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The Laser Plasma Division at the Centre for Advanced Technology is engaged in a variety of research and development activities on laser-plasma interaction with special emphasis on laser-matter interaction at ultra-high intensities. An important aspect of our future work is studies in laser-plasma based acceleration using an elaborate infrastructural set-up of ultra-fast laser and plasma diagnostic systems and recently acquired 10TW, 50fs Ti:Sapphire laser system. This paper presents outline of the planned studies in this field. [ABSTRACT FROM AUTHOR]

Experimental measurements of ablation parameters in a KrF laser produced aluminum plasma are presented for the laser intensity range of 10[sup 11]-10[sup 13] W/cm². Plasma created from large focal spots shows single broad ion current pulses as compared to two groups of ions observed from small focal spots. Detailed analysis of the velocity distributions of emitted ions is carried out to understand this behavior and its effect on measurements of the ablation parameters. Scalings with laser intensity of the mass ablation rate (m ∼ I[sup 0.42, sub L]) and of the ablation pressure (P[sub a] ∼ I[sup 0.81, sub L) 8 - IL') giving m = 1.5 × 10[sup 5] g cm[sup -2] sec[sup -1] and P[sup a] = 5.5 Mbar at a laser intensity of 10[sup 13] W/cm² are obtained. The results are found to be in good agreement with the predictions of a plasma ablation model based on inverse bremsstrahlung dominated absorption. [ABSTRACT FROM AUTHOR]

A simple and compact x-ray luminosity time-of-flight spectrometer for measurements of the velocity distribution of highly charged ions in laser-produced plasmas is presented. The spectrometer is based on making time- and space-resolved measurements of x-ray line emission from these ions in the expansion zone. Design, operation, and performance of this system are described. The spectrometer is useful in studying processes of interaction between highly charged ions and buffer gases, and nonthermal behavior of laser-produced plasma expansion. [ABSTRACT FROM AUTHOR]

Experimental measurements are presented of space and time integrated x-ray emission from aluminum plasma produced by 2 ns (full width at half-maximum), 268 nm Raman compressed KrF laser pulses. A single temperature spectrum was measured giving temperatures of 75–440 eV over the range of intensities 9×1011–2×1013 W/cm2. This scaling agrees well with a simple self-regulating model. Conversion of laser energy into x rays above 1 keV energy was found to scale as I2.37L reaching 0.35% at 3.5×1013 W/cm2. [ABSTRACT FROM AUTHOR]

Ion generation in the cathode plasma jet of a moderate-current (∼2.3 kA), low-energy (≤20 J) vacuum spark discharge triggered by ∼27 ps, 10 mJ laser pulses is studied using time of flight technique. Fastest ion velocity and velocity corresponding to the peak of the time of flight signals for Al cathode were measured to be ∼5.25×108 cm/s (energy of ∼143 keV/u) and ∼8.1×107 cm/s (energy of ∼3.4 keV/u), respectively. Corresponding velocities in the case of ions generated from laser-produced Al plasma (energy of ∼550 mJ, intensity of ∼1014 W/cm2) were found to be much smaller, viz., ∼1.05×108 cm/s (energy of ∼5.75 keV/u) and ∼2.63×107 cm/s (energy of ∼0.36 keV/u), respectively. Study shows efficient acceleration of ions in a current-carrying cathode plasma jet of a small-energy multipicosecond laser-triggered spark discharge as compared with that in a high-energy multipicosecond laser-produced plasma plume. [ABSTRACT FROM AUTHOR]

Characteristics of cathode plasma jet pinching and x-ray emission from a multipicosecond laser-triggered vacuum discharge are presented. Discharge was created in between a planar Al cathode and a conical point-tip Ti anode (separation: 2–15 mm, circuit inductance of ∼0.53 μH, peak discharge current of ∼3 kA, and rise time of ∼400 ns). For anode-cathode separation of ∼13.5 mm, only a single pinched plasma point was formed in the cathode plasma jet at a distance of ∼9.5 mm from the cathode. Quantitative analysis of the x-ray signals recorded using a pin diode with different filters and viewing different regions of the discharge, shows soft (∼keV photon energy) x-ray emission from the plasma point with a flux of ∼(3–5)×1010 photons/sr, and multi-keV x-ray emission from the Ti anode with Kα (∼4.51 keV) photon flux of ∼1010 photons/sr. [ABSTRACT FROM AUTHOR]

A simple method that uses envelope functions of unbalanced interferometric (auto/cross) correlation signals has been presented for simultaneous visual detection of pulse chirp and asymmetry, without direction-of-time ambiguity in ultrashort laser pulses. The ambiguity issues of unbalanced interferometric correlation envelope (ICE) difference signals have been studied. It is found that unbalanced ICE difference signals are visually different corresponding to practically indistinguishable unbalanced interferometric autocorrelation (IAC) signals for a distinct symmetric–asymmetric pulse pair with identical intensity autocorrelation and power spectra. The theoretical analysis of ICE signals is supported by experimental unbalanced IAC signals obtained using 200 fs laser pulses from a cw mode-locked Nd:phosphate laser oscillator. [ABSTRACT FROM AUTHOR]

The quadratic photocurrent in commercial light emitting diodes (LEDs) has been studied in reverse bias mode for autocorrelation measurement of ultrashort laser pulses. It is found that the photocurrent can be greatly enhanced by operating the LED biased just below the reverse bias breakdown threshold. The effect of aging of LEDs on laser exposure in this mode of operation is found to be similar to that for the photovoltaic mode. The large internal gain in LED junction has enabled the recording of the second order autocorrelation signal of ∼200 fs laser pulses from 100 MHz laser oscillator with two orders of magnitude smaller average and peak power product compared to the case of the photovoltaic mode. [ABSTRACT FROM AUTHOR]

A simple method of generating multi-keV, monochromatic, twin x-ray point sources based on laser-driven vacuum diode is presented. Electrons from a laser-produced aluminum plasma cathode were accelerated towards two point-tip titanium anodes placed symmetrically with respect to the cathode. Temporal and spatial characterizations of the K-shell x-ray emission from the twin anodes are carried out. Each source (∼300 μm diameter) emits ∼109 Ti Kα (hν≈4.51 keV) photons in a pulse of ∼20 ns duration with a peak brightness of ∼4×1018 photons/cm2 s sr. Single-shot twin x-ray shadowgrams of physical objects were recorded at a distance of several centimeters from the two sources. Such a system may be useful for single-shot stereoradiography for differential imaging in the multi-keV energy region. [ABSTRACT FROM AUTHOR]

A new method to calculate the high order harmonic generation from the interaction of ultra-short laser pulses with plasma plumes containing solid nanoparticles has been presented. It is shown that enhancement for a particular harmonic order is almost independent of the nanoparticle size, in agreement with the observed experimental results which are not explainable by the existing theories applicable to high order harmonic generation from gases or gas clusters. [ABSTRACT FROM AUTHOR]

Near complete absorption of the energy of intense ultra-short laser pulses (45 fs, intensity ∼1.6 × 1016 to 2.5 × 1017 W/cm2) is observed in carbon nanotubes deposited on a planar molybdenum substrate. The hollow structure of the nanotube plasma facilitates resonant electric field enhancement during its ionization phase. This resonantly enhanced localized field at a density much larger than the critical density nc leads to efficient hot electron generation, which results in enhanced Kα emission of Mo at 17.5 keV. It is observed that for nanotubes, depending on the degree of hollowness, there is an optimum laser intensity for maximum x-ray enhancement compared to a planar uncoated target. [ABSTRACT FROM AUTHOR]

A study of the conversion efficiency and line shape of the K-α x-ray line radiation from a planar titanium target irradiated by an ultra-short laser pulse is performed. The conversion efficiency and spectral broadening are studied as a function of laser intensity (5 × 1016-1018 W cm-2), laser pulse duration (45 fs-800 fs), and laser fluence (2 × 103-4.2 × 104 J cm-2). The K-α1 line (4510 eV) is observed to be broadened (up to ∼9 eV), predominantly towards the higher energy side and strongly depends on the laser fluence rather than on laser intensity. The reason for the spectral broadening is attributed to K-α emission in warm dense plasma. The role of hot electrons and direct laser heating on spectral broadening is outlined. In addition to this, our observations indicates that the presence of pre-plasma strongly contribute to the observed broadening through the inner-shell transitions in multiply charged titanium ions in the pre-plasma. The appropriate laser irradiation parameters to achieve high conversion efficiency and minimum spectral width of the K-α radiation are identified. The study is important, since the control of the spectral profile is of general interest for diffraction or scattering experiments in view of its potential in increasing temporal resolution. [ABSTRACT FROM AUTHOR]

An optical diagnostic method to record the evolution of plasma density on picosecond time scale with ∼8 ps resolution in a single shot is presented. A chirped laser pulse of 200 ps is used for obtaining the time resolved interferograms to provide the temporal evolution of plasma density (1017/cm3) during the probe beam duration. It is shown that the plasma expansion velocity can also be measured from the diffraction pattern superimposed on the interferograms. Plasma velocity of ∼1.2 × 107 cm/s was estimated along with its temporal profile. The details of the method and the results are presented. [ABSTRACT FROM AUTHOR]

Nano-ripple formation from ultrashort laser pulse irradiation of semiconductors of different band gaps has been studied using a Ti-sapphire laser with 8 mJ energy, 45 fs pulse duration and 800 nm wavelength (1.5eV) at a fluence in the range of ∼100 mJ/cm2 -1J/cm2. The effects of the number of laser shots, angle of incidence, laser polarization, fluence, incident laser wavelength, bandgap, and ambient medium on the ripple period, have been studied. Depending upon the experimental parameters nano-ripple sizes varied in the range of λ-λ/9. The studies clearly show that narrower nano-ripples are formed from wide bandgap semiconductors. In addition, the width of the nano-ripples decreases with the laser wavelength and fluence. The observed results are explained considering the transient metallic nature of the semiconductor surface on irradiation with intense femtosecond pulse which excites surface plasmon leading to the nano-ripple formation. The critical role of the surface plasma electron density in deciding ripple period is identified which helps in generation of narrow sub-wavelength nano-ripples. [ABSTRACT FROM AUTHOR]

In this paper, we present a comparative study of the laser energy absorption, soft x-ray emission (in the water window region: 2.3-4.4 nm) and hard x-ray emission (in the 2-20 keV range) from planar aluminum and nanohole alumina of 40 nm average diameter, when irradiated by Ti:sapphire laser pulses. The laser pulse duration was varied from 45 to 500 fs, and the focused intensity on the target ranged from ∼3 × 1016 W/cm2 to 3×1017 W/cm2. The x-ray yield enhancement from the nanoholes shows an increased coupling of the laser energy to the target. The effect of laser pulse duration on the x-ray emission was also studied, where a resonance like phenomenon was observed. The laser energy absorption measurements in the nanoholes showed a marginal enhancement in absorption as compared to planar Al. The integrated keV x-ray yield, from nanohole alumina and planar Al, at an intensity of 3 × 1017 W/cm2, was 25 and 3.5 μJ, respectively. The results can be explained by considering the hydrodynamic expansion of the laser irradiated structure and field enhancement in the nanoholes. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study on generation of nanoparticle various sizes using Ti:sapphire laser pulses, is reported. Nanoparticle formation in plasma plumes of metals like silver and copper, expanding in vacuum, has been studied using stretched pulses of 300 ps duration [subnanoseconds (sub-ns)] from a Ti:sapphire laser. It has been compared with the nanoparticle formation (of the same materials) when compressed pulses of 45 fs duration were used under similar focusing conditions. Nanoparticle formation is observed at intensities as high as 2×1016 W/cm2. The structural analysis of the nanoparticle deposition on a silicon substrate showed that, using 45 fs pulses, smaller nanoparticles of average size ∼20 nm were generated, whereas on using the sub-ns pulses, larger particles were produced. Also, the visible light transmission and reflection from the nanoparticle film of Ag on glass substrate showed surface plasmon resonance (SPR). The SPR curves of the films of nanoparticles deposited by femtosecond pulses were always broader and reflection/transmission was always smaller when compared with the films formed using the sub-ns pulses, indicating smaller size particle formation by ultrashort pulses. Thus, it has been demonstrated that variation in the laser pulse duration of laser offers a simple tool for varying the size of the nanoparticles generated in plasma plumes. [ABSTRACT FROM AUTHOR]

The morphologies of the fullerene targets and the ablated fullerenes to determine the optimal conditions of excitation of the C60-containing targets have been analyzed. The optimization of fullerene-containing plasma conditions allowed the enhanced harmonic generation in these plasmas using laser radiation of different wavelengths, pulse durations, and phase modulation. A comparison between the harmonic generation in single-atom/ion-containing plasmas (using bulk carbon, silver, and indium targets) and fullerene-rich plasma plumes showed better conversion efficiency for the latter medium. The influence of phase modulation of the fundamental radiation in fullerene plasmas on the spectral properties of harmonics has been studied. [ABSTRACT FROM AUTHOR]

The dependence of the high-order harmonic intensity on the laser focal spot position in laser produced plasma plumes is experimentally studied. High order harmonics up to the 59th order (λ∼13.5 nm) were generated by focusing 48 fs laser pulses from a Ti:sapphire laser system in silver plasma plume produced using 300 ps uncompressed laser radiation as the prepulse. The intensity of harmonics nearly vanished when the best focus was located in the plume center, whereas it peaked on either side with unequal intensity. The focal spot position corresponding to the peak harmonic intensity moved away from the plume center for higher order harmonics. The results are explained in terms of the variation of phase mismatch between the driving laser beam and harmonics radiation produced, relativistic drift of electrons, and defocusing effect due to radial ionization gradient in the plasma for different focal spot positions. [ABSTRACT FROM AUTHOR]

An experimental study on hard x-ray Bremsstrahlung radiation due to fast electron generation from a planar solid copper target irradiated by 150 mJ, 45 fs Ti:sapphire laser pulses at I∼1018 W/cm2 is reported. Angular distribution of the hard x-ray dose rate (hν≥40 keV) is observed to be strongly forward peaked in the direction of target normal with a measured peak value of 40 μSv/h, at a distance of 500 mm from the target. Two sources of this radiation, one at the laser irradiated target and the other at the chamber wall facing the target, are inferred from the measurements of angular distribution and distance dependence of the x-ray dose rate. [ABSTRACT FROM AUTHOR]

The effect of gold on keV x-ray emission from gold-copper (Au–Cu) mix-Z plasma has been experimentally studied. The intensity of the copper L-shell line radiation (λ∼7.8–10.9 Å) as well as the integrated keV x-ray yield were observed to decrease sharply with increasing atomic fraction of gold in the mix-Z target. The decrease was observed to be by a factor of ∼2.1 for 0.12 atomic fraction of gold in the Au–Cu mix-Z target with respect to the pure copper target. The results can be explained from physical consideration of the high value of free-bound opacity of gold in the spectral region of the L-shell emission of copper ions and downconversion of the absorbed keV line radiation. [ABSTRACT FROM AUTHOR]

A comparative study of the characteristics of moderate-current (∼10 kA), low-energy (<=20 J) vacuum discharge triggered by multipicosecond and nanosecond duration laser pulses is performed. Temporal profiles of the x-ray emission, discharge current, and anode voltage measured in vacuum discharge created between a planar titanium cathode and a conical point-tip anode are observed to be quite different for the two regimes of the laser pulse duration. While cathode plasma jet pinching is clearly observed in the discharge created by low-energy (∼5 mJ), 27 ps full width at half-maximum (FWHM) laser pulses, a feeble pinching occurred for 4 ns (FWHM) laser pulses only above a threshold energy of ∼250 mJ. In addition to the multiple K-shell x-ray pulses emitted from the titanium anode up to 100 ns, evidence of a much harder x-ray component (hν>100 keV) is also seen in the discharge triggered by picosecond laser pulses. [ABSTRACT FROM AUTHOR]

An experimental study of parametric dependence of space-resolved x-ray line emission from laser-produced magnesium plasma expanding in a helium background gas has been performed for different pressures up to 125 mbar. A strong enhancement in MgXI He-α resonance (1s² ¹S[sub 0]-1 s2p ¹P[sub 1] at 9.17 Å) and intercombination (1s² ¹S[sub 0]-1 s2p ³P[sub 1] at 9.23 Å) line emission is observed in the expansion region extending up to ∼6 mm from the target. At the optimum background gas pressure of about 40 mbar, there is an order of magnitude increase in the x-ray line intensity compared to that observed for a gas pressure of few mbar. The results of measurements of the distance of the peak emission region from the target and width of the emission zone for different pressures are presented, and the role of the background gas for the observed behavior is discussed. [ABSTRACT FROM AUTHOR]

We report here energetic (>lOOkeV) negative hydrogen ions (H-) generated in the interaction of moderately intense (1018 W cm-2) ultrashort laserpulses (45 fs) with transparent hydrogen containing solid targets. An unambiguous and consistent detection of negative hydrogen ions, with a flux of 8 x 1011 H- ions/sr, has been observed in every single laser shot, using a Thomson parabola ion spectrograph. Simple estimates based on charge transfer cross sections match well with experimental observations. Our method offers the implementation of an intense, ultrashort laser based negative-ion source at a higher repetition rate, which can be important for various applications. [ABSTRACT FROM AUTHOR]

We report on the proton acceleration studies from thin metallic foils of varying atomic number (Z) and thicknesses, investigated using a 45 fs, 10 TW Ti:sapphire laser system. An optimum foil thickness was observed for efficient proton acceleration for our laser conditions, dictated by the laser ASE prepulse and hot electron propagation behavior inside the material. The hydrodynamic simulations for ASE prepulse support the experimental observation. The observed maximum proton energy at different thicknesses for a given element is in good agreement with the reported scaling laws. The results with foils of different atomic number Z suggest that a judicious choice of the foil material can enhance the proton acceleration efficiency, resulting into higher proton energy. [ABSTRACT FROM AUTHOR]

A μ-XRF study to assess the distribution of uranium and thorium in (U,Th)O2 pellets covering the composition of advanced heavy water reactor (AHWR) fuel pellets prepared by powder metallurgical compaction (PMC) and coated agglomerate pelletization (CAP) routes was made using micro-focus beam line (BL-16) of Indus-2 synchrotron radiation facility. The methodology thus developed was successfully applied to these pellets. The study reveals that the uranium distribution in pellets prepared by PMC route is uniform, whereas the pellets prepared through CAP route have a wide range of compositional variation. In addition, the uniformity in CAP route-prepared pellets improves with increase in the relative amount of uranium in the pellets. The sample preparation in present methodology is very simple compared with scanning electron microscopy. The study reveals the utility of synchrotron-based μ-XRF for fuel pellet characterization of AHWR reactors. Alhough CAP route of fuel pellet preparation requires less exposure of personnel to high radiation dose, the non-uniformity in the fuel pellet must be considered when using these pellets in reactors. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We report an experimental study on the optimization of a laser plasma based x-ray source of ultra-short duration K-a line radiation. The interaction of pulses from a CPA based Ti:sapphire laser (10 TW, 45 fs, 10 Hz) system with magnesium, titanium, iron and copper solid target generates bright 1-8 keV K-a x-ray radiation. The x-ray yield was optimized with the laser pulse duration (at fixed fluence) which is varied in the range of 45 fs to 1.4 ps. It showed a maximum at laser pulse duration of ~740 fs, 420 fs, 350 and 250 fs for Mg (1.3 keV), Ti (4.5 keV), Fe (6.4 keV) and Cu (8.05 keV) respectively. The x-ray yield is observed to be independent of the sign of the chirp. The scaling of the K-a yield (Ix α ILß ) for 45 fs and optimized pulse duration were measured for laser intensities in the region of 3 × 1014 - 8 × 1017. The x-ray yield shows a much faster scaling exponent ß = 1.5, 2.1, 2.4 and 2.6 for Mg, Ti, Fe and Cu respectively at optimized pulse duration compared to scaling exponent of 0.65, 1.3, 1.5, and 1.7 obtained for 45 fs duration laser pulses. The laser to x-ray energy conversion efficiencies obtained for different target materials are nMg = 1.2 × 10-5, nTi = 3.1 × 10-5, nFe = 2.7 × 10-5, nCu = 1.9 × 10-5. The results have been explained from the efficient generation of optimal energy hot electrons at longer laser pulse duration. The faster scaling observed at optimal pulse duration indicates that the x-ray source is generated at the target surface and saturation of x-ray emission would appear at larger laser fluence. An example of utilization of the source for measurement of shock-wave profiles in a silicon crystal by time resolved x-ray Sdiffraction is also presented. [ABSTRACT FROM AUTHOR]

Enhanced water window x-ray emission (23-44 Å) from carbon clusters, formed in situ using a pre-pulse, irradiated by intense (I > 1017 W/cm2) ultra-short laser pulse, is demonstrated. An order of magnitude x-ray enhancement over planar graphite target is observed in carbon clusters, formed by a sub-ns pre-pulse, interacting with intense main pulse after a delay. The effect of the delay and the duration of the main pulse is studied for optimizing the x-ray emission in the water window region. This x-ray source has added advantages of being an efficient, high repetition rate, and low debris x-ray source. [ABSTRACT FROM AUTHOR]

Uranium and thorium in their mixed oxides were determined by synchrotron-induced energy dispersive X-ray fluorescence (EDXRF) spectrometry. Mixed oxide calibration mixtures containing uranium and thorium in different relative amounts and approximately fixed amount of yttrium were prepared in the form of pellets. The EDXRF spectra of the pellets were measured at the microfocus XRF (BL-16) beam line of Indus-2 synchrotron radiation facility (Raja Ramanna Centre of Advanced Technology, Indore, India). Characteristic X-ray lines U Lα, Th Lα, and Y Kα were used as analyte lines. Calibration plot for the determination of uranium was prepared by plotting amount ratios of U and Y against the intensity ratios of U Lα and Y Kα. Similarly, a calibration plot for thorium analysis was also made. The amounts of uranium and thorium in the sample mixtures prepared in similar way as the calibration mixtures were determined using the aforementioned calibration plots. The precision values obtained for uranium and thorium determinations were found to be 0.3 and 0.2% (relative standard deviation, 1 σ), respectively. The EDXRF results deviated from the expected values by 1% for uranium and 0.9% for thorium determinations. These analytical features are much superior compared with laboratory-based analysis results. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Self-generated magnetic fields produced in laser plasmas at moderate laser intensities have been measured using a three-channel polaro-interferometer. The main elements of this device are two birefringent calcite wedges placed between two crossed polarizers. Using this device, the spatial profiles of (a) the rotation angle (polarometry), (b) the electron density (interferometry), and (c) the transmitted probe beam intensity (shadowgraphy) are recorded simultaneously using a digital camera with a large format CCD in a single laser shot. Magnetic fields of 2-4 MG had been estimated in aluminum plasma at laser intensities ∼1013 W/cm2. It is also possible to use this device in other configurations to get time resolved information. [ABSTRACT FROM AUTHOR]

A diagnostic system to detect electrical faults inside a coaxial high current discharge device is presented here. This technique utilizes two biconical antennas picking up electromagnetic radiation from the discharge device, a voltage divider sensing input voltage, and a Rogowski coil measuring the main discharge current. A computer program then analyses frequency components in these signals and provides information as to whether the discharge event was normal or any breakdown fault occurred inside the coaxial device. The diagnostic system is developed for a 450 kV and 50 kA capillary discharge plasma setup. For the setup various possible faults are analyzed by electrical simulation, followed by experimental results. In the case of normal discharge through the capillary load the dominant frequency is ∼4 MHz. Under faulty conditions, the peak in magnitude versus frequency plot of the antenna signal changes according to the fault position which involves different paths causing variation in the equivalent circuit elements. [ABSTRACT FROM AUTHOR]

The results of enhanced harmonic generation in fullerene-containing plasma plumes using laser radiation of different wavelengths, pulse durations, and phase modulation are presented. Odd and even harmonics up to the 29th order of 800 nm radiation have been extended in the case of two-color orthogonal polarization pump of C60-containing plasma plumes. The conversion efficiency for the odd and even harmonics in the vicinity of SPR of the C60-containing plasma (40–70 nm) was estimated to be in the range of 10−4. [ABSTRACT FROM AUTHOR]

The wavelength conversion of femtosecond laser pulses in laser plasmas containing clusters of different nature and dimension (fullerenes, metal nanoparticles) is studied. Pulses of a titanium-sapphire laser are used in combination with orthogonally polarized second-harmonic pulses as radiation to be converted. Variations in the generation efficiency of higher harmonics are analyzed under conditions of phase-modulated pulses. It is shown that the optimization of components of a nonlinear optical plasma medium, of plasma excitation conditions by single- and two-color pumps, and of phase and spectral parameters of radiation to be converted leads to a considerable increase in the generation efficiency of higher harmonics. [ABSTRACT FROM AUTHOR]

A large aperture disc amplifier has been designed, set-up and characterized for its performance on small signal gain, spatial variation of gain, and thermal recovery time. This amplifier, consisting of three elliptical Nd: phosphate glass discs of size 214 x 114 x 20mm mounted at Brewster angle and pumped by ten xenon filled flash lamps of 600mm arc length, provided a small signal gain of 6 at electrical pump energy of 36 kJ (in a pulse of 450 μs) using an in-house developed dual-polarity capacitor bank based power supply. It was coupled to a high power Nd: phosphate glass laser chain and a maximum output pulse energy exceeding 100 J in a 1·5 ns (FWHM) pulse has been measured. A dry nitrogen gas based cooling system was developed for cooling the glass discs with a thermal recovery time of ∼ 20 minutes. [ABSTRACT FROM AUTHOR]

A laser plasma X-ray source of narrow spectral range in the water-window region, is reported using a 50–50 (atomic fraction) mixture of gold-copper mix-Z planar target. Plasma was produced using the second harmonic beam of an Nd:glass laser focused to an intensity ∼1013 W/cm2 on the target. The spectrum of the plasma radiation transmitted through a free-standing 0.4 μm aluminium/0.9 μm vanadium X-ray filter foil was measured to lie in the narrow-band of 24–26 Å. This provides a debris-free X-ray dose of 2–3 mJ/sr which can be used for single shot X-ray imaging of live biological samples. [ABSTRACT FROM AUTHOR]

A study of indium nanoparticles prepared by two laser ablation techniques is reported. The suspensions of indium nanoparticles were prepared using the laser ablation of bulk indium in liquids. The prepared suspensions of indium nanoparticles were analyzed by the X-ray fluorescence spectroscopy and absorption spectroscopy. The position of the surface plasmon resonance of In-containing suspensions (350 nm) was consistent with the estimations taking into account the average size of In nanoparticles (43 nm) measured using the X-ray fluorescence spectroscopy. The nonlinear optical parameters of indium nanoparticles-containing liquids were studied by the z-scan technique using a picosecond Nd:YAG laser. We compare the laser ablation in liquids with the laser ablation of indium in vacuum at the tight and weak focusing conditions of a Ti:sapphire laser and analyze the 60 nm indium nanoparticles synthesized in the latter case. [ABSTRACT FROM AUTHOR]

Parametric dependence of the intensity of 182 Å Balmer-α line (C5+; n = 3 → 2), relevant to xuv soft X-ray lasing schemes, from laser-produced carbon plasma is studied in circular spot focusing geometry using a flat field grating spectrograph. The maximum spectral intensity for this line in space integrated mode occurred at a laser intensity of 1.2 x 1013 W cm-2. At this laser intensity, the space resolved measurements show that the spectral intensity of this line peaks at ∼1.5 mm from the target surface indicating the maximum population of C5+ ions (n = 3), at this distance. From a comparison of spatial intensity variation of this line with that of C5+ Ly-α (n = 2 → 1) line, it is inferred that n = 3 state of C5+ ions is predominantly populated through three-body recombination pumping of C6+ ions of the expanding plasma consistent with quantitative estimates on recombination rates of different processes. [ABSTRACT FROM AUTHOR]

A single-frame X-ray framing camera has been set up for fast imaging of X-ray emissions from pulsed plasma sources. It consists of two parts, viz. an X-ray pin-hole camera using an open-ended microchannel plate (MCP) detector coupled to a CCD camera, and a high voltage short duration gate pulse for the MCP. The camera uses a 10-µm pin-hole aperture for imaging on the MCP detector with a magnification of 6X. The high voltage pulser circuit generates a pulse of variable duration from 5 to 30 ns (at 70% of peak amplitude) with variable amplitude from 800V to 1·25 kV, and is triggered through a laser pulse synchronized with the event to be recorded. The performance of the system has been checked by recording X-ray emission from a laser-produced copper plasma. A reduction factor of ∼6·5 is seen in the dark current contribution as the MCP gate pulse is decreased from 250µs to 5 ns duration. [ABSTRACT FROM AUTHOR]

A lateral shearing interferometer has been set up and coupled to an S-20 optical streak camera to obtain time resolved spatial electron density profiles of laser produced plasmas in a single shot. The electron density profiles are recorded with a temporal resolution of 70 ps for a continuous time duration of 15 ns. Performance of the system is demonstrated by recording temporal evolution of one dimensional density profiles of an expanding laser produced aluminum plasma. The system can be especially useful in characterization of expanding plasma for thin film deposition using laser ablated plumes. [ABSTRACT FROM AUTHOR]

A simple technique of single shot x-ray microscopic imaging is reported using two simultaneously produced laser-plasma x-ray sources. Twin x-ray images are recorded by placing the sample in a proximity gap with a photoresist and exposing it to two equivalent point x-ray sources. Characterization of the technique is carried out using a carbon fiber as a test sample and x-ray emission (hν>=1 keV) from laser produced copper plasmas. A spatial resolution of ∼300 nm is observed, consistent with the geometrical parameters of the imaging system. This method may be useful in obtaining a single shot comparison of spectral and temporal characteristics of a sample by using different x-ray filters or plasma targets for the two imaging paths and a temporal delay in generation of the two x-ray sources, respectively. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spectral, temporal, and spatial characteristics of harmonics generated at solid surfaces interacting with laser radiation t = 27 ps and I ≤ 1.5 × 10[sup 15] W/cm²) are studied. Spectral broadening and a long-wavelength shift of the second harmonic were observed for laser radiation intensities exceeding 5 × 10[sup 14] W/cm². Results of the study of the conversion of spectral parameters and polarization features for the generation of second and third harmonics are presented. Conversion efficiencies for the second, third, and fourth harmonic are 2 × 10[sup -8], 10[sup -10], and 5 × 10[sup -12], respectively. The results obtained are compared with data of analogous studies utilizing shorter pulses. Spectral, temporal, and spatial characteristics of harmonics generated at solid surfaces interacting with laser radiation t = 27 ps and I ≤ 1.5 × 10[sup 15] W/cm²) are studied. Spectral broadening and a long-wavelength shift of the second harmonic were observed for laser radiation intensities exceeding 5 × 10[sup 14] W/cm². Results of the study of the conversion of spectral parameters and polarization features for the generation of second and third harmonics are presented. Conversion efficiencies for the second, third, and fourth harmonic are 2 × 10[sup -8], 10[sup -10], and 5 × 10[sup -12], respectively. The results obtained are compared with data of analogous studies utilizing shorter pulses. [ABSTRACT FROM AUTHOR]

A study of the spatial coherence of the high order harmonic radiation generated by the interaction of 45 fs Ti:sapphire laser beam with carbon (graphite) plasma plume has been carried out using Young's double slit interferometry. It is observed that the spatial coherence varies with harmonic order, laser focal spot size in plasma plume, and peaks at an optimal spot size. It is also observed that the spatial coherence is higher when the laser pulse is focused before the plasma plume than when focused after the plume, and it decreases with increase in the harmonic order. The optimum laser parameters and the focusing conditions to achieve good spatial coherence with high harmonic conversion have been identified, which is desirable for practical applications of the harmonic radiation. [ABSTRACT FROM AUTHOR]

An experimental study on the time resolved x-ray diffraction from laser shocked silicon crystal, carried out using a 10 TW Ti:sapphire laser system, is presented. The characteristic Kα x-ray line radiation generated by 45 fs laser produced plasmas of two different target materials (iron and copper) is used as the probe, whereas the stretched pulse of sub-nanosecond duration (pump), derived from the same laser, is used to compress the sample. The use of x-ray probe of different photon energies yields information about the strain over a greater crystal depth. The dynamics of the strain propagation is inferred by monitoring the evolution of rocking curve width of the shocked sample at different time delays between the pump and the probe pulse. The shock velocity deduced from these measurements is ∼106 cm/s, consistent with the sound velocity in bulk silicon. The maximum elastic compression observed is 0.4%, indicating a pressure of 0.8 GPa. [ABSTRACT FROM AUTHOR]

The dynamics of ultrashort laser plasmas demand single shot temporal measurements on fast time scales. We describe a method to record the plasma expansion on picosecond (ps) timescales continuously over hundreds of ps, in single shot. The method uses the chirp of a Ti:sapphire laser as a time-resolved optical diagnostic tool. Using this technique, the evolution of the plasma expansion had been recorded with ps time resolutions, by probing with a chirped laser pulse of 200 ps duration. A peak expansion velocity of 1.8×107 cm/s is observed and its evolution in time is obtained for ∼300 ps. [ABSTRACT FROM AUTHOR]

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 ∼10[SUP13]W cm[SUP-2]. Radiation intensity in the spectral region ∼15-150Å 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. [ABSTRACT FROM AUTHOR]