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We present optical Thomson scattering measurements of electron density and temperature in high Mach number laser-driven blast waves in homogeneous gases. Taylor–Sedov blast waves are launched in nitrogen (N2) or helium (He) at pressures between 0.4 mTorr and 10 Torr by ablating a solid plastic target with a high energy laser pulse (10 J, 1012 W/cm2). Experiments are performed at high repetition rate (1 Hz), which allows one-dimensional and two-dimensional Thomson scattering measurements over an area of several cm2 by automatically translating the scattering volume between shots. Electron temperature and density in the blast wave fronts were seen to increase with increasing background gas pressure. Measured electron density and temperature gradients were used to calculate ∂ B / ∂ t ∝ ∇ T e × ∇ n e . The experimentally measured ∂ B / ∂ t showed agreement with the magnetic field probe (B-dot) measurements, revealing that magnetic fields are generated in the observed blast waves via the Biermann battery effect. The results are compared to numerical three-dimensional collisional magnetohydrodynamic simulations performed with FLASH, and are discussed in the context of spontaneous magnetic field generation via the Biermann battery effect. [ABSTRACT FROM AUTHOR]

We present measurements of ion velocity distribution profiles obtained by laser induced fluorescence (LIF) on an explosive laser produced plasma. The spatiotemporal evolution of the resulting carbon ion velocity distribution was mapped by scanning through the Doppler-shifted absorption wavelengths using a tunable, diode-pumped laser. The acquisition of these data was facilitated by the high repetition rate capability of the ablation laser (1 Hz), which allowed for the accumulation of thousands of laser shots in short experimental times. By varying the intensity of the LIF beam, we were able to explore the effects of fluorescence power against the laser irradiance in the context of evaluating the saturation vs the non-saturation regime. The small size of the LIF beam led to high spatial resolution of the measurement compared to other ion velocity distribution measurement techniques, while the fast-gate operation mode of the camera detector enabled the measurement of the relevant electron transitions. [ABSTRACT FROM AUTHOR]

We have developed a non-collective Thomson scattering diagnostic for measurements of electron density and temperature on the Large Plasma Device. A triple grating spectrometer with a tunable notch filter is used to discriminate the faint scattering signal from the stray light. In this paper, we describe the diagnostic and its calibration via Raman scattering and present the first measurements performed with the fully commissioned system. Depending on the discharge conditions, the measured densities and temperatures range from 4.0 × 1012 to 2.8 × 1013 cm−3 and from 1.2 to 6.8 eV, respectively. The variation of the measurement error with plasma parameters and discharges averaged is also discussed. [ABSTRACT FROM AUTHOR]

The laminar coupling of energy between a laser-produced plasma and a background magnetized plasma was investigated via planar laser induced fluorescence diagnostic and magnetic flux probes. Experiments performed on the Large Plasma Device at the University of California, Los Angeles, mapped out the two-dimensional spatiotemporal evolution of the laser-plasma (debris) ion velocity distribution function (VDF) to assess debris-background coupling in a sub-Alfvénic regime. The acquisition of these data necessitates high repetition rate (1 Hz) as each dataset is the accumulation of thousands of laser shots, which would not be feasible in single-shot experiments. Fully kinetic, three-dimensional particle-in-cell simulations are compared to the measured VDFs to provide a framework in which we can understand the coupling of a sub-Alfvénic plasma flow through a preformed, magnetized plasma. The simulations display the same departure from the expected gyromotion of the debris plasma as observed in the experimental data, and in conjunction with the measured magnetic field traces, have led to the direct observation of the collisionless coupling via laminar fields. [ABSTRACT FROM AUTHOR]

Magnetospheres are a ubiquitous feature of magnetized bodies embedded in a plasma flow. While large planetary magnetospheres have been studied for decades by spacecraft, ion-scale "mini" magnetospheres can provide a unique environment to study kinetic-scale, collisionless plasma physics in the laboratory to help validate models of larger systems. In this work, we present preliminary experiments of ion-scale magnetospheres performed on a unique high-repetition-rate platform developed for the Large Plasma Device at the University of California, Los Angeles. The experiments utilize a high-repetition-rate laser to drive a fast plasma flow into a pulsed dipole magnetic field embedded in a uniform magnetized background plasma. 2D maps of the magnetic field with high spatial and temporal resolution are measured with magnetic flux probes to examine the evolution of magnetosphere and current density structures for a range of dipole and upstream parameters. The results are further compared to 2D particle-in-cell simulations to identify key observational signatures of the kinetic-scale structures and dynamics of the laser-driven plasma. We find that distinct 2D kinetic-scale magnetopause and diamagnetic current structures are formed at higher dipole moments, and their locations are consistent with predictions based on pressure balances and energy conservation. [ABSTRACT FROM AUTHOR]

Experiments, analytic modeling, and numerical simulations are presented to characterize carbon plasmas produced by high-intensity (109 - 1013 W cm-2) lasers relevant to experimental laboratory astrophysics. In the large-scale limit, the results agree well with a self-similar isentropic, adiabatic fluid model. Laser-target simulations, however, show small-scale structure in the velocity distribution of different ion species, which is also seen in experiments. These distributions indicate that most of the plasma energy resides in moderate charge states (C+3-C+4), most of the mass resides in the lowest charge states, and the highest charge states move fastest. [ABSTRACT FROM AUTHOR]

We present optical Thomson scattering measurements of electron density and temperature in a large-scale (∼2 cm) exploding laser plasma produced by irradiating a solid target with a high-energy (5–10 J) laser pulse at a high repetition rate (1 Hz). The Thomson scattering diagnostic matches this high repetition rate. Unlike previous work performed in single shots at much higher energies, the instrument allows for point measurements anywhere inside the plasma by automatically translating the scattering volume using motorized stages as the experiment is repeated at 1 Hz. Measured densities around 4 × 1016 cm−3 and temperatures around 7 eV result in a scattering parameter near unity, depending on the distance from the target. The measured spectra show the transition from collective scattering close to the target to non-collective scattering at larger distances. Densities obtained by fitting the weakly collective spectra agree to within 10% with an irradiance calibration performed via Raman scattering in nitrogen. [ABSTRACT FROM AUTHOR]

Laser-produced plasma velocity distributions are an important, but difficult quantity to measure. We present a non-invasive technique for measuring individual charge state velocity distributions of laser-produced plasmas using a high temporal and spectral resolution monochromator. The novel application of this technique is its ability to detect particles up to 7 m from their inception (significantly larger than most laboratory plasma astrophysics experiments, which take place at or below the millimeter scale). The design and assembly of this diagnostic is discussed in terms of maximizing the signal to noise ratio, maximizing the spatial and temporal resolution, and other potential use cases. The analysis and results of this diagnostic are demonstrated by directly measuring the time-of-flight velocity of all ion charge states in a laser produced carbon plasma. [ABSTRACT FROM AUTHOR]

The creation of a repeatable collisionless quasi-parallel shock in the laboratory would provide a valuable platform for experimental studies of space and astrophysical shocks. However, conducting such an experiment presents substantial challenges. Scaling the results of hybrid simulations of quasi-parallel shock formation to the laboratory highlights the experimentally demanding combination of dense, fast, and magnetized background and driver plasmas required. One possible driver for such experiments is high-energy laser-produced plasmas (LPPs). Preliminary experiments at the University of California, Los Angeles, have explored LPPs as drivers of quasi-parallel shocks by combining the Phoenix Laser Laboratory [Niemann et al., J. Instrum. 7, P03010 (2012)] with a large plasma device [Gekelman et al., Rev. Sci. Instrum. 87, 025105 (2016)]. Beam instabilities and waves characteristic of the early stages of shock formation are observed, but spatial dispersion of the laser-produced plasma prematurely terminates the process. This result is illustrated by experimental measurements and Monte Carlo calculations of LPP density dispersion. The experimentally validated Monte Carlo model is then applied to evaluate several possible approaches to mitigating LPP dispersion in future experiments. [ABSTRACT FROM AUTHOR]

Ion-beam transport in space charge neutralizing discharge channels has been proposed for the final focus and chamber transport in a heavy-ion fusion reactor. A driver scenario with two-sided target illumination requires a system of two intersecting discharges to transport beams of the same charge from opposite sides towards the fusion target. In this article we report on experiments on the creation of free-standing, intersecting high-current discharge channels. The discharges are initiated in ammonia gas (NH[SUB3]) in a metallic chamber by two perpendicular CO[SUB2]-laser beams, which resonantly heat and subsequently rarefy the gas along the laser paths before the breakdown. These low density channels guide the discharges along the predefined paths and also around the 90° angles without any mechanical guiding structures. In this way stable X-, T-, and L-shaped discharges with currents in excess of 40 kA, at pressures of a few mbar were created with a total length of 110 cm. An 11.4 A MeV [SUP58]Ni[SUP+12] beam from the UNILAC (Universal Linear Accelerator) linear accelerator was used to probe the line-integrated ion-optical properties of the central channel in a T-shaped discharge. [ABSTRACT FROM AUTHOR]

High current discharge channels can neutralize both current and space charge of very intense ion beams. Therefore, they are considered an interesting solution for final focus and beam transport in a heavy ion beam fusion reactor. At the Gesellschaft fuer Schwerionenforschung accelerator facility, 50 cm long, free-standing discharge channels were created in a 60 cm diameter metallic chamber. Discharges with currents of 45 kA in 2 to 25 mbar ammonia (NH[sub 3]) gas are initiated by a CO[sub 2] laser pulse along the channel axis before the capacitor bank is triggered. Resonant absorption of the laser, tuned to the υ[sub 2] vibration of the ammonia molecule, causes strong gas heating. Subsequent expansion and rarefaction of the gas prepare the conditions for a stable discharge to fulfill the requirements for ion beam transport. The influence of an electric prepulse on the high current discharge was investigated. This article describes the laser-gas interaction and the discharge initiation mechanism. We found that channels are magnetohydrodynamic stable up to currents of 45 kA, measured by fast shutter and streak imaging techniques. The rarefaction of the laser heated gas is studied by means of a one-dimensional Lagrangian fluid code (CYCLOPS) and is identified as the dominant initiation mechanism of the discharge. [ABSTRACT FROM AUTHOR]

High current discharge channels are a promising option for the final stage of beam transport in a heavy ion fusion reactor. The creation and the properties of the channel are studied at the Gesellschaft fuer Schwerionenforschung accelerator facility. This article discusses a diagnostic technique to determine the neutral gas density channel created by a laser in a neutral gas. The concept is to measure the spreading of an ion beamlet due to collisional scattering with the gas, and thereby deduce the average line-integrated gas density. The results show a density reduction near the axis, which compares favorably with one-dimensional code calculations. [ABSTRACT FROM AUTHOR]

The explosive expansion of a localized plasma cloud into a relatively tenuous, magnetized, ambient plasma characterizes a variety of astrophysical and space phenomena. In these rarified environments, collisionless electromagnetic processes rather than Coulomb collisions typically mediate the transfer of momentum and energy from the expanding "debris" plasma to the surrounding ambient plasma. In an effort to better understand the detailed physics of collisionless coupling mechanisms, compliment in situ measurements of space phenomena, and provide validation of previous computational and theoretical work, the present research jointly utilizes the Large Plasma Device and the Raptor laser facility at the University of California, Los Angeles to study the super-Alfvénic, quasi-perpendicular expansion of laser-produced carbon (C) and hydrogen (H) debris plasma through preformed, magnetized helium (He) ambient plasma via a variety of diagnostics, including emission spectroscopy, wavelength-filtered imaging, and a magnetic flux probe. Doppler shifts detected in a He1+ ion spectral line indicate that the ambient ions initially accelerate transverse to both the debris plasma flow and the background magnetic field. A qualitative analysis in the framework of a "hybrid" plasma model (kinetic ions and inertia-less fluid electrons) demonstrates that the ambient ion trajectories are consistent with the large-scale laminar electric field expected to develop due to the expanding debris. In particular, the transverse ambient ion motion provides direct evidence of Larmor coupling, a collisionless momentum exchange mechanism that has received extensive theoretical and numerical investigation. In order to quantitatively evaluate the observed Doppler shifts, a custom simulation utilizing a detailed model of the laser-produced debris plasma evolution calculates the laminar electric field and computes the initial response of a distribution of ambient test ions. A synthetic Doppler-shifted spectrum constructed from the simulated test ion velocities excellently reproduces the experimental measurements, verifying that the observed ambient ion motion corresponds to collisionless coupling through the laminar electric field. [ABSTRACT FROM AUTHOR]

Collisionless shocks are common phenomena in space and astrophysical systems, and in many cases, the shocks can be modeled as the result of the expansion of a magnetic piston though a magnetized ambient plasma. Only recently, however, have laser facilities and diagnostic capabilities evolved sufficiently to allow the detailed study in the laboratory of the microphysics of piston-driven shocks. We review experiments on collisionless shocks driven by a laser-produced magnetic piston undertaken with the Phoenix laser laboratory and the Large Plasma Device at the University of California, Los Angeles. The experiments span a large parameter space in laser energy, background magnetic field, and ambient plasma properties that allow us to probe the physics of piston-ambient energy coupling, the launching of magnetosonic solitons, and the formation of subcritical shocks. The results indicate that piston-driven magnetized collisionless shocks in the laboratory can be characterized with a small set of dimensionless formation parameters that place the formation process in an organized and predictive framework. [ABSTRACT FROM AUTHOR]

Recent experiments at the University of California, Los Angeles have successfully generated subcritical magnetized collisionless shocks, allowing new laboratory studies of shock formation relevant to space shocks. The characteristics of these shocks are compared with new data in which no shock or a pre-shock formed. The results are consistent with theory and 2D hybrid simulations and indicate that the observed shock or shock-like structures can be organized into distinct regimes by coupling strength. With additional experiments on the early time parameters of the laser plasma utilizing Thomson scattering, spectroscopy, and fast-gate filtered imaging, these regimes are found to be in good agreement with theoretical shock formation criteria. [ABSTRACT FROM AUTHOR]

The collision-less transfer of momentum and energy from explosive debris plasma to magnetized background plasma is a salient feature of various astrophysical and space environments. While much theoretical and computational work has investigated collision-less coupling mechanisms and relevant parameters, an experimental validation of the results demands the measurement of the complex, collective electric fields associated with debris-background plasma interaction. Emission spectroscopy offers a non-interfering diagnostic of electric fields via the Stark effect. A unique experiment at the University of California, Los Angeles, that combines the Large Plasma Device (LAPD) and the Phoenix laser facility has investigated the marginally super-Alfvénic, quasi-perpendicular expansion of a laser-produced carbon (C) debris plasma through a preformed, magnetized helium (He) background plasma via emission spectroscopy. Spectral profiles of the He II 468.6nm line measured at the maximum extent of the diamagnetic cavity are observed to intensify, broaden, and develop equally spaced modulations in response to the explosive C debris, indicative of an energetic electron population and strong oscillatory electric fields. The profiles are analyzed via time-dependent Stark effect models corresponding to single-mode and multi-mode monochromatic (single frequency) electric fields, yielding temporally resolved magnitudes and frequencies. The proximity of the measured frequencies to the expected electron plasma frequency suggests the development of the electron beam-plasma instability, and a simple saturation model demonstrates that the measured magnitudes are feasible provided that a sufficiently fast electron population is generated during C debris-He background interaction. Potential sources of the fast electrons, which likely correspond to collision-less coupling mechanisms, are briefly considered. [ABSTRACT FROM AUTHOR]

We present optical Thomson scattering results that image for the first time in a single measurement the spatial transition from collective to non-collective scattering. Data were taken in the Phoenix laser laboratory at the University of California, Los Angeles. The Raptor laser was used to ablate a carbon plasma, which was diagnosed with the frequency-doubled Phoenix laser serving as a Thomson scattering probe. Scattered light was collected from the laser plasma up to 10 cm from the target surface and up to 10 us after ablation, and imaged with high spatial and spectral resolutions. The results show a strong Thomson collective feature close to the target surface that smoothly transitions to a non-collective feature over several mm. [ABSTRACT FROM AUTHOR]

The interaction of a laser-driven super-Alfvénic magnetic piston with a large, preformed magnetized ambient plasma has been studied by utilizing a unique experimental platform that couples the Raptor kJ-class laser system [Niemann et al., J. Instrum. 7, P03010 (2012)] to the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles. This platform provides experimental conditions of relevance to space and astrophysical magnetic collisionless shocks and, in particular, allows a detailed study of the microphysics of shock formation, including piston-ambient ion collisionless coupling. An overview of the platform and its capabilities is given, and recent experimental results on the coupling of energy between piston and ambient ions and the formation of collisionless shocks are presented and compared to theoretical and computational work. In particular, a magnetosonic pulse consistent with a low-Mach number collisionless shock is observed in a quasi-perpendicular geometry in both experiments and simulations. [ABSTRACT FROM AUTHOR]

Two-dimensional hybrid simulations of perpendicular collisionless shocks are modeled after potential laboratory conditions that are attainable in the LArge Plasma Device (LAPD) at the University of California, Los Angeles Basic Plasma Science Facility. The kJ class 1053 nm Nd:Glass Raptor laser will be used to ablate carbon targets in the LAPD with on-target energies of 100-500 J. The ablated debris ions will expand into ambient, partially ionized hydrogen or helium. A parameter study is performed via hybrid simulation to determine possible conditions that could lead to shock formation in future LAPD experiments. Simulation results are presented along with a comparison to an analytical coupling parameter. [ABSTRACT FROM AUTHOR]

Backscattered light via laser-plasma instabilities has been measured in early NIF hohlraum experiments on two beam quads using a suite of detectors. A full aperture backscatter system and near backscatter imager (NBI) instrument separately measure the stimulated Brillouin and stimulated Raman scattered light. Both instruments work in conjunction to determine the total backscattered power to an accuracy of ∼15%. In order to achieve the power accuracy we have added time-resolution to the NBI for the first time. This capability provides a temporally resolved spatial image of the backscatter which can be viewed as a movie. [ABSTRACT FROM AUTHOR]

A three-axis, 2.5 mm overall diameter differential magnetic probe (also known as B-dot probe) is discussed in detail from its design and construction to its calibration and use as diagnostic of fast transient effects in exploding plasmas. A design and construction method is presented as a means to reduce stray pickup, eliminate electrostatic pickup, reduce physical size, and increase magnetic signals while maintaining a high bandwidth. The probe’s frequency response is measured in detail from 10 kHz to 50 MHz using the presented calibration method and compared to theory. The effect of the probe’s self-induction as a first order correction in frequency, O(ω), on experimental signals and magnetic field calculations is discussed. The probe’s viability as a diagnostic is demonstrated by measuring the magnetic field compression and diamagnetism of a sub-Alfvénic (∼500 km/s,MA∼0.36) flow created from the explosion of a high-density energetic laser plasma through a cooler, low-density, magnetized ambient plasma. [ABSTRACT FROM AUTHOR]

Experiments show that application of laser smoothing schemes including smoothing by spectral dispersion and polarization smoothing effectively suppresses stimulated Raman scattering from a 2ω (527 nm) laser beam in a low average-gain plasma with a steep density gradient. Full-wave simulations reproduce the observed trends in the data and show that the scattering reduction is an indirect result of suppressing active filamentation. [ABSTRACT FROM AUTHOR]

Experiments are described in which a 1 mJ, 1 ps, 1200 nm seed laser beam is amplified by the interaction with an intersecting 350 J, 1 ns, 1054 nm pump beam in a low density (1×1019/cm3) plasma. The transmission of the seed beam is observed to be enhanced by >=25× when the plasma is near the resonant density for stimulated Raman scattering, compared to measured transmissions at wavelengths just below the resonant value. The amplification is observed to increase rapidly with increases in both pump intensity and plasma density. [ABSTRACT FROM AUTHOR]

Efforts to predict and control laser-plasma interactions (LPI) in ignition hohlraum targets for the National Ignition Facility [G. H. Miller et al., Opt. Eng. 43, 2841 (2004)] are based on plasma conditions provided by radiation hydrodynamic simulations. Recent experiments provide compelling evidence that codes such as HYDRA [M. M. Marinak et al., Phys. Plasmas 8, 2275 (2001)] can accurately predict the plasma conditions in laser-heated targets such as gas-filled balloon (gasbag) and hohlraum platforms for studying LPI. Initially puzzling experimental observations are found to be caused by bulk hydrodynamic phenomena. Features in backscatter spectra and transmitted light spectra are reproduced from the simulated plasma conditions. Simulations also agree well with Thomson scattering measurements of the electron temperature. The calculated plasma conditions are used to explore a linear-gain based phenomenological model of backscatter. For long plasmas at ignition-relevant electron temperatures, the measured backscatter increases monotonically with gain and is consistent with linear growth for low reflectivities. These results suggest a role for linear gain postprocessing as a metric for assessing LPI risk. [ABSTRACT FROM AUTHOR]

A near backscattering imaging diagnostic system has been implemented, qualified, and fielded on the first quad of beams on the National Ignition Facility [E. M. Campbell and W. J. Hogan, Plasma Phys. Controlled Fusion 41, B39 (1999)]. This diagnostic image diffusing scatter plates, placed around the final focus lenses on the NIF target chamber, to quantitatively measure the fraction of light backscattered outside of the incident cone of the focusing optics. The imaging system consists of a wide-angle lens coupled to a gated charged coupled device camera, providing 3 mm resolution over a 2 m field of view. To account for changes of the system throughput due to exposure to target debris the system was routinely calibrated in situ at 532 and 355 nm using a dedicated pulsed laser source. The diagnostic and calibration methods will be described together with recent results from the NIF early light shots. [ABSTRACT FROM AUTHOR]

In experiments at the Omega Laser Facility [J. M. Soures et al., Fusion Technol. 30, 492 (1996)], stimulated Brillouin backscatter (SBS) from gasbags filled with krypton and xenon gases was ten times lower than from CO2-filled gasbags with similar electron densities. The SBS backscatter was a 1%–5% for both 527 and 351 nm interaction beams at an intensity of ∼1015 W/cm2. The SRS backscatter was less than 1%. The 351 nm interaction beam is below the threshold for filamentation and the SBS occurs in the density plateau between the blast waves. Inverse bremsstrahlung absorption of the incident and SBS light account for the lower reflectivity from krypton than from CO2. The 527 nm interaction beam filaments in the blowoff plasma before the beam propagates through the blast wave, where it is strongly absorbed. Thus, most of the 527 nm SBS occurs in the flowing plasma outside the blast waves. [ABSTRACT FROM AUTHOR]

Experiments in plasmas produced with 2 mm diameter gas filled targets preheated with 10 kJ of laser energy have shown that the stimulated Raman scattering (SRS) of a high intensity, 351 nm, beam is affected by the presence of a second, counterpropagating, high intensity beam and that has its polarization aligned to the first when the plasma conditions are relevant to ignition by indirect drive. Separate experiments with the crossing beam's polarization rotated to be normal to the first beam's polarization show little change in the SRS backscatter when the second beam is added, consistent with the reduction in the SRS being caused by low frequency waves driven by the ponderomotive force produced by the beating of the two beams. [ABSTRACT FROM AUTHOR]

The first hohlraum experiments on the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)] using the first four laser beams have activated the indirect-drive experimental capabilities and tested radiation temperature limits imposed by hohlraum plasma filling. Vacuum hohlraums have been irradiated with laser powers up to 9 TW, 1 to 9 ns long square pulses and energies of up to 17 kJ to study the hohlraum radiation temperature scaling with the laser power and hohlraum size, and to make contact with hohlraum experiments performed previously at other laser facilities. Furthermore, for a variety of hohlraum sizes and pulse lengths, the measured x-ray flux shows signatures of plasma filling that coincide with hard x-ray emission from plasma streaming out of the hohlraum. These observations agree with hydrodynamic simulations and with analytical modeling that includes hydrodynamic and coronal radiative losses. The modeling predicts radiation temperature limits on full NIF (1.8 MJ) that are significantly greater than required for ignition hohlraums. [ABSTRACT FROM AUTHOR]

Experiments done at the National Ignition Facility laser [J. A. Paisner, E. M. Campbell, and W. Hogan, Fusion Technol. 26, 755 (1994)] using gas-filled hohlraums demonstrate a key ignition design feature, i.e., using plasma pressure from a gas fill to tamp the hohlraum-wall expansion for the duration of the laser pulse. Moreover, our understanding of hohlraum energetics and the ability to predict the hohlraum soft-x-ray drive has been validated in ignition-relevant conditions. Finally, the laser reflectivity from stimulated Raman scattering in the fill plasma, a key threat to hohlraum performance, is shown to be suppressed by choosing a design with a sufficiently high ratio of electron temperature to density. [ABSTRACT FROM AUTHOR]

Experiments show that power is transferred between two copropagating 351 nm laser beams crossing in an Al plasma when the frequency of the driven ion wave is shifted by a Mach 1 flow. The resonant amplification of a low-intensity (<=2.5×1014 W/cm2) beam intersected by a high-intensity (7.0×1014 W/cm2) pump beam is determined by comparing the transmitted beam power to that measured in experiments where the plasma flow direction is reversed and the ion wave is evidently detuned. The polarization of the amplified light is also observed to align to the pump polarization consistent with ion-wave scattering. The amplification is found to reduce with probe-beam intensity demonstrating a nonlinear saturation mechanism that is effective when the ion-wave damping is weak, which is modeled with a calculation including both the nonlinear ion-wave frequency shifts due to ion trapping and whole-beam pump depletion. [ABSTRACT FROM AUTHOR]

A near backscattering imaging diagnostic system is being implemented on the first quad of beams on the National Ignition Facility. This diagnostic images diffusing scatter plates, placed around the final focus lenses on the National Ignition Facility target chamber, to quantitatively measure the fraction of light backscattered outside of the focusing cone angle of incident laser beam. A wide-angle imaging system relays an image of light scattered outside the lens onto a gated charge coupled device camera, providing 3 mm resolution over a 2 m field of view. To account for changes of the system throughput due to exposure to target debris the system will be routinely calibrated in situ at 532 and 355 nm using a dedicated pulsed laser source. [ABSTRACT FROM AUTHOR]

We present a sensitive density diagnostic based on the deflection of a laser beam by refractive index gradients. The method is used to investigate stress waves in plexiglass, created by the irradiation of multilayered metal–plexiglass targets with intense relativistic heavy-ion beams. Measured laser deflection angles are of the order of 1 mrad, with a resolution of the apparatus of 50 μrad. Results are in excellent agreement with interferometric measurements. The deflection technique is superior to an imaging interferometer in terms of simplicity and sensitivity. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A series of underdense laser plasma interaction experiments performed on the Helen laser [M. J. Norman et al., Appl. Opt. 41, 3497 (2002)] at the Atomic Weapons Establishment (AWE), U.K., using 2ω light have uncovered a strong dependence of laser backscatter and hot electron production on plasma composition. Using low-Z materials, we find a behavior familiar from previous 3ω work, the interchange of stimulated Raman scattering for Brillouin scattering as we change from gases that have high ion wave damping (e.g., C5H12) to gases with low ion wave damping (e.g., CO2). However, as Z is increased, we find that Brillouin scattering drops while Raman scattering remains low. For gases with Z greater than 18, it is possible to have long scalelength, underdense plasmas with both low Brillouin and Raman backscatter losses. Complementary measurements of hot electron production show efficient production of hot electrons in C5H12 plasmas approaching 0.25ncr, but changing the plasma composition can greatly suppress the hot electron production, even near 0.25ncr. Additional experiments indicate that by adding small amounts of high Z dopant, significant changes to the backscatter and hot electron production in cr targets may be produced. [ABSTRACT FROM AUTHOR]

A scalable setup using injection by frequency conversion to establish a multipassing cavity for noncollective Thomson scattering on low density plasmas is presented. The cavity is shown to support >10 passes through the target volume with a 400% increase in energy on target versus a single-pass setup. Rayleigh scattering experiments were performed and demonstrate the viability of the cell to study low density plasmas of the order of 1012-1013 cm-3. A high-repetition, low-energy, single-pass Thomson scattering setup was also performed on the University of California, Los Angeles Large Plasma Device and shows that the multipass cavity could have a significant advantage over the high-repetition approach due to the cavity setup's inherently higher signal per shot. [ABSTRACT FROM AUTHOR]

Two-dimensional monochromatic images of fast-electron stimulated Ar Kα and He-α x-ray self-emission have recorded a time-integrated map of the extent of Ar≈6+ and Ar16+ ions, respectively, within a high density (1020 cm-3 atomic density) Ar plasma. This plasma was produced by irradiating a 2 mm wide clustering Ar gas jet with an ultrahigh intensity (1019 W/cm2, 50 TW) Ti:sapphire laser operating at 800 nm. Spherically bent quartz crystals in the 200 (for Kα) and 201 (for He-α) planes were used as near-normal incidence reflective x-ray optics. We see that a large (830 μm long) region of plasma emits Kα primarily along the laser axis, while the He-α emission is confined to smaller hot spot (230 μm long) region that likely corresponds to the focal volume of the f/8 laser beam. X-ray spectra from a Bragg spectrometer operating in the von Hamos geometry indicate that the centroids of the Kα and He-α emission regions are separated by approximately 330 μm along the laser axis. [ABSTRACT FROM AUTHOR]

We used Kr Kα (12.6 keV), Zr Kα (15.7 keV), and Ag Kα (22.2 keV) x-rays, produced by petawatt-class laser pulses, to measure the integrated crystal reflectivity Rint of flat highly oriented pyrolytic graphite (HOPG) up to the fifth order. The maximum Rint was observed in first order (3.7 mrad at 12.6 keV), decreasing by a factor of 3–5 for every successive order, and dropping by a factor of 2–2.5 at 22.2 keV. The current study indicates that HOPG crystals are suitable for measuring scattering signals from high energy x-ray sources (E≥20 keV). These energies are required to penetrate through the high density plasma conditions encountered in inertial confinement fusion capsule implosions on the National Ignition Facility. [ABSTRACT FROM AUTHOR]

A high contrast 12.6 keV Kr Kα source has been demonstrated on the petawatt-class Titan laser facility using strongly clustering Kr gas jet targets. The contrast ratio (Kα to continuum) is 65, with a competitive ultrashort pulse laser to x-ray conversion efficiency of 10-5. Filtered shadowgraphy indicates that the Kr Kα and Kβ x rays are emitted from a roughly 1×2 mm2 emission volume, making this source suitable for area backlighting and scattering. Spectral calculations indicate a typical bulk electron temperature of 50–70 eV (i.e., mean ionization state 13–16), based on the observed ratio of Kα to Kβ. Kr gas jets provide a debris-free high energy Kα source for time-resolved diagnosis of dense matter. [ABSTRACT FROM AUTHOR]

The absolute laser conversion efficiency to Kα-like inner shell x-rays (integrated from Kα to Kβ) is observed to be an order of magnitude higher in argon gas jets than in solid targets due to enhanced emission from higher ionization stages following ultrashort pulse laser irradiation. Particle-in-cell and spectral simulations indicate that these observations are consistent with Kα emission from a warm Ar plasma subject to hot electron inner-shell ionization. These results demonstrate that gas jet targets are bright, high conversion efficiency, high repetition rate, debris-free multi-keV x-ray sources for spectrally resolved scattering and backlighting of rapidly evolving dense matter. [ABSTRACT FROM AUTHOR]

Ultralow density 1 mg/cc aerogel foam reduces the thermal conductivity of pure helium gas by about a factor of 3 at 1 Torr and 20 K, with smaller reductions at higher temperatures and pressures. The measurements were made using the dc hot wire method over temperatures ranging between 15 and 300 K and pressures ranging from 0.05 to 100 Torr. The results are generally consistent with a simple model that accounts for a reduction in the He mean free path due to the foam. [ABSTRACT FROM AUTHOR]

We have developed and commissioned a transmitted beam diagnostic (TBD) for the 2ω high intensity interaction beam at the Omega laser facility. The TBD consists of a bare-surface reflector mounted near the target, which collects and reflects 4% of the transmitted light to a detector assembly outside the vacuum chamber. The detector includes a time integrating near-field camera that measures beam spray, deflection, and the absolute transmitted power. We present a detailed description of the instrument and the calibration method and include first measurements on laser heated gas bag targets to demonstrate the performance of the diagnostic. [ABSTRACT FROM AUTHOR]

The full aperture backscatter system provides a measure of the spectral power, and integrated energy scattered by stimulated Brillouin (348–354 nm) and Raman (400–800 nm) scattering into the final focusing lens of the first four beams of the NIF laser. The system was designed to provide measurements at the highest expected fluences with: (1) spectral and temporal resolution, (2) beam aperture averaging, and (3) near-field imaging. This is accomplished with a strongly attenuating diffusive fiber coupler and streaked spectrometer and separate calibrated time integrated spectrometers, and imaging cameras. A new technique determines the wavelength dependent sensitivity of the complete system with a calibrated Xe lamp. Data from the calibration system are combined with scattering data from targets to produce the calibrated power and energy measurements that show significant corrections due to the broad band calibrations. [ABSTRACT FROM AUTHOR]

Collisionless coupling between super Alfvénic ions and an ambient plasma parallel to a background magnetic field is mediated by a set of electromagnetic ion/ion-beam instabilities including the resonant right hand instability (RHI). To study this coupling and its role in parallel shock formation, a new experimental configuration at the University of California, Los Angeles utilizes high-energy and high-repetition-rate lasers to create a super-Alfvénic field-aligned debris plasma within an ambient plasma in the Large Plasma Device. We used a time-resolved fluorescence monochromator and an array of Langmuir probes to characterize the laser plasma velocity distribution and density. The debris ions were observed to be sufficiently super-Alfvénic and dense to excite the RHI. Measurements with magnetic flux probes exhibited a right-hand circularly polarized frequency chirp consistent with the excitation of the RHI near the laser target. We compared measurements to 2D hybrid simulations of the experiment. [ABSTRACT FROM AUTHOR]

The dynamics of an exploding laser-produced plasma in a large ambient magneto-plasma was investigated with magnetic flux probes and Langmuir probes. Debris-ions expanding at super-Alfvénic velocity (up to MA=1.5) expel the ambient magnetic field, creating a large (>20 cm) diamagnetic cavity. We observe a field compression of up to B/B0=1.5 as well as localized electron heating at the edge of the bubble. Two-dimensional hybrid simulations reproduce these measurements well and show that the majority of the ambient ions are energized by the magnetic piston and swept outside the bubble volume. Nonlinear shear-Alfvén waves (δB/B0>25%) are radiated from the cavity with a coupling efficiency of 70% from magnetic energy in the bubble to the wave. [ABSTRACT FROM AUTHOR]

Paramagnetic Faraday rotator glass (rare-earth doped borosilicate) with a high Verdet constant will be used to measure the magnetic field inside of low density Helium plasmas (Te ∼ 5 eV, Ti ∼ 1 eV) with a density of n ∼ 1012 cm-3. Linearly polarized light is sent through the glass such that the plane of polarization is rotated by an angle that depends on the strength of the magnetic field in the direction of propagation and the length of the crystal (6 mm). The light is then passed into an analyzer and photo-detector setup to determine the change in polarization angle. This setup can detect magnetic fields up to 5 kG with a resolution of <5 G and a temporal resolution on the order of a nanosecond. The diagnostic will be used to characterize the structure and evolution of laser-driven collisionless shocks in large magnetized plasmas. [ABSTRACT FROM AUTHOR]

Planar laser induced fluorescence (PLIF) imaging can potentially assess ion distributions and coupling in the context of super-Alfvénic ablation plasma expansions into magnetized background plasmas. In this feasibility study, we consider the application of PLIF to rapidly expanding carbon plasmas generated via energetic laser ablation of graphite. By utilizing hydrodynamic and collisional-radiative simulations, we identify schemes accessible to commercially available tunable lasers for the C I atom, the C II ion, and the C V ion. We then estimate the signal-to-noise ratios yielded by the schemes under reasonable experimental configurations. [ABSTRACT FROM AUTHOR]

Laser-produced plasmas are often used as bright x-ray backlighters for time-resolved plasma diagnostics, but such backlighters simultaneously generate damaging electromagnetic pulse (EMP). A laser-driven Ar gas jet x-ray source has been measured with magnetic flux B-dot probes to produce 20 times ±37% less integrated EMP in the 0.5-2.5 GHz band than a solid chlorinated plastic foil, while retaining 85% of the laser to ≈3 keV x-ray conversion efficiency. These results are important for future backlighter development, since tailoring target density may provide a way to reduce EMP even as laser power increases. [ABSTRACT FROM AUTHOR]

We present experiments on the Trident laser facility at Los Alamos National Laboratory which demonstrate key elements in the production of laser-driven, magnetized, laboratory-scaled astrophysical collisionless shocks. These include the creation of a novel magnetic piston to couple laser energy to a background plasma and the generation of a collisionless shock precursor. We also observe evidence of decoupling between a laser-driven fast ion population and a background plasma, in contrast to the coupling of laser-ablated slow ions with background ions through the magnetic piston. 2D hybrid simulations further support these developments and show the coupling of the slow to ambient ions, the formation of a magnetic and density compression pulses consistent with a collisionless shock, and the decoupling of the fast ions. [ABSTRACT FROM AUTHOR]

A high contrast 12.6 keV Kr K alpha source has been demonstrated on the petawatt-class Titan laser facility using strongly clustering Kr gas jet targets. The contrast ratio (K alpha to continuum) is 65, with a competitive ultrashort pulse laser to x-ray conversion efficiency of 10(-5). Filtered shadowgraphy indicates that the Kr K alpha and K beta x rays are emitted from a roughly 1x2 mm(2) emission volume, making this source suitable for area backlighting and scattering. Spectral calculations indicate a typical bulk electron temperature of 50-70 eV (i.e., mean ionization state 13-16), based on the observed ratio of K alpha to K beta. Kr gas jets provide a debris-free high energy K alpha source for time-resolved diagnosis of dense matter.