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In complex environments, the ability to identify the constituent chemicals within a mixture is extremely important. By utilizing a Pearson correlation algorithm to compare sets of multi-wavelength resonance-Raman signatures, we demonstrate the automated identification of chemicals within a mixture. Applying a linear mixture model, we are also able to estimate the fractional volumetric abundances contained therein. The multi-wavelength resonance-Raman signature used for identification is obtained by illuminating the unknown mixture with a series of 21 sequential laser wavelengths. This signature is then compared with the signatures of a set of known chemicals. By maximizing the Pearson correlation coefficient between the signature of the mixture and a weighted superposition of the signatures of the pure chemicals, we are able to determine the mixture components with 100% accuracy. The linear superposition of the selected chemicals, which minimizes the least squares distance between the signatures of the mixture, and its mathematical recreation determines the corresponding fraction, by volume, of each chemical within the mixture. Copyright © 2012 John Wiley & Sons, Ltd.

Raman and other spectroscopy systems operating in the deep ultraviolet (DUV) spectral region below 250 nm lack scattering standards. Such standards are particularly important for experiments that use multiwavelength excitation and/or when results are compared across different experiment platforms. Teflon has been used as an external standard in the visible and near-IR spectral regions and has been suggested for use in the ultraviolet (UV). Comparison of the relative amplitudes of prominent Teflon Raman lines indicates a significant effect on line ratios when the excitation wavelength is below 250 nm. This dependence on excitation wavelength has been commented on previously and attributed to pre-resonance effects, but no detailed examination had been undertaken to date. We present the results of a study of Teflon Raman line ratios obtained from closely spaced excitation wavelengths in the DUV from 210 to 320 nm. The 731 cm−1 line is identified as well suited for a standard. Electronic transition energies associated with resonance of principal Teflon Raman lines are obtained. Copyright © 2010 John Wiley & Sons, Ltd.

Athermal annealing in phosphorus, boron, and aluminum/boron implanted 4H-SiC is explored by launching mechanical shock waves, induced by exposure to a laser pulse. Annealing is observed in a 3.2 mm outer diameter donut-shaped area surrounding the laser exposed spot. The minimum sheet-resistance within this area in phosphorus-implanted sample is close to the thermally annealed value of 400 Ω/□. Unlike thermal annealing, shock annealing did not cause any redistribution of the implant, including boron, a known transient enhanced diffuser in SiC. Optimization of parameters like laser power and/or pulse duration or investigation of an alternate mechanical shockwave launching method is required to achieve athermal annealing far away from the center of the exposed region and similar to that of the thermal annealed material. Results of this study might help in the manufacturing of silicon carbide high-power, high-temperature, and radiation hardened devices in the future.

The time evolution of electrical discharges induced and guided between the cathode of a Van de Graf generator and a ground sphere was studied using a 100 fs Ti:Sapphire laser. Nonlinear focusing and ionization effects produce optical and plasma filaments in the discharge region. Streak camera images often exhibit streamers that propagate towards the cathode, but sudden discharge triggering is frequently observed with no streamer precursors. The typical discharge triggering delay time was measured to be 150 ns. Similar time delays were obtained from an air chemistry code used to model the long time behavior of the plasma induced by the short laser pulse. The model shows that ohmic heating of the filament plasma persists over long time scales and inhibits the decay of electron density due to recombination and attachment processes. Eventually the rise in electron temperature causes the avalanche rate to increase to the point where breakdown occurs. The hydrodynamic density reduction process reported by Tzortzakis et al. [Phys. Rev. E 64, 057401 (2001)] is also taken into consideration. Its main effect is found to be a hastening of the breakdown process.

At the frontier of plasma physics and technology are applications of laser-generated plasmas to laboratory simulations of astrophysical phenomena and to industrial processing. This article presents work at the Naval Research Laboratory in both of these areas. We show how laser plasmas are used to measure a blast wave corrugation overstability important in astrophysics. Detailed atomic physics calculations of radiative cooling within the blast front are used to develop a criterion of the existence of the overstability and are used to explain the experimental results. The criterion depends on quantities such as element abundances, densities, temperatures, and blast wave velocities—quantities which can be measured spectroscopically—and therefore used to infer whether astrophysical blast wave nonuniformities are the result of this instability. In other experiments, high-velocity jets are formed in the laboratory using miniature hollow cones. Jets produced by these cones are used to study the physics of jets occurring in supernovae and in star-forming accretion disks. In industrial semiconductor processing, annealing, that is, removing crystal damage and electrically activating the semiconductor, is a critical step. Industrial annealing techniques most often utilize heat generated by an oven, flash lamps, or a low-power laser. During such heating dopants within the semiconductor lattice diffuse and spread. This degrades the performance of circuits in which the individual circuit elements are very close to each other. We are developing an annealing technique in which shock or sound waves generated by a laser plasma are used to anneal the semiconductor. We have demonstrated that the method works over small areas and that it does not lead to significant dopant diffusion.

Underwater shocks and bubbles were generated using a high energy pulsed laser system. The advantages of this experimental approach are: (1) precisely controlled and measured experimental conditions; (2) improved diagnostics, including extensive imaging capabilities; (3) unique experiments, including a simultaneously detonated line charge; and (4) the ability to provide validation quality data for hydrodynamic simulation codes. Bubble sensitivity to variation of several experimental parameters was examined. Numerical simulations were performed corresponding to the experimental shots, showing that empirical bubble theory, experimental bubble data, and simulations were all in good agreement.

A shock precursor in a thin thermal layer above a 3000 K hot surface is measured in a small-scale laboratory experiment. Precursor and thermal layer characteristics are diagnosed using spectroscopy, interferometry, and dark-field shadowgraphy. The experiment is successfully simulated with a code previously validated using large-scale high-explosive tests, thereby demonstrating its applicability to events that differ in energy by 12 orders of magnitude. The simulation shows that a small-scale laboratory experiment can be a good surrogate for infrequent, expensive, and hard to diagnose large-scale tests.

Simultaneous measurements of high energy electrons and plasma wave characteristics have been conducted in a self-modulated laser wakefield accelerator. Approximately 10(exp 8) electrons were accelerated from the background plasma to energies greater than 1 MeV with a peak energy of approximately 30 MeV. A strong correlation between the plasma wave amplitude and electron production was measured with no evidence of wave breaking. Simulations indicate plasma electrons are trapped by the low phase velocity beat waves produced by backward Raman scattering.

A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wakefields produced by a sub-picosecond, high intensity laser pulse (7×1018 W/cm2) in an underdense plasma (ne≈1019 cm−3) were measured with a pump–probe coherent Thomson scattering (CTS) technique to last for less than 5 ps, consistent with the decay of large amplitude plasma waves due to the modulational instability. A plasma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump–probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using an electro-magnetic spectrometer, with the energy spectra and divergence of lower energy (up to 4 MeV) electrons obtained using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were obser...

Physical Review Letters, February 24, 1997. College Park, MD: American Physical Society. Vol.78, No.8, pp. 1584-1587.

Thin film optical coatings are susceptible to damage by high intensity x rays. Time-resolved measurements of this damage are required to better understand the mechanism, so that more rugged coatings can be developed. In the present experiment, dark-field shadowgraphy was used to temporally map the x-ray damage across the surface of certain anti-reflecting (AR) coatings. Two beams from the NRL PHAROS III high power Nd:glass laser system were utilized to generate a point source of plasma x rays, which in turn was used to irradiate and damage the optical coatings. Thin, opaque filters, coupled with permanent magnets and pinholes, were used to shield the optical samples from ultraviolet and charged-particle damage, respectively. The absolute, time-integrated x-ray fluence was measured with a crystal spectrograph, and also was temporally resolved with an x-ray diode. The surface morphology of the damaged optical samples was examined after each shot visually, and later with a profilometer as well as with both s...

We present results of the design and testing of a modified optical Solc notch filter for use in the deep ultraviolet (DUV, 190-300 nm) spectral range. The filter was designed to block a specific wavelength in this region. In addition, a sequence of blocked wavelengths occurs at wavelengths both shorter and longer than the specified wavelength. For Raman applications utilizing tunable lasers, the provision of multiple blocked wavelengths by a single filter may be especially useful. The filter design presented here produces extinction ratios >240 with transmission minima ~1 nm full width at half-maximum. Specific results are shown for the Raman spectra of Teflon excited at 248.4 nm.

Experiments examining nonlinear scattering mechanisms of high intensity (2 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ W/${\mathrm{cm}}^{2}$) laser light in underdense plasmas were performed. Redshifted second harmonic emission at 45\ifmmode^\circ\else\textdegree\fi{} from the directly backscatterd direction was observed from field ionized plasmas having electron densities ranging up to ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. This result is consistent with the doubling of stimulated Raman scattered light from large plasma electron density gradients produced by ponderomotive cavitation.

The Swept Wavelength Optical Resonant Raman Detector (SWOrRD) at the Naval Research Laboratory is scanned through the 220 nm to 260 nm window of the ultraviolet and generates two dimensional spectral maps allowing for rapid identification of chemical materials. We create a library of chemicals whose Raman spectral features partially overlap and then measure the mixtures created by combinations of the library set. The library and mixtures are identically processed and loaded into a detection program. This system, utilizing a linear combination model enables the deconvolution of the multi chemical liquid mixture and a reconstitution of the fractional molecular abundances.

X‐ray damage to optical surfaces consisting of melting, fracturing, cracking, and cratering is produced, and then analyzed using interference contrast microscopy and stylus profilometery. The test samples are irradiated by 0.5–1.6 keV x rays at fluence levels up to 5.5 cal/cm2. The x rays originate from L‐shell transitions in copper ions, produced when 1.25‐μm‐thick targets are irradiated at 1.3 kJ energy by a 1.054 μm wavelength laser. The x‐ray emission is found to be nearly isotropic over 2π sr, while the plasma mass‐flow distribution is peaked along the laser axis. Hence, contamination of the test sample by target debris or plasma is greatly reduced by placing the samples off axis from the laser beam, in addition to the use of beryllium shields.

We are developing tunable-multi-wavelength resonance-Raman spectroscopy and algorithms to enable rapid detection and identification of bacteria and chemicals in complex environments. The system, dubbed SWOrRD, is capable of illuminating a sample containing many chemicals or biological agents with a sequence of laser wavelengths between 210nm and 2000nm; a range which encompasses the resonant frequencies of cells, microorganisms, cellular metabolites, and many chemicals; and measures the resonance-Raman spectra of light scattered from the sample at each laser wavelength. These multiple spectra, which contain much more information about the bond structure that is contained in a single spectrum, are analyzed by a linear-mixture algorithm, based on NRL's ORASIS, to determine the chemical and bacteriological constituents of the sample. The current status of the research will be described.

Recently, researchers at the Naval Research Laboratory have developed the SWORrD system for measuring two-dimensional Raman Spectra. The device consists of a tunable 2d ultraviolet laser that illuminates the sample at various wavelengths (210-300 nm) and collects a single Raman spectrum at each laser wavelength. The single spectra are combined to form a two-dimensional spectrum (laser wavelength by scattered wavenumber). In this paper we introduce a novel method for the detection of known agents ('targets') within measured 2d spectra. Our method is bases on 'linear mixed pixel' techniques from hyperspectral imagery; in particular, we generalize the Adaptive Subspace Detector (ASD) to a form suitable for SWORrD samples. Our detector uses the individual laser runs to define a set of points within wavenumber space; the set of points corresponding to a 2d spectra defines a particular subspace that contains each material. These subspaces are then used with ASD to identify targets. We include experimental results using real-world data to illustrate our results.

A 1 cm2 area in phosphorus-implanted silicon samples is annealed by irradiation of a much smaller 0.002 cm2 area with a single laser pulse. Resistivity of the annealed region is uniform and similar to that measured after thermal annealing. Electrically activated donors did not diffuse into the sample and only slightly towards the sample surface. The process is 100% reproducible. We present evidence that the annealing is not caused by heat.

Subpicosecond laser pulses at power levels in excess of 1 TW were channeled through hollow microcapillary tubes by use of a combination of grazing-incidence dielectric and plasma-wall reflection mechanisms. Maximum input and output intensities were 10(17) and 10(16) W/cm(2) through 50-microm radius by 3-cm-long glass microcapillary tubes with as few as two waveguide modes being excited. 133-microm radius tubes as long as 13 cm resulted in successful channeling with an extinction coefficient of 0.2 cm(-1) and a plasma-wall reflectivity of 80%.

We present the first measurements of an instability in Taylor-Sedov blast waves propagating through a uniform gas. The instability occurred in a gas whose adiabatic index was low. Amplitude perturbations grew as a power of time. Our observations are compared to theory.

The first two-dimensional (2D) resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm. Two-dimensional spectra reflect variations in resonance Raman scatter with illumination wavelength, adding information not available from single or few one-dimensional spectra, thereby increasing the number of variables available for use in identification, which is especially useful in environments with contaminants and interferents. We have recently shown that 2D resonance Raman spectra can identify bacteria. Thus, a single device that identifies the presence of explosives, bacteria, and other chemicals in complex backgrounds may be feasible.

Detection of microorganisms and chemicals using multi-wavelength resonance-Raman spectroscopy is described. Combined with a proper algorithm, this technique may lead to contactless sensor, capable of real-time detection of biological and chemical threats in complex environments.

The authors have investigated the second-harmonic conversion efficiency of broadband Nd:glass laser light ( Delta nu /c >

Laser-produced plasmas have many properties similar to, or which can be scaled to, those encountered in space, magnetospheric, ionospheric, and astrophysical situations. We describe several such experiments performed with the PHAROS III Nd-laser facility at NRL.

We present evidence that the energy introduced by a short laser pulse focused to high intensity on a small spot on the surface of neutron-transmutation-doped silicon electrically activates impurities far away from the focal spot. The activation of the impurities is measured by far-infrared spectroscopy of shallow donor levels and by magnetotransport characterization. Electrical activity is comparable to that obtained with conventional thermal annealing. X-ray rocking curve measurements show strain in the area of the focal spot, but none at large distances from the focal spot.

Summary form only given. The time evolution of electrical discharges induced and guided between the cathode of a 200 kV Van de Graf generator and a ground sphere was studied using a 100 fs Ti:sapphire laser. Nonlinear focusing and ionization effects produce optical and plasma filaments in the discharge region. Streak camera images often exhibit streamers that propagate towards the cathode, but sudden discharge triggering is frequently observed with no streamer precursors. The typical discharge triggering delay time was measured to be 150 ns. Similar time delays were obtained from an air chemistry code used to model the long time behavior of the plasma induced by the short laser pulse including the external electric field. The model shows that ohmic heating of the filament plasma persists over long time scales and inhibits the decay of electron density due to recombination and attachment processes. Eventually the rise in electron temperature causes the avalanche rate to increase to the point where breakdown occurs. The hydrodynamic density reduction process reported by Tzortzakis, et al. is also taken into consideration. Its main effect is found to be a hastening of the breakdown process.

Thin film coatings are susceptible to high intensity x‐ray damage. The PHAROS III laser was utilized to generate a point source of x‐ray emission used to determine the damage threshold of AR‐coated space optics. Thin filters coupled with magnets were used to shield the specimens from thermal radiation and plasma debris. Grids supporting the thin filters could be patterned into the coatings. The surface morphology of damaged specimens has been examined with SEM and AFM microscopes to determine the nature of the damage in multilayer AR coatings. Microscopic techniques were used to measure the depths of coating damage and edge sharpness in the patterned region.

Large amplitude plasma wakefields produced by a high power laser pulse in an underdense plasma were studied in a self-modulated laser wakefield accelerator (SM-LWFA) experiment. A pump-probe coherent Thomson scattering (CTS) technique was used and the lifetime of the wakefield was measured to be less than 5 psec. A plasma channel was observed to form in the wake of the pump laser pulse. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using a magnetic spectrometer. Highly non-linear plasma waves were also detected using forward Raman scattering diagnostics and were observed to correlate with the electron signals.

Phosphorus, arsenic, and boron dopants in neutron-transmutation-doped and ion-implanted silicon were electrically activated by a technique that does not involve the direct application of heat as in conventional thermal annealing or pulsed laser annealing. Dopants activated by this method exhibit much less diffusion than donors activated by standard thermal methods.

We observe, for the first time in a laboratory, the formation of decursors at intersections of planar surfaces with powerful blast waves. The blast waves, which have hundreds‐kilobar overpressures, are created by rapid ablation of material heated with an intense laser beam.

High energy laser pulses (up to 150 joules at 527 nm) were directed onto underwater targets to produce a heated volume which, upon expansion, created underwater shocks and bubbles. These experiments were a feasibility study for a laboratory experimental program studying two phenomena: (1) bubble collapse dynamics in close proximity to a solid interface, and (2) shock propagation dynamics in aerated sand/water mixtures. The laser was focused both onto a point in the water and onto the surface of a solid thin foil target immersed in the water. For both types of targets, the focal volume of the laser beam was in water. Solid laser targets were observed to effectively localize laser energy deposition, providing a point shock source and producing spherical shocks. Shock pressures were inferred from shock propagation speeds to be 30 Kbar at 2 mm, and measured by carbon piezoresistive gauges to be 200 bar at 13 mm. Bubbles were formed with maximum radii of 15 mm and oscillation times of 3 ms. Key features of a design for a permanent experimental chamber are given in the report.

We experimentally demonstrate the annealing and electrical activation of doped silicon without the direct application of heat as in conventional thermal annealing or pulsed laser annealing. 25 × 25 × 3 mm samples of silicon doped by neutron transmutation were irradiated with a short pulse from a 1.06-micron laser. The few joule laser pulse was focused to mm-diameter surface spot which resulted in high power (∼ 1011W/cm2) capable of launching shocks into the entire sample. In a few instances the entire silicon slab, including regionsfar outside the illuminated spot, was annealed and electrically activated. In the annealed samples electrical activation of donors throughout the slab, measured with a four-point probe, was uniform and comparable to that of thermally annealed control samples. Far-infrared spectroscopy and Hall measurements also showed that the donor species was activated and Raman spectroscopy demonstrated marked improvement in the crystal structure. We conjecture that the annealing was caused by the mechanical energy that was launched into the slab by the laser pulse. Results of experiments on an ion-implanted silicon sample are also discussed.

High energy electrons (up to 30 MeV) have been measured in the self-modulated laser wakefield accelerator using a 2.5 TW laser pulse and a high sensitivity detector, a scintillator coupled to a photo-multiplier tube (PMT). Highly non-linear plasma waves have been detected using forward Raman scattering as a plasma diagnostic and a correlation between the non-linear plasma waves and electron signal has been observed.

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ=248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ=351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns≤τ≤1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for ...

We present results of experiments on very-high Mach number (> 100) shocks and very-high Mach number and Reynolds number (> 100 × 106) turbulence, which are initiated with a powerful laser driver. In one experiment we examined laser-produced-shock solid-surface interactions and Mach stems and triple points, and also measured a new phenomenon termed a blast wave decursor. In a second experiment we found that shocks become unstable if they propagate through a gas which has a low adiabatic index and we measured the growth rate of the instability. In a third experiment we have shown that a high Mach number shock dramatically enhances the structure of a turbulent field through which it passes and that the shock itself is badly distorted. This result is unexpected since common wisdom has it that high Mach number shocks self-heal as they pass through a turbulent field.

We present results of experiments on very-high Mach number (> 100) shocks and very-high Mach number and Reynolds number (> 100, 106) turbulence. Such high Mach number hydrodynamics are initiated with a powerful laser pulse driver. We show that shocks created with a laser driver follow the Taylor-Sedov self-similar solution and scale via the Sachs scaling law just like shocks created by more traditional methods. In one experiment we examined laser- produced-shock solid-surface interactions and observed expected phenomena such as Marsh stems and triple points, and also measured a new phenomenon termed a blast wave decursor. In second experiment we found that shocks become unstable if they propagate through a gas which has a low adiabatic index and we measured the growth rate of the instability. In a third experiment we have shown that a high Mach number shock dramatically enhances the structure of a turbulent field through which it passes and that the shock is itself badly distorted. This result is unexpected since common wisdom has it that high Mach number shocks would self-heal as they pass through a turbulent field. Turbulence, Shock.

Visible spectral lines from [ital n]=3, [Delta][ital n]=0 transitions in N[sup +] and N[sup 2+] ions are used for measuring the plasma electron density and temperature in a region of two colliding blast waves, propagating through a 1.5--10-Torr nitrogen atmosphere. The blast waves originate at the tips of two aluminum rods irradiated with two beams of the Naval Research Laboratory (NRL) Pharos-III 1.054-[mu]m-wavelength Nd:glass laser operated at an energy of 200--430 J for each beam in 5-ns pulses. An electron density in the colliding-blast-wave region of [ital N][sub [ital e]][approx]10[sup 18] cm[sup [minus]3] was deduced from Stark broadening of spectral lines from N[sup +] ions. An electron temperature of [ital T][sub [ital e]][approx]4 eV was measured in this region from a spectral-line intensity ratio between N[sup 2+] and N[sup +] ions. Near one target, an electron density of [ital N][sub [ital e]][approx]8[times]10[sup 20] cm[sup [minus]3] was determined from series-limit x-ray spectral-line merging; a mean electron temperature of [ital kT][sub [ital e]][approx]225 eV was determined from x-ray line-intensity ratios. Some evidence was found for enhanced velocities for blast waves propagating through a plasma formed by a preceding blast wave.

A scattered-light spectroscopy system for collecting light scattered from a sample, e.g. Raman-scattered light, to produce a spectrum of the sample, includes a cylindrical cell for holding the sample that is transparent and coated on either its inside surface or outside surface with a reflective coating, e.g. aluminum. The reflective coating has an opening for aligning with an aperture in a spectrometer for receiving the sample-scattered light. Light from a source such as a laser illuminates the sample to produce a scattered light having a first part received directly at the opening and a second part reflected by the reflective coating one or more times prior to arrival at the opening, thereby adding to the total scattered light entering the aperture of the spectrometer to improve its collection efficiency.

Cumulative plasma jets formed by hollow cones imploded via laser ablation of their outer surfaces were observed. The velocity, shape, and density of the jets are measured with monochromatic 0.65keV x-ray imaging. Depending on cone geometry, cumulative jets with ion density ∼2×1020cm−3 and propagation velocities >10km∕s are formed. Similar results are observed when jets are formed by imploding wedges. Such jets can be used to simulate hydrodynamics of astrophysical jets interacting with stellar or interstellar matter.

We present images from the first observation and measurements of an instability in Taylor-Sedov blast waves propagating through a uniform background gas. The instability occurs when the adiabatic index of the background gas is low. Images of the expanding blast waves are dark field shadowgraphs obtained using a frequency-doubled (527 nm) pulsed YAG laser and black and white photographic film. The film images are digitized, and image processing routines are used for analysis and presentation.

We performed high-intensity subpicosecond laser-plasma interaction experiments to examine nonlinear scattering mechanisms in underdense plasmas. At incident laser intensities of 2 x 10(18) W/cm(2) the stimulated-Raman-backscattered spectrum exhibited an extremely broad, supercontinuumlike structure (Deltaomega/omega(0)1) extending from ~500 to1200 nm (limited only by detector sensitivity). Large-amplitude modulations in the spectrum of the backscattered light were measured and are attributed to an interaction of the stimulated-Raman-scattered radiation with ion plasma waves.

We observed, for the first time to our knowledge, soft-x-ray 3p–3s lasing in neonlike zinc and arsenic in the wavelength interval 212–267 A, with gain coefficients ranging from 2.0 to 5.4 cm−1. Also, spectral lines were observed in neonlike gallium at the wavelengths predicted for lasing. A single-beam, driving laser (λ = 1.05 μm) at pulse energies of up to 500 J, with rise times of ~0.6 ns and pulse durations of 1–2 ns, was used with slab targets. Time-integrated diagnostics were used. We discuss the refractive bending of the lasing beams, the importance of the rise time of the driving laser pulse, and the interferometric measurement of the electron density gradients and density distributions of the lasant plasma.

Growth rates of the Rayleigh-Taylor instability in foils accelerated with a laser beam smoothed by induced spatial incoherence were measured and compared with hydrodynamic code simulations. Modes with 150- and 100-\ensuremath{\mu}m wavelengths grew at predicted rates. However, no growth was experimentally observed at a 50-\ensuremath{\mu}m wavelength. Code simulations suggest that induced spatial incoherence can influence the 50-\ensuremath{\mu}m Rayleigh-Taylor mode by delaying the start of its growth.

An optical technique called induced spatial incoherence can produce the highly uniform laser beams required for direct-illumination laser fusion. Here we present experimental data on laser-plasma interactions showing that the technique, when used in combination with short laser wavelength, also improves the coupling physics.

We observe linear and nonlinear features of a strong plasma/magnetic field interchange Rayleigh-Taylor instability in the limit of large ion Larmor radius. The instability undergoes rapid linear growth culminating in free-streaming flute tips.

Using a fast-gated (120 psec-5 nsec) microchannel-plate optical camera (Gated Optical Imager), framing photographs have been taken of the rapidly streaming, laser plasma (~5 x 10 7 cm/sec) passing through a vacuum or a background gas, with and without a magnetic field. Observations of Large-Larmor-Radius Interchange Instabilities are presented.

The interaction of two interstreaming plasmas is of interest from several points of view. As a basic plasma physics topic, there are a myriad of possible collisionless and hydrodynamic instabilities that may occur when two beams interpenetrate. Some aspects of these instabilities are well established and have been observed, but others are poorly understood. Interpenetrating plasmas related to our choice of experimental parameters appear in several natural and man-made situations. The sun, for instance, emits a solar wind plasma which forms a flowing interplanetary drifting plasma with drift velocities in the hundreds of kilometer per second range; interestingly, laser-produced plasmas have comparable ablation velocities; laser-produced-plasmas also stream from coronas having temperatures very close to the solar coronas (1–2keV). The solar wind interactions with the earth’s magnetosphere and cometary plasmas have been observed to be very complex. An even more dramatic natural example of interacting energetic plasmas is a supernova explosion. Man-made counter streaming plasma situations also abound, e.g., in some proposed inertial confinement fusion reactors, theta pinch devices, ion beam ICF schemes, and in some ionospheric modifications.

Stimulated Brillouin scattering spectra have been measured from plasmas produced by spatially and temporally incoherent laser light. The measured backscatter spectra and intensities differ substantially from measurements of Brillouin scattering with coherent illumination. Backscatter is strongly suppressed and the Brillouin signature in the spectra disappears. This quenching of stimulated Brillouin scattering is observed even when the laser coherence time is much larger (10\ifmmode\times\else\texttimes\fi{}) than the predicted convective saturation time of the stimulated Brillouin scattering instability.

Hot-electron fractions measured in Nd-laser-plasma interactions at 1014W/cm2 increase from 0.02% to 0.3% as the plasma scalelength at quarter-critical density (nc/4)is systematically increased from 140 microns to 320 microns. Correlated x-ray and 3w/2 emissions indicate that the hot electrons are produced by a nc/4 instability. Originator-supplied key words include: Laser-plasma interactions, Long-scalelength plasma, Hot-electron temperature, Plasma instabilities.