accurate diagnostics for the X-ray measurements in a wide (few eV up to MeV) X-ray spectral region. It is extremely sensitive, capable of measuring an X-ray dose as low as μGy and also has a dynamic range of almost 10 6 , which is several orders of magnitude higher compared to the XVPD. Therefore, TLD is very well suited for integrated X-ray measurements from laser plasmas. Multi-channel arrays of TLD and XVPD are used with various X-ray filters to cover the desired spectral range. A comparative measurement was also carried out between X-ray yield from laser-produced plasma and a conventional X-ray generator operated at low voltage. It was observed that the X-ray dose in laser-produced plasma is almost 10 9 orders higher than the conventional X-ray generator.

We have observed laser induced birefringence in optically isotropic glasses as well as in Nd:YAG crystal using polarized pulses of wavelength 1.06 µm and duration 100 ps with an intensity near 109 W cm−2. The laser used in the experiment was a Nd:YAG oscillator–amplifier system. The change in the material induced by the laser was not a permanent change but existed for as long as the laser pulse lasted. We observed conoscopic patterns when the laser was passed through glass or the Nd:YAG crystal. These patterns are typically seen when light is passed through birefringent materials kept between a polarizer and analyser. This observation immediately suggests that the glass or the crystal possesses optical birefringence. This birefringence is demonstrated in fused silica glass, Nd:glass and Nd:YAG crystal. The circular symmetric conoscopic patterns show that the optic axis is formed along the direction of laser propagation. The laser induced birefringence depended on the laser intensity and we characterized the magnitude of this change by measuring the output pulse at the end of the crossed polarizer.

Laser driven shock wave transit time in thin aluminium targets was experimentally estimated by determining the shock emergence time at the rear of thin aluminium foils of varying thickness from 5 to 35 μm. A 20 J, 5 ns Nd:glass laser was focused to produce laser intensity of 1012 to 5 × 1013 W/cm2 on the targets which were placed in vacuum. Target foil movement was measured to an accuracy of 10 μm using optical shadowgraphy technique. This technique was used to accurately measure the shock transit time by recording the optical shadowgrams at various instants of time and thus identify the instant at which the foil is just set into motion. Shock transit time measured in foils of different thickness can give the value of shock velocity at a given laser intensity. Target motion recorded by shadowgraphy can also give the target foil velocity from which shock pressure can be estimated. Experimental values of shock transit time, shock velocity and shock pressure were observed to agree well with the values using one-dimensional multi-group radiation hydrodynamic simulations.

A scheme to enhance the target foil velocity has been investigated for a direct drive inertial fusion target. Polymer PVA (polyvinyl alcohol or (C2H4O)n) target foils of thickness 15–20 μm were used in plain form and also embedded with gold in the nano-particle (Au-np) or micro-particle (Au-mp) form. Nano-particles were of 20–50 nm and micro-particles of 2–3 μm in size. 17% higher target velocity was measured for foils embedded with nano-particle gold (Au-np) as compared to targets embedded with micro-particles gold (Au-mp). The weight of gold in both cases was in the range 40–55% of the full target weight (atomic percentage of about 22%). Experiments were performed with the single beam of the Prague Asterix Laser System (PALS) at 0.43 μm wavelength (3ω of the fundamental wavelength), 120 Joule energy and 300 psec pulse duration. Laser intensity on the target was about 1015 W/cm2. A simple model has been proposed to explain the experimental results.

The idea of direct-indirect target is experimentally tested with planar targets from LPI made of low-density metal foam (nanosnow) or of nanoparticles (30%) in plastic aerogels. The laser experiments in BARC have shown that compared to plain targets the foams of/with nanoparticles demonstrate: 1) better x-ray emissivity ; 2) smoother energy distribution over the target surface; 3) material flow from low-density wall being slower and more uniform; 4) active transverse energy transport over the target plane. These properties could be useful for hohlraums and direct-indirect concept.

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...

A simple method of production of supersonic plasma jets with parameters relevant to laboratory astrophysics applications is proposed and demonstrated. The method consists in using a cylindrical channel for guiding and collimating the plasma generated from a laser-irradiated thin foil target. In the experiment, a 120 J, 1.315 um, 0.3 ns laser pulse irradiated a thin (10 or 20 um) CH foil placed at the entrance of the cylindrical channel (made in the aluminum cylinder) of diameter dCh = 0.3 mm and length of 2 mm. It was found that when the focal spot diameter (dL) is well matched to the channel diameter (dL ≈ (2/3)dCh), the channel can form a collimated, high-density (up to 1020 cm3) plasma jet of the Mach number ≥10. The method seems to be flexible in the generation of jets of various hydrodynamic parameters and atomic numbers and does not require high-energy lasers for the jet production.

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...

Plasma jets produced from targets with atomic numbers ranging from 5 to 82 have been investigated in detail. Theoretical modeling based on a radiative cooling instability has been used to successfully explain the experimental results. Optical shadowgrams and x‐ray pinhole pictures strongly indicate a correlation between the jets and x‐ray emission from plasma. Plasma jets are intense in materials such as copper, molybdenum, tungsten, and gold which are characterized by strong x‐ray emission. Interferograms indicate a large increase in electron density in the region of plasma jets.

An experimental study of laser-driven shock wave propagation in a transparent material such as Plexiglas using a high-speed optical shadowgraphy technique is presented in this paper. A Nd:glass laser was used to produce laser intensity in the range 1012-1014 W/cm2 on the target. Optical shadowgrams of the propagating shock front were recorded with a second-harmonic (0.53-μm) optical probe beam. Shock pressures were measured at various laser intensities, and the scaling was found to agree with the theoretically predicted value. Shock pressure values have also been obtained from a one-dimensional Lagrangian hydrodynamic simulation, and they match well with experimental results. Shadowgrams of shock fronts produced by nonuniform spatial laser beam irradiation profiles have shown complete smoothing when targets with a thin coating of a material of high atomic number such as gold were used. Shock pressures in such coated targets are also found to be considerably higher compared with those in uncoated targets.

A simple and reliable technique to temporally shorten a multinanosecond Nd:glass laser pulse to less than nanosecond duration at the second harmonic wavelength is described in this article. Using this technique a short probe pulse synchronized with the main laser was generated for optical probing of laser‐produced plasmas. Experiments reported were conducted with a Nd:glass laser of wavelength 1.06 μm and of 20 ns duration to yield a temporally shortened pulse of duration less than a nanosecond at a wavelength of 0.53 μm. This technique would be particularly useful and give better results for shorter wavelength lasers (UV and VUV) for which the conventional techniques of pulse slicing are sophisticated as well as add to the expense and complexity of the laser system.

Acceleration of a thin (10 or 20μm) plastic foil by 120J, 0.438μm, 0.3ns laser pulse of intensity up to 1015W∕cm2 has been investigated. It is shown that the introducing a high-Z dopant to the foil causes an increase in the ablating plasma density, velocity, and collimation which, in turn, results in a remarkably higher kinetic energy and energy fluence of the flyer foil.

A large energy output and a wider spectral bandwidth , not reported earlier from Nd:glass based regenerative amplifiers, is demonstrated. The energy extracted from the regenerative amplifier is 15 mJ, with a bandwidth of 3.8 nm aid an overall gain of 2.1X108. This is achieved by a new cavity optimization technique.

The generation of subnanosecond Nd:YAG laser pulses by the technique of periodic Q-switching using a simple electrically triggered spark gap is described. The construction and some operating characteristics of the spark gap are given. The measurement of very short rise time high-voltage pulses using a V probe is also described.

Presence of nano-particles on target surface has been observed to lead to increased laser absorption of laser pulse in plasma. Therefore, a coating of nano-particles on foil targets could lead to an enhanced ablative acceleration. The work presented in this paper concerns this possibility. The results of experiments performed with PALS laser system (125 J, ~250 ps at 1.3 ?m) with a focused intensity of about 1014 W/cm2 are presented. 15 ?m thick Polyethylene teraphthalate (C10 H8 O4)n or PET foils show an almost 40% increase in target movement when coated with a layer of gold nano-particles. Comparison between targets with coating of bulk gold and nano-gold shows about 15% higher target movement in gold nano-particle coated PET targets as compared to bulk gold coating. This result is a clear indication of enhanced laser energy absorption in targets with nano-structured surface of gold. We also present evidence to show the effect of nano-particle coating on lateral thermal conduction.

study of Equation of State (EOS) at high pressures is of interest for several fields of Physics, in particular astrophysics, material science and inertial confinement fusion. The Laser driven shock studies on various targets have been done with the 10 J/ 500 ps laser system. The focused intensity at the target was 1013 to 1014 W/cm2. The shock velocity was measured by using an optical streak camera. We have used various thicknesses of Al foil and 10 μm gold foil targets. Scaling of target velocity, with laser intensity has been done. The scaling of shock transit time with the foil thickness has been done. It is observed that at laser intensity > 1 × 1014 W/cm2 the shock pressure measured from various target material are more than 10 Mbar. The scaling of shock transit time with target thickness has been measure. Also the scaling of shock velocity with laser intensity has been done for Aluminium and gold targets. The experimental observations are seen to match well with the simulations.

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.

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

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.

Filamentary structures or jetting in the plasma flow generated from laser irradiated thin foil targets have been observed using an optical probe technique. Laser wavelength, pulse duration, and intensity range used in the experiments were 1.06 μm, 5 ns, and 5×1012 W/cm2, respectively. A 0.53‐μm‐wavelength optical pulse of 50‐μJ energy and 1‐ns duration was used as an optical probe. Optical shadowgrams showed that the filamentary structures or jets in the plasma flow were strongest in the region about 300 μm away from the target surface, and in time, much later than the peak of the main laser pulse. It was also observed that jetting does not affect the stability of the ablatively accelerated thin foils. Jetting was found to be stronger for targets with a higher atomic number. A time‐resolved interferometric study of the plasma produced from targets with high atomic numbers also showed the existence of an abrupt change in fringe shift or plasma density in the region of these jets.

An enhanced spatial smoothing of ablative motion of thin plastic foil targets coated with high atomic number ablators such as gold or aluminium, irradiated by a spatially modulated Nd: glass laser beam was observed. Optical shadowgraphy coupled with double foil technique was used to observe the laser-irradiated foil motion. Laser irradiance used for the experiments was in the range of 1011–1013 watts/cm2. A 60–80% enhancement in the smoothing was observed for a laser beam modulation (width 75–150 µm) at the target surface.