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Efficient generation of 13.5 nm light with increased conversion efficiency and output power is important for Extreme Ultraviolet (EUV) lithography applications. In this study, we present a computational investigation of plasma dynamics and EUV generation from laser-driven plasma, with specific focus on the influence of magnetic fields, ranging up to 50 T. Simulations show that the plasma expansion is restricted based on the direction and strength of the magnetic field, resulting in an anisotropic plasma confinement, which in turn allows for radiation escape with a reduced loss. Moreover, angle-dependent measurements show an increase in in-band EUV (2% bandwidth around 13.5 nm) yield, reaching a peak enhancement of up to 40% when a magnetic field is applied, particularly when it is oriented perpendicular to the laser axis. The ability to control plasma dynamics by magnetic field offers exciting prospects for optimizing EUV radiation sources. [ABSTRACT FROM AUTHOR]

Studies a two kJ, 200 kA, table top plasma focus device as an intense x-ray source. Measurement of the x-ray yield from gases as a function of filling pressure and in neon as a function of anode length; Temperature estimated from spectroscopic observations.

A study on the neutron production from single and double gas puff Z-pinches on the CESZAR linear transformer driver with ∼0.45 MA current and 170 ns rise time is presented. Total neutron yield measurements made with a LaBr activation detector are compared for three configurations, using a double nozzle setup. When a single, hollow, deuterium gas shell was used, reliable implosions could only be attained at higher load mass than the optimal value to match implosion time with the driver rise time, with neutron yields of ∼106 per pulse. The use of a double gas puff configuration with a deuterium center jet allowed a reduction in the shell density and operation closer to machine-matched conditions, recording up to (4.1 ± 0.3) × 107 neutrons/pulse when either Kr or D2 was used in the shell. For a comparable mass and implosion time, using a higher atomic-number gas in the outer shell results in more unstable plasma surface and smaller plasma radius at the location of instability bubbles, which, however, do not seem to consistently correlate with a higher neutron yield. Comparing implosion dynamics with models and neutron yields with literature scaling suggests that the machine current is not well coupled to the plasma during the final stages of compression. Optimizing current and energy coupling to the pinched plasma is critical to improving performance, particularly in low-impedance drivers. [ABSTRACT FROM AUTHOR]

Measurement of the temporal evolution of laser-produced plasma temperature is very important for many of its applications, and several plasma diagnostic tools are routinely used by researchers. However, it is very challenging to measure the properties of the plasma at the early and late times of its evolution using a single diagnostic tool. In this study, we combined emission and laser absorption spectroscopy to compare the excitation temperatures of a laser-produced uranium plasma system. Several U I transitions in the near-infrared spectral range (775–800 nm) were considered, and the Boltzmann plot method was used to measure the excitation temperatures using both emission and absorption spectroscopy. Emission spectroscopy provided early-time temperature measurements of the plasma up to times 2–20 µs, while absorption spectroscopy provided temperature measurements at late times of plasma evolution (for times 5–80 µs). The emission and absorbance of U I transitions were found to follow the Boltzmann distribution, indicating the plasma is likely in the state of local thermodynamic equilibrium even at late times of its lifetime. The emission and absorption-based time-resolved excitation temperatures demonstrated good agreement at earlier times (≤15 µs) in the overlapped temporal region, while a deviation in the measured values was seen at times (≥15 µs), and potential reasons for such a disagreement are discussed. [ABSTRACT FROM AUTHOR]

A review is presented here of a number of invited papers presented at the 2008 American Physical Society April meeting [held jointly with High Energy Density Physics/High Energy Density Laboratory Astrophysics (HEDP/HEDLA) Conference] devoted to intense laser-matter interactions. They include new insights gained from wave-kinetic theory into laser-wakefield accelerators and drift wave turbulence interacting with zonal flows in magnetized plasmas; interactions with cluster media for the generation of radiative blast waves; fast electron energy transport in cone-wire targets; numerical investigations into Weibel instability in electron-positron-ion plasmas and the generation of gigabar pressures with thin foil interactions. [ABSTRACT FROM AUTHOR]

Observations of X-pinch discharges driven by a 160-kA 80-ns pulse current are reported. X-pinches consisting of aluminum and molybdenum mounted at different angles (120° and 83°) were studied. Coronal-plasma streaming, which is perpendicular to the wires toward the midplane of the X-pinch, was observed to form jets moving toward the electrodes on the either side of the cross point. The jet velocity (2.5 × 106 cm · s-1) is similar for both molybdenum and aluminum wires and is slightly larger for the narrow angle (83°). The jets were observed to show kink (m = 1 mode) instability at late times, after the jets had come into contact with the electrodes. Tiffs is due to a transfer of current to the jet. The experimental results agree qualitatively with three-dimensional resistive MHD simulations, which include radiation. The jets produced in X-pinches may be relevant to the study of astrophysical jets. [ABSTRACT FROM AUTHOR]

Recent experiments using Terawatt lasers to accelerate protons deposited on thin wire targets are modeled with a new type of gridless plasma simulation code. In contrast to conventional mesh-based methods, this technique offers a unique capability in emulating the complex geometry and open-ended boundary conditions characteristic of contemporary experimental conditions. Comparisons of ion acceleration are made between the tree code and standard particle-in-cell simulations for a typical collisionless “hole boring” scenario in slab geometry. The utility of the gridless approach is emphasized by a series of simulations in “wire” geometry, in which electrons are permitted to circulate around the target at arbitrary distances from the focal region. The simulations reveal a number of features in common with recent experimental observations, including a disclike emission pattern of the MeV protons accelerated away from the wire. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Results are presented from laser-plasma interaction experiments using the VULCAN laser at the Rutherford Appleton Laboratory. Wire targets were used to elucidate the role of the return currents generated by the relativistic electron beam leaving the target at laser intensities up to 5×1019W cm². For some shots an additional wire or a foil was placed near the target wire. In other shots, a foil was used as the target with a wire behind. Three main observations were made: (i) Z-pinch behavior in the wires due to the return currents, (ii) optical transition radiation (OTR) at the second harmonic of the laser, and (iii) proton emission. The OTR and the proton emission were observed from both the primary wire target and the adjacent wire. The OTR emission is associated with electron bunches at twice the laser frequency due to ponderomotive J×B acceleration by the laser. The proton emission from the adjacent target was likely due to field emission of electrons by the large potential produced from charging of the primary wire target. The observations agree with simulations using the three-dimensional tree code PEPC and the two-and-one-half-dimensional particle-in-cell code OSIRIS. [ABSTRACT FROM AUTHOR]

The effect of discrete wires on the implosion dynamics of wire array Z-pinch experiments at ∼1 MA current level is discussed. The data show that the formation of a core–corona structure leads to gradual radial redistribution of mass by precursor plasma flow from the stationary wire cores during the first ∼80% of the implosion time. This phase ends with the formation of gaps in the wire cores, which occurs due to the nonuniformity of ablation rate along the wires. The final phase of the implosion starting at this time occurs as a rapid snowplow-like implosion of the plasma, previously injected into the interior of the array. The density distribution of the precursor plasma being peaked on the array axis could be a key factor providing stability of the wire array implosions operating in the regime of discrete wires. The implications of this implosion scenario to the operation of nested wire arrays and foam targets on the array axis are also discussed. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports the experiments on the implosion dynamics of single and nested nickel wire arrays driven by the MAGPIE generator with a 1-MA, 240-ns current pulse. Experimental setup and diagnostics; Discussion of implosion trajectory; Information on precursor plasma.

Analyzes Z-pinch plasma discharges in aluminum and tungsten wires. Expansion of the coronal plasma of various diameters of aluminum and tungsten wires; Persistence of short wavelength perturbations for a longer time for larger diameter tungsten wires; Decay time of bright spots.

The ion emission properties of laser-produced plasmas as a function of laser intensities between 4–50 GW cm − 2 and varying angles with respect to the target normal were investigated. The plasmas were produced by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser on various metal targets. The targets used for this study include Ti, Mo, and Gd (Z = 22 , 42 , 64). It is noted that all ion profiles are composed of multiple peaks—a prompt emission peak trailed by three ion peaks (ultrafast, fast, and thermal). Experimentally, it is shown that each of these ion peaks follows a unique trend as a function of laser intensity, angle, and distance away from the target. Theoretically, it is shown that simple analytical models can be used to explain the properties of the ions. The variations in the ion velocity and density as a function of laser intensity are found to be in good agreement with theoretical models of sheath acceleration, isothermal self-similar expansion, and ablative plasma flow for various ion peaks. [ABSTRACT FROM AUTHOR]

Three independent methods (extreme ultraviolet spectroscopy, imaging at 68 and 256 eV) have been used to measure planar target rear surface plasma temperature due to heating by hot electrons. The hot electrons are produced by ultraintense laser-plasma interactions using the 150 J, 0.5 ps Titan laser. Soft x-ray spectroscopy in the 50–400 eV region and imaging at the 68 and 256 eV photon energies give a planar deuterated carbon target rear surface pre-expansion temperature in the 125–150 eV range, with the rear plasma plume averaging a temperature approximately 74 eV. [ABSTRACT FROM AUTHOR]

Results from experiments performed to characterize plastic capsules containing foam layers are presented. A compact X-pinch pulser with a footprint <1 m2 having a peak current of 80 kA and a rise time of 50 ns was used. Various wire materials including tungsten, molybdenum, and aluminum were employed. Results with plastic capsules (1 mm diameter, 20 μm thick wall with 80 μm foam inside the capsule) show phase contrast effects at the edges of the wall due to the foam, which mimics the ice inside the shell. The sharpness of the image reveals a source less than 2 μm in size and x-ray diodes show a pulse length of ∼10 ns. The small source size allows high-resolution phase contrast imaging of capsules. The x-ray pulse from an X-pinch is sufficiently short to avoid the motional blurring due to cryogenic system vibrations, which is not possible with low flux sources. [ABSTRACT FROM AUTHOR]

We report results from experiments performed at the Rutherford Appleton Laboratory using the VULCAN laser facility (I>5×1019 W cm-2). Single wire targets were used, and on some shots additional objects were placed near the target. These were positioned so that they were not irradiated by the laser. Proton emission from single wire targets was observed as radially symmetric structures (“stripes”) in both the forward and backward directions, and was due to plasma sheath acceleration around the wire. The presence of objects in the vicinity of the interaction had a significant effect on the angular emission pattern of protons from the primary target. Importantly, the secondary object was also observed to be a source of energetic proton emission. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurements of the optical and x-ray emission from a small 40-kA, 30-ns (10%-90%) rise-time X-pinch plasma discharge are reported. This device is truly "table-top," having a laboratory footprint of less than 1 m[SUP2]. With tungsten wires, the total energy (hν>800 eV) is 60 mJ in a pulse of less than 10 ns. The size of the emitting region is less than 10 μm. The emission is reproducible with a jitter relative to the current pulse less than 5 ns. Initial experiments demonstrating the use of this X-pinch for applications in x-ray radiography are presented. [ABSTRACT FROM AUTHOR]

A table-top plasma focus device is shown to be an ideal neutron source for the calibration and characterization of dark matter detectors and has been optimized to produce a maximum yield of 2.0×10[sup 7] neutrons per shot. The interaction of energetic neutrons is similar to that expected from weakly interacting massive particles (WIMPs)—a favored candidate for the dominant component of dark matter in the universe. The weak interaction of a neutron with liquid xenon gas was measured in a prototype xenon two-phase detector. We have developed a detector system in which both the primary scintillation and ionization from the initial interaction can be detected. Both measurements are critical for identifying WIMP’s. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of a prepulse current on the behavior of wire array Z pinches is investigated. The experiments were performed on the MAGPIE generator (1 MA peak current at 240 ns) [I. Mitchell, J. M. Bayley, J. P. Chittenden et al., Rev. Sci. Instrum. 67, 1533 (1996)]. A linear ramp current, ∼500-ns long, was used as a prepulse. The array consisted of 32 15-μm aluminum wires, 23-mm long arranged in a 16-mm diameter circle. With a prepulse of 1 kA/wire, a low density precursor plasma column (n[sub e]∼2×10[sup 17] cm[sup -3]) is formed on the array axis before the start of the main current. Later, the soft x-ray emission shows the growth of an m=1 helical instability in the precursor plasma, which indicates the presence of a current. Without a prepulse current, the precursor plasma on axis is uniform and does not show any instability. The x-ray pulse at stagnation on axis is at least 30 times smaller with a prepulse current than without. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of wire arrays have been studied using a point-projection X-pinch x-ray backlighter installed in one of the return posts of the MAGPIE generator. Variations of diameter (15–50 μm aluminum) and number of wires (two or four) in the X-pinch enabled backlit images in the range of 140–200 ns after the current start. A temporal and spatial resolution of <1 ns and 5 μm is achieved. The radiographic images of aluminum wire array show that the wire cores are present at the original position until 80% of the implosion time and the size of the wire cores is 0.25 mm for aluminum and 0.1 mm for tungsten. A very fine structure of the order of 10 μm has been observed in titanium wire arrays. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Relativistically intense laser light interacting with solid density targets can accelerate protons to multi-MeV energies via the target normal sheath acceleration process. The use of hollow hemisphere targets with a hollow conical region to focus protons of selected energies, for applications such as isochoric heating of matter and for the fast ignition approach to inertial confinement fusion, is explored for laser intensity ∼ 10 20 Wcm−2. Specifically, the effects of having the cone tip open or closed is investigated experimentally and via a programme of scaled particle-in-cell simulations. The open cone configuration is found to result in proton focusing in the energy range of 9 to 24 MeV, and produce an annular profile for higher energy components, up to 55 MeV, while the spatial distribution of lower energy components remains unchanged. By contrast, for the closed cone case, the focusing effect is diminished by the fields present on the inner wall of the cone tip. Simulations reveal that strong electrostatic and magnetic fields present on the inner surfaces of the target induce the focusing effect with the open cone, but also result in proton divergence in the case of the closed cone. Additionally, the simulations demonstrate the possibility to tailor the cone geometry to select the energy range over which the focusing occurs. [ABSTRACT FROM AUTHOR]

We demonstrate that laser reflection acts as a catalyst for superponderomotive electron production in the preplasma formed by relativistic multipicosecond lasers incident on solid density targets. In 1D particle-in-cell simulations, high energy electron production proceeds via two stages of direct laser acceleration: an initial stochastic backward stage and a final nonstochastic forward stage. The initial stochastic stage, driven by the reflected laser pulse, provides the preacceleration needed to enable the final stage to be nonstochastic. Energy gain in the electrostatic potential, which has been frequently considered to enhance stochastic heating, is only of secondary importance. The mechanism underlying the production of high energy electrons by laser pulses incident on solid density targets is of direct relevance to applications involving multipicosecond laser-plasma interactions. [ABSTRACT FROM AUTHOR]

We investigate the ablation of silicon as a function of laser pulse length at a constant fluence using time-resolved x-ray spectroscopy data obtained from OMEGA EP experiments at the University of Rochester's Laboratory for Laser Energetics. Our targets consisted of three-layer planar structures composed of Si (50 μm), Cu (25 μm), and SiO2 (500 μm) layers. The Si layer was irradiated by a 351-nm laser with varying pulse widths of 250 ps, 500 ps, 1 ns, and 10 ns while maintaining a constant fluence of ∼27.9 kJ/cm2. Electron temperatures and densities of the ablated plasma were determined by analyzing the time-resolved x-ray spectroscopy data through a comparison of experimental measurements with synthetic results obtained from Si atomic calculations in a steady state and non-local thermodynamic equilibrium. These calculations were computed using PrismSPECT [MacFarlane et al., High Energy Density Phys. 3, 181 (2007)]. Additionally, radiation-hydrodynamics simulations with FLASH are used to generate simulated plasma-density and plasma-temperature profiles, which are then compared with the experimental measurements. Our analyses reveal that increasing the laser pulse length at a constant fluence results in a decrease in electron temperatures and densities. Furthermore, the longer pulses with lower intensities lead to deeper ablation regions before reaching the peak ablation but lower ionization balances in the silicon layer. These findings emphasize the critical role of laser pulse length in plasma ablation and shock generation for laser-impulse studies. [ABSTRACT FROM AUTHOR]

Gas puff Z-pinch experiments with annular Ar and Ne gas shells have been conducted on the Compact Experimental System for Z-pinch and Ablation Research (CESZAR) linear transformer driver (LTD) with 500 kA current and 160 ns rise time. Here, we present results from the first systematic gas puff Z-pinch experiments using a fast (≤ 200 ns) LTD as a driver, in which we show that 7% of the stored energy in the capacitors is coupled to plasma kinetic energy as estimated via self-emission and laser schlieren images. 0D and 1D simulations—which do not allow instability growth and thus reach greater maximum average velocities—using initial conditions inferred from experimental implosion trajectories predict coupling in excess of 10% of the stored energy. The Ar and Ne implosions were comparably massed and thus achieved similar maximum kinetic energies, though the Ne pinches were more stable and the x-ray pulses were longer and produced higher yield: 2–5 ns and 0.21–0.52 J (0.15–0.37 J/cm) of Ar K-shell and 12–25 ns and 2.2–3.9 J (1.6–2.6 J/cm) of Ne K-shell, respectively. The difference in stability is most likely attributed to variations in initial conditions such as density distribution and gas breakdown initiation. [ABSTRACT FROM AUTHOR]

The dense plasma focus (DPF) can be an intense source of x rays, wherein the insulator sleeve strongly dictates the electrical breakdown, which subsequently affects the formation of a plasma sheath and a collapse phase. Experiments on a 25 kJ DPF (operated at 4.4 kJ) are carried out to demonstrate the influence of insulator surface morphology on the pinch structure, dynamics, and x-ray yield using a Ne fill. Two borosilicate insulators are directly compared, one with a smooth finish and the other machined with four circumferential grooves traversing the perimeter of the exterior insulator surface. Comparisons are made through same-shot imaging diagnostics of the evolving plasma sheath during breakdown, rundown, and at the pinch in addition to the time-resolved measurements of emitted x rays via filtered photodiodes. The presence of structures on the insulator sleeve reduces x-ray production across all fill pressures by a factor of 2.8 ± 2.4 on average and reduces the highest x ray producing shots by a factor of 5.5 ± 1.8. Observations of sheath asymmetry and inhomogeneity at lift-off are observed and correlated with subsequent observations of off-axis radial collapse. Taken together, this suggests that local variations in the insulator surface decrease the spatial uniformity of the sheath, leading to an azimuthally asymmetric focus, reduced electron densities, and, ultimately, degraded x-ray production. [ABSTRACT FROM AUTHOR]

X-pinches are one of the most localized sources of X-rays and are potentially useful for a number of applications. An investigation was carried out into the plasma dynamics of a "tabletop" X-pinch plasma generated by passing a 40-ns 35-kA current pulse through two or four 5-µm-thick tungsten wires. It was found that the number of wires significantly changes the dynamics of the discharge, as evidenced by images of the emission in the visible spectral region. It was also found that increasing the number of wires enhanced the reproducibility of the discharge and the X-ray pulse. [ABSTRACT FROM AUTHOR]

Wire X-pinches (WXPs) have been studied comprehensively as fast (∼ 1 ns pulse width), small (∼ 1 μ m) x-ray sources, created by twisting two or more fine wires into an "X" to produce a localized region of extreme magnetic pressure at the cross-point. Recently, two alternatives to the traditional WXP have arisen: the hybrid X-pinch (HXP), composed of two conical electrodes bridged by a thin wire or capillary, and the laser-cut foil X-pinch (LCXP), cut from a thin foil using a laser. We present a comparison of copper wire, hybrid, and laser-cut foil X-pinches on a single experimental platform: UC San Diego's ∼ 200 kA, 150 ns rise time GenASIS driver. All configurations produced 1–2 ns pulse width, ≤ 5 μ m soft x-ray (Cu L-shell, ∼ 1 keV) sources (resolutions diagnostically limited) with comparable fluxes. WXP results varied with linear mass and wire count, but consistently showed separate pinch and electron-beam-driven sources. LCXPs produced the brightest (∼ 1 MW), smallest (≤ 5 μ m) Cu K-shell sources, and spectroscopic data showed both H-like Cu K α lines indicative of source temperatures ≥ 2 keV, and cold K α (∼ 8050 eV) characteristic of electron beam generated sources, which were not separately resolved on other diagnostics (within 1–2 ns and ≤ 200 μ m). HXPs produced minimal K-shell emission and reliably single, bright, and small L-shell sources after modifications to shape the early current pulse through them. Benefits and drawbacks for each configuration are discussed to provide potential X-pinch users with the information required to choose the configuration best suited to their needs. [ABSTRACT FROM AUTHOR]

Experiments on a MA-class Dense Plasma Focus (DPF) device have been carried out to investigate changes in neutron production by adding moderate amounts of krypton to a deuterium fill gas. The neutron yield from Z-pinch devices, including DPFs, conventionally scales as the peak current to the fourth power. However, a dramatic drop-off from ∼I4 scaling occurs above 3 MA, which recent modeling [D. T. Offermann et al., Phys. Rev. Lett. 116, 195001 (2016)] attributed to the transition in the predominant neutron production mechanism from beam-target fusion to thermonuclear fusion. Previously, the addition of Kr (and other high-Z) dopants has been shown to enhance beam-target fusion yields at currents below 300 kA, with optimal concentrations at 1%–2% Kr, whereas here we show that the optimal concentration of Kr at the MA level is near 0.1% by volume—elucidating a trend in the optimal Kr doping concentration as a function of the device scale. The neutron time-of-flight data reveal that Kr doping creates shorter and more intense neutron bursts, likely due a tighter but unstable pinch, highlighting a key trade-off for Kr doping. [ABSTRACT FROM AUTHOR]

Fast electron transport and spatial energy deposition are investigated in integrated cone-guided Fast Ignition experiments by measuring fast electron induced copper K-shell emission using a copper tracer added to deuterated plastic shells with a geometrically reentrant gold cone. Experiments were carried out at the Laboratory for Laser Energetics on the OMEGA/OMEGA-EP Laser where the plastic shells were imploded using 54 of the 60 OMEGA60 beams (3ω, 20 kJ), while the high intensity OMEGA-EP (BL2) beam (1 ω, 10 ps, 500 J, Ipeak > 1019 W/cm²) was focused onto the inner cone tip. A retrograde analysis using the hybrid-PIC electron transport code, ZUMA, is performed to examine the sensitivity of the copper Kα spatial profile on the laser-produced fast electrons, facilitating the optimization of new target point designs and laser configurations to improve the compressed core areal density by a factor of 4 and the fast electron energy coupling by a factor of 3.5. [ABSTRACT FROM AUTHOR]

Azimuthal magnetic field measurements obtained during the implosion phase of an oxygen gas-puff Z-pinch on a 500 kA peak current and 180 ns rise time linear transformer driver are presented. While a fraction of the driver current was measured within the imploding plasma, key initial conditions were found to significantly impact the delivery of current to the plasma load. The electrode geometry was modified to assist the initial dielectric breakdown and resulted in improved shot reproducibility. Optimization of the gas injection plenum pressure and timing resulted in an increase in the current coupling parameter, defined as the ratio of the measured value of B θ to the expected value, from 50% to 75%. The degree of radial expansion of the gas puff in the load region, which is suspected to lead to the observed current loss during the implosion, was reduced by shortening the valve opening duration. Additionally, a pre-embedded axial magnetic field of up to 0.2 T was found to have no significant impact on the plasma-current coupling of the oxygen implosions. [ABSTRACT FROM AUTHOR]

A phenomenological model of wire array Z-pinch implosions, based on the analysis of experimental data obtained on the mega-ampere generator for plasma implosion experiments (MAGPIE) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)], is described. The data show that during the first ∼80% of the implosion the wire cores remain stationary in their initial positions, while the coronal plasma is continuously jetting from the wire cores to the array axis. This phase ends by the formation of gaps in the wire cores, which occurs due to the nonuniformity of the ablation rate along the wires. The final phase of the implosion starting at this time occurs as a rapid snowplow-like implosion of the radially distributed precursor plasma, previously injected in the interior of the array. The density distribution of the precursor plasma, being peaked on the array axis, could be a key factor providing stability of the wire array implosions operating in the regime of discrete wires. The modified “initial” conditions for simulations of wire array Z-pinch implosions with one-dimension (1D) and two-dimensions (2D) in the r–z plane, radiation-magnetohydrodynamic (MHD) codes, and a possible scaling to a larger drive current are discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Talbot–Lau x-ray interferometry is a refraction-based diagnostic that can map electron density gradients through phase-contrast methods. The Talbot–Lau x-ray deflectometry (TXD) diagnostics have been deployed in several high energy density experiments. To improve diagnostic performance, a monochromatic TXD was implemented on the Multi-Tera Watt (MTW) laser using 8 keV multilayer mirrors (Δθ/θ = 4.5%-5.6%). Copper foil and wire targets were irradiated at 1014–1015 W/cm2. Laser pulse length (∼10 to 80 ps) and backlighter target configurations were explored in the context of Moiré fringe contrast and spatial resolution. Foil and wire targets delivered increased contrast <30%. The best spatial resolution (<6 μm) was measured for foils irradiated 80° from the surface. Further TXD diagnostic capability enhancement was achieved through the development of advanced data postprocessing tools. The Talbot Interferometry Analysis (TIA) code enabled x-ray refraction measurements from the MTW monochromatic TXD. Additionally, phase, attenuation, and dark-field maps of an ablating x-pinch load were retrieved through TXD. The images show a dense wire core of ∼60 μm diameter surrounded by low-density material of ∼40 μm thickness with an outer diameter ratio of ∼2.3. Attenuation at 8 keV was measured at ∼20% for the dense core and ∼10% for the low-density material. Instrumental and experimental limitations for monochromatic TXD diagnostics are presented. Enhanced postprocessing capabilities enabled by TIA are demonstrated in the context of high-intensity laser and pulsed power experimental data analysis. Significant advances in TXD diagnostic capabilities are presented. These results inform future diagnostic technique upgrades that will improve the accuracy of plasma characterization through TXD. [ABSTRACT FROM AUTHOR]

Magnetization of inertial confinement implosions is a promising means of improving their performance, owing to the potential reduction of energy losses within the target and mitigation of hydrodynamic instabilities. In particular, cylindrical implosions are useful for studying the influence of a magnetic field, thanks to their axial symmetry. Here, we present experimental results from cylindrical implosions on the OMEGA-60 laser using a 40-beam, 14.5 kJ, 1.5 ns drive and an initial seed magnetic field of B0 = 30 T along the axes of the targets, compared with reference results without an imposed B-field. Implosions were characterized using time-resolved x-ray imaging from two orthogonal lines of sight. We found that the data agree well with magnetohydrodynamic simulations, once radiation transport within the imploding plasma is considered. We show that for a correct interpretation of the data in these types of experiments, explicit radiation transport must be taken into account. [ABSTRACT FROM AUTHOR]

Current and future applications of intense proton sources abound, including radiography, cancer therapy, warm dense matter generation, and inertial confinement fusion. With increasingly efficient acceleration and focusing mechanisms, proton current densities may soon approach and exceed 10 10 A / cm 2 , e.g., via intense laser drivers. Simulations have previously shown that in this current density regime, beam-induced field generation plays a significant role in beam transport through dense plasmas. Here, we present a theoretical model for the generation of resistive magnetic fields by intense proton beam transport through solid density plasmas. The theoretical evolution of the magnetic field profile is calculated using an analytic model for aluminum resistivity, heat capacity, and stopping power, applicable from cold matter to hot plasma. The effects of various beam and material parameters on the field are investigated and explained for both monoenergetic and Maxwellian proton beams. For a proton beam with Maxwellian temperature 5 MeV and total energy 10 J, the model calculates resistive magnetic fields up to 150 T in aluminum. The calculated field profiles from several beam cases are compared with 2D hybrid particle-in-cell simulations, with good agreement found in magnitude and time scale. [ABSTRACT FROM AUTHOR]

In this study, we present the absolute calibration of the conical crystal for the zinc spectrometer (ZSPEC), an x-ray spectrometer at the OMEGA laser facility at the Laboratory for Laser Energetics. The ZSPEC was originally designed to measure x-ray Thomson scattering using flat or cylindrically curved highly oriented pyrolytic graphite crystals centered around Zn He-alpha emission at 9 keV. To improve the useful spectral range and collection efficiency of the ZSPEC, a conical highly annealed pyrolytic graphite crystal was fabricated for the ZSPEC. The conically bent crystal in the Hall geometry produces a line focus perpendicular to the spectrometer axis, corresponding to the detector plane of electronic detectors at large scale laser facilities such as OMEGA, extending the useful range of the spectrometer to 7–11 keV. Using data collected using a microfocus Mo x-ray source, we determine important characteristics of ZSPEC such as the dispersion, spatial resolution, and absolute sensitivity of the instrument. A ray-trace model of ZSPEC provides another point of agreement in calculations of the ZSPEC dispersion and crystal response. [ABSTRACT FROM AUTHOR]

Optimization of neutron yields from dense plasma focus devices is a complex multi-faceted challenge that necessitates the prudent selection of mechanical constraints such as the electrode and insulator geometries. Here, the neutron yield is found to significantly depend on the insulator length. As the length of the insulator increases, the exposed anode length traveled by the sheath during the run-down phase decreases. This suggests an increase in the optimal fill pressure with increasing insulator length to maintain the pinch time near peak current. However, in the present study, the opposite trend is observed—the optimal fill pressure for neutron production decreases with increasing insulator length. Optical probing of the sheath from run-down to the pinch reveals significant plasma filamentation with increasing pressure and a dependence of insulator length on filamentation onset. A direct consequence of increased filamentation is a reduction in mass sweeping efficiency, directly quantified as a function of fill pressure for the first time. [ABSTRACT FROM AUTHOR]

The role of the longitudinal ambipolar electric field, present inside a pre-formed plasma, in electron heating and beam generation is investigated by analyzing single electron motion in the presence of one electromagnetic plane wave and 'V' shaped potential well (constant electric field) in a one dimensional slab approximation. It is shown that for the electron confined in an infinite potential well, its motion becomes stochastic when the ratio of normalized laser electric field a0, to normalized longitudinal electric field Ez, exceeds unity, i.e., a0/Ez>=1. For a more realistic potential well of finite depth, present inside the pre-formed plasma, the condition for stochastic heating of electrons gets modified to 1<=a0/Ez<=L, where L is the normalized length of the potential well. The energy of electron beam leaving such a potential well and entering the solid scales ∼a02/Ez, which can exceed the laser ponderomotive energy (∼a0) in the stochastic regime. [ABSTRACT FROM AUTHOR]

We report spatially and temporally resolved measurements of self-generated multi-megagauss magnetic fields produced during ultrahigh intensity laser plasma interactions. Spatially resolved measurements of the magnetic fields show an asymmetry in the distribution of field with respect to the angle of laser incidence. Temporally resolved measurements of the self-generated third harmonic suggest that the strength of the magnetic field is proportional to the square root of laser intensity (i.e., the laser B-field) during the rise of the laser pulse. The experimental results are compared with numerical simulations using a particle-in-cell code which also shows clear asymmetry of the field profile and similar magnetic field growth rates and scalings. [ABSTRACT FROM AUTHOR]

The ignition concept for electron fast ignition inertial confinement fusion requires sufficient energy be transferred from an ∼20 ps laser pulse to the compressed fuel via approximately MeV electrons. We have assembled a suite of diagnostics to characterize such transfer, simultaneously fielding absolutely calibrated extreme ultraviolet multilayer imagers at 68 and 256 eV; spherically bent crystal imagers at 4.5 and 8 keV; multi-keV crystal spectrometers; MeV x-ray bremmstrahlung, electron and proton spectrometers (along the same line of sight), and a picosecond optical probe interferometer. These diagnostics allow careful measurement of energy transport and deposition during and following the laser-plasma interactions at extremely high intensities in both planar and conical targets. Together with accurate on-shot laser focal spot and prepulse characterization, these measurements are yielding new insights into energy coupling and are providing critical data for validating numerical particle-in-cell (PIC) and hybrid PIC simulation codes in an area crucial for fast ignition and other applications. Novel aspects of these diagnostics and how they are combined to extract quantitative data on ultrahigh intensity laser-plasma interactions are discussed. [ABSTRACT FROM AUTHOR]

A theoretical study complemented with published experimental data of proton acceleration from sub-micron (thickness < 1 µm) foils irradiated by ultra-high contrast (>1010) short pulse lasers is presented. The underlying physics issues pertinent to proton acceleration are addressed using two-dimensional particle-in-cell simulations. For laser energy ε ≤ 4 J (intensity I ≤ 5 x 1020 W/cm²), simulation predictions agree with experimental data, both exhibiting scaling superior to Target Normal Sheath Acceleration's model. Anomalous behavior was observed for ε > 4 J (I > 5 x 1020 W/cm²), for which the measured maximum proton energies were much lower than predicted by scaling and these simulations. This unexpected behavior could not be explained within the frame of the model, and we conjecture that pre-pulses preceding the main pulse by picoseconds may be responsible. If technological issues can be resolved, energetic proton beams could be generated for a wide range of applications such as nuclear physics, radiography, and medical science. [ABSTRACT FROM AUTHOR]

Talbot-Lau x-ray interferometry has been implemented to map electron density gradients in High Energy Density Physics (HEDP) experiments. X-ray backlighter targets have been evaluated for Talbot-Lau X-ray Deflectometry (TXD). Cu foils, wires, and sphere targets have been irradiated by 10–150 J, 8–30 ps laser pulses, while two pulsed-power generators (∼350 kA, 350 ns and ∼200 kA, 150 ns) have driven Cu wire, hybrid, and laser-cut x-pinches. A plasma ablation front generated by the Omega EP laser was imaged for the first time through TXD for densities >1023 cm−3. Backlighter optimization in combination with x-ray CCD, image plates, and x-ray film has been assessed in terms of spatial resolution and interferometer contrast for accurate plasma characterization through TXD in pulsed-power and high-intensity laser environments. The results obtained thus far demonstrate the potential of TXD as a powerful diagnostic for HEDP. [ABSTRACT FROM AUTHOR]

The interaction of a relativistic laser pulse (1018 W cm−2µm2) with foil targets can accelerate ions to energies of tens of MeV u−1 with optimized laser and target parameters. We report the results on simulations of the interaction of a 6.0 × 1020 W cm−2 laser pulse incident on ultrathin (10–500 nm) titanium foils to investigate the roles of laser pulse duration and ionization mechanisms in the acceleration of titanium ions. While holding peak intensity constant, two laser pulse durations were investigated, 140 and 650 fs. The optimum thickness is dependent on pulse duration, as it requires the concurrence of target transparency with the incidence of the peak laser intensity. The collisional processes do not play a significant role in Ti ion beam generation from the 140 fs laser pulse duration at the optimum thickness (30 nm). However, for the 650 fs laser optimum, collisions improve the conversion efficiency of highly energetic (10 MeV u−1), high charge titanium (Ti21 − 22 +) by a factor of 20, and the titanium ion cutoff energy by ∼15%. This improvement is due to the fact that collisional ionization increases the electron density of the plasma, which delays the time of relativistic transparency, causing collisions to decrease the optimum foil thickness from 150 to 100 nm. Additionally, collisional ionization increases the charge-to-mass ratio of the titanium, and injects more electrons into the accelerating sheath field. At the optimum thickness, target normal sheath acceleration is the dominant mechanism of acceleration, with additional contributions from radiation pressure and shock wave acceleration. [ABSTRACT FROM AUTHOR]

Focusing effect of laser-driven positron jets by self-generated target sheath fields has been observed for the first time experimentally and the results are supported by the computational studies. In the experiment, OMEGA EP short-pulse (0.7 ps, 500 J) irradiates mm-size gold targets with a concave back surface and reference flat-surface targets. Both targets exhibited positrons with quasi-monoenergetic energy peaks while targets with concave curvature also showed increased number of positrons at the detector. The data is consistent with hybrid-PIC simulations confirming that the time-varying electric fields driven by electrons escaping from the target significantly change the trajectories of positrons. These simulations show a small radius of curvature on the rear side increases the relative focusing effect and the positrons to electrons ratio in the escaping plasma. For the smallest radius of curvature, positron jets that are up to 10 times denser can be achieved. [ABSTRACT FROM AUTHOR]

Proton beams driven by chirped pulse amplified lasers have multi-picosecond duration and can isochorically and volumetrically heat material samples, potentially providing an approach for creating samples of warm dense matter with conditions not present on Earth. Envisioned on a larger scale, they could heat fusion fuel to achieve ignition. We have shown in an experiment that a kilojoule-class, multi-picosecond short pulse laser is particularly effective for heating materials. The proton beam can be focussed via target design to achieve exceptionally high flux, important for the applications mentioned. The laser irradiated spherically curved diamond-like-carbon targets with intensity 4 × 1018 W/cm2, producing proton beams with 3 MeV slope temperature. A Cu witness foil was positioned behind the curved target, and the gap between was either empty or spanned with a structure. With a structured target, the total emission of Cu Kα fluorescence was increased 18 fold and the emission profile was consistent with a tightly focussed beam. Transverse proton radiography probed the target with ps order temporal and 10 μm spatial resolution, revealing the fast-acting focussing electric field. Complementary particle-in-cell simulations show how the structures funnel protons to the tight focus. The beam of protons and neutralizing electrons induce the bright Kα emission observed and heat the Cu to 100 eV. [ABSTRACT FROM AUTHOR]

We report experimental results on relativistic electron beam (REB) transport in a set of cold and shock-heated carbon samples using the high-intensity kilojoule-class OMEGA EP laser. The REB energy distribution and transport were diagnosed using an electron spectrometer and x-ray fluorescence measurements from a Cu tracer buried at the rear side of the samples. The measured rear REB density shows brighter and narrower signals when the targets were shock-heated. Hybrid PIC simulations using advanced resistivity models in the target warm-dense-matter (WDM) conditions confirm this observation. We show that the resistivity response of the media, which governs the self-generated resistive fields, is of paramount importance to understand and correctly predict the REB transport. [ABSTRACT FROM AUTHOR]

We report on the characterization of the conditions of an imploding cylindrical plasma by time-resolved x-ray emission spectroscopy. Knowledge about this implosion platform can be applied to studies of particle transport for inertial confinement fusion schemes or to astrophysical plasmas. A cylindrical Cl-doped CH foam within a tube of solid CH was irradiated by 36 beams (Itotal ∼ 5 × 1014 W/cm2, 1.5 ns square pulse, and Etotal ∼ 16.2 kJ) of the OMEGA-60 laser to radially compress the CH toward the axis. The analysis of the time-resolved spectra showed that the compression can be described by four distinct phases, each presenting different plasma conditions. First the ablation of the cylinder is dominant; second, the foam is heated and induces a significant jump in emission intensities; third, the temperature and density of the foam reaches a maximum; and finally, the plasma expands. Ranges for the plasma temperature were inferred with the atomic physics code SCRAM (Spectroscopic Collisional-Radiative Atomic Model) and the experimental data have been compared to hydrodynamic simulations performed with the 2D code FLASH, which showed a similar implosion dynamic over time. [ABSTRACT FROM AUTHOR]

In the shock ignition (SI) laser fusion scheme, hot electrons generated by the laser spike pulse can either preheat the fuel or strengthen the ignition shock, depending on the hot electron characteristics. We conducted a planar target experiment on the OMEGA-EP laser facility and characterized the temperature and total energy of hot electrons generated from a kilojoule-class 100-ps infrared (IR) or a 1-ns ultraviolet (UV) laser interacting with a large ( L n ∼ 330 − 450 μm) and hot ( T e ∼ 1 − 2 keV) coronal plasma at the SI-relevant intensities (∼ 10 16 W / cm 2 ). The IR laser converts ∼2.5% energy into hot electrons with T hot ∼ 60–90 keV, while the UV laser couples 0.8 % ± 0.7 % energy into T hot = 27 ± 9 keV hot electrons. The IR-produced hot electrons yield five times higher Cu K α emission than the UV case, confirming the higher electron conversion efficiency with the IR laser. The low energy conversion from the UV laser to hot electrons may be due to the refraction of the off-normal incident laser in the large coronal plasma. These findings are the first comparisons of hot electron generation between the IR and UV pulses at kilojoule scales in SI-relevant large-scale plasmas. The findings may expand the SI design space to include IR lasers as the possible spike lasers. [ABSTRACT FROM AUTHOR]

Gas puff Z-pinches are intense sources of X-rays and neutrons but are highly susceptible to the magneto-Rayleigh-Taylor instability (MRTI). MRTI mitigation is critical for optimal and reproducible yields, motivating significant attention toward various potential mitigation mechanisms. One such approach is the external application of an axial magnetic field, which will be discussed here in the context of recent experiments on the Zebra generator (1 MA, 100 ns) at the University of Nevada, Reno. In these experiments, an annular Kr gas liner is imploded onto an on-axis deuterium target with a pre-embedded axial magnetic field B z 0 ranging from 0 to 0.3 T. The effect of B z 0 on the stability of the Kr liner is evaluated with measurements of plasma radius, overall instability amplitude, and dominant instability wavelength at different times obtained from time-gated extreme ultraviolet pinhole images. It was observed that the external axial magnetic field does not affect the implosion velocity significantly and that it reduces the overall instability amplitude and the presence of short-wavelength modes, indicating improved pinch stability and reproducibility. For the highest applied B z 0 = 0.3 T, the stagnation radius measured via visible streak images was found to increase. These findings are consistent with experiments reported in the literature, but here, the B z 0 required for stability, B z 0 = 0.13 I p k / R 0 (where Ipk is the driver peak current and R0 is the initial radius), is lower. This could be attributed to the smaller load geometry, both radially and axially. Consistent with other experiments, the cause of decreased convergence cannot be explained by the additional axial magnetic pressure and remains an open question. [ABSTRACT FROM AUTHOR]