Advanced laser development and plasma-physics studies on the multiterawatt laser.

The multiterawatt (MTW) laser, built initially as the prototype front end for a petawatt laser system, is a 1053 nm hybrid system with gain from optical parametric chirped-pulse amplification (OPCPA) and Nd:glass. Compressors and target chambers were added, making MTW a complete laser facility (output energy up to 120 J, pulse duration from 20 fs to 2.8 ns) for studying high-energy-density physics and developing short-pulse laser technologies and target diagnostics. Further extensions of the laser support ultrahigh-intensity laser development of an all-OPCPA system and a Raman plasma amplifier. A short summary of the variety of scientific experiments conducted on MTW is also presented.

Magnetic Signatures of Radiation-Driven Double Ablation Fronts.

In experiments performed with the OMEGA EP laser system, magnetic field generation in double ablation fronts was observed. Proton radiography measured the strength, spatial profile, and temporal dynamics of self-generated magnetic fields as the target material was varied between plastic, aluminum, copper, and gold. Two distinct regions of magnetic field are generated in mid-Z targets-one produced by gradients from electron thermal transport and the second from radiation-driven gradients. Extended magnetohydrodynamic simulations including radiation transport reproduced key aspects of the experiment, including field generation and double ablation front formation.

Energy Flow in Thin Shell Implosions and Explosions.

Energy flow and balance in convergent systems beyond petapascal energy densities controls the fate of late-stage stars and the potential for controlling thermonuclear inertial fusion ignition. Time-resolved x-ray self-emission imaging combined with a Bayesian inference analysis is used to describe the energy flow and the potential information stored in the rebounding spherical shock at 0.22 PPa (2.2 Gbar or billions of atmospheres pressure). This analysis, together with a simple mechanical model, describes the trajectory of the shell and the time history of the pressure at the fuel-shell interface, ablation pressure, and energy partitioning including kinetic energy of the shell and internal energy of the fuel. The techniques used here provide a fully self-consistent uncertainty analysis of integrated implosion data, a thermodynamic-path independent measurement of pressure in the petapascal range, and can be used to deduce the energy flow in a wide variety of implosion systems to petapascal energy densities.

Precision mapping of laser-driven magnetic fields and their evolution in high-energy-density plasmas.

The magnetic fields generated at the surface of a laser-irradiated planar solid target are mapped using ultrafast proton radiography. Thick (50 µm) plastic foils are irradiated with 4-kJ, 2.5-ns laser pulses focused to an intensity of 4×10^{14} W/cm^{2}. The data show magnetic fields concentrated at the edge of the laser-focal region, well within the expanding coronal plasma. The magnetic-field spatial distribution is tracked and shows good agreement with 2D resistive magnetohydrodynamic simulations using the code draco when the Biermann battery source, fluid and Nernst advection, resistive magnetic diffusion, and Righi-Leduc heat flow are included.

Magnetic reconnection between colliding magnetized laser-produced plasma plumes.

Observations of magnetic reconnection between colliding plumes of magnetized laser-produced plasma are presented. Two counterpropagating plasma flows are created by irradiating oppositely placed plastic (CH) targets with 1.8-kJ, 2-ns laser beams on the Omega EP Laser System. The interaction region between the plumes is prefilled with a low-density background plasma and magnetized by an externally applied magnetic field, imposed perpendicular to the plasma flow, and initialized with an X-type null point geometry with B=0 at the midplane and B=8 T at the targets. The counterflowing plumes sweep up and compress the background plasma and the magnetic field into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing the first detailed observations of a stretched current sheet in laser-driven reconnection experiments. The dynamics of current sheet formation are in good agreement with first-principles particle-in-cell simulations that model the experiments.

Filamentation instability of counterstreaming laser-driven plasmas.

Filamentation due to the growth of a Weibel-type instability was observed in the interaction of a pair of counterstreaming, ablatively driven plasma flows, in a supersonic, collisionless regime relevant to astrophysical collisionless shocks. The flows were created by irradiating a pair of opposing plastic (CH) foils with 1.8 kJ, 2-ns laser pulses on the OMEGA EP Laser System. Ultrafast laser-driven proton radiography was used to image the Weibel-generated electromagnetic fields. The experimental observations are in good agreement with the analytical theory of the Weibel instability and with particle-in-cell simulations.

Observation of self-similarity in the magnetic fields generated by the ablative nonlinear Rayleigh-Taylor instability.

Magnetic fields generated by the nonlinear Rayleigh-Taylor growth of laser-seeded three-dimensional broadband perturbations were measured in laser-accelerated planar targets using ultrafast proton radiography. The experimental data show self-similar behavior in the growing cellular magnetic field structures. These observations are consistent with a bubble competition and merger model that predicts the time evolution of the number and size of the bubbles, linking the cellular magnetic field structures with the Rayleigh-Taylor bubble and spike growth.

Contrast optimization of Fresnel zone plate imaging.

Fresnel zone plates (FZPs) are circular diffractive elements that operate as a lens for x-rays. They have gained interest in the field of laser-plasma physics due to their ability to achieve higher spatial resolution than pinholes. Their design and implementation are complicated by the fact that a significant amount of the x-rays passing through the FZP will not diffract (zeroth order) and present a background to the measurement. This background can be large and inhomogeneous depending on the geometric setup of the experiment. Here, we present calculations of the diffracted (first order) and un-diffracted (zeroth order) flux profiles, which makes it possible to optimize the contrast between the first order imaging rays and the zeroth order background. Calculations for the implementation of a central block in the FZP, designed to block the zeroth from the entire field of view, are also presented.

High-resolution x-ray spectrometer for x-ray absorption fine structure spectroscopy.

Two extended x-ray absorption fine structure flat crystal x-ray spectrometers (EFX's) were designed and built for high-resolution x-ray spectroscopy over a large energy range with flexible, on-shot energy dispersion calibration capabilities. The EFX uses a flat silicon [111] crystal in the reflection geometry as the energy dispersive optic covering the energy range of 6.3-11.4 keV and achieving a spectral resolution of 4.5 eV with a source size of 50 µm at 7.2 keV. A shot-to-shot configurable calibration filter pack and Bayesian inference routine were used to constrain the energy dispersion relation to within ±3 eV. The EFX was primarily designed for x-ray absorption fine structure (XAFS) spectroscopy and provides significant improvement to the Laboratory for Laser Energetics' OMEGA-60 XAFS experimental platform. The EFX is capable of performing extended XAFS measurements of multiple absorption edges simultaneously on metal alloys and x-ray absorption near-edge spectroscopy to measure the electron structure of compressed 3d transition metals.

Magnetic field generation by the Rayleigh-Taylor instability in laser-driven planar plastic targets.

Magnetic fields generated by the Rayleigh-Taylor instability were measured in laser-accelerated planar foils using ultrafast proton radiography. Thin plastic foils were irradiated with ∼4-kJ, 2.5-ns laser pulses focused to an intensity of ∼10(14) W/cm(2) on the OMEGA EP Laser System. Target modulations were seeded by laser nonuniformities and amplified during target acceleration by the Rayleigh-Taylor instability. The experimental data show the hydrodynamic evolution of the target and MG-level magnetic fields generated in the broken foil. The experimental data are in good agreement with predictions from 2-D magnetohydrodynamic simulations.

Time-resolved measurements of hot-electron equilibration dynamics in high-intensity laser interactions with thin-foil solid targets.

Time-resolved K(α) spectroscopy has been used to infer the hot-electron equilibration dynamics in high-intensity laser interactions with picosecond pulses and thin-foil solid targets. The measured K(α)-emission pulse width increases from ~3 to 6 ps for laser intensities from ~10(18) to 10(19) W/cm(2). Collisional energy-transfer model calculations suggest that hot electrons with mean energies from ~0.8 to 2 MeV are contained inside the target. The inferred mean hot-electron energies are broadly consistent with ponderomotive scaling over the relevant intensity range.

Transport of an intense proton beam from a cone-structured target through plastic foam with unique proton source modeling.

Laser-accelerated proton beams are applicable to several research areas within high-energy density science, including warm dense matter generation, proton radiography, and inertial confinement fusion, which all involve transport of the beam through matter. We report on experimental measurements of intense proton beam transport through plastic foam blocks. The intense proton beam was accelerated by the 10ps, 700J OMEGA EP laser irradiating a curved foil target, and focused by an attached hollow cone. The protons then entered the foam block of density 0.38g/cm^{3} and thickness 0.55 or 1.00mm. At the rear of the foam block, a Cu layer revealed the cross section of the intense beam via proton- and hot electron-induced Cu-K_{α} emission. Images of x-ray emission show a bright spot on the rear Cu film indicative of a forward-directed beam without major breakup. 2D fluid-PIC simulations of the transport were conducted using a unique multi-injection source model incorporating energy-dependent beam divergence. Along with postprocessed calculations of the Cu-K_{α} emission profile, simulations showed that protons retain their ballistic transport through the foam and are able to heat the foam up to several keV in temperature. The total experimental emission profile for the 1.0mm foam agrees qualitatively with the simulated profile, suggesting that the protons indeed retain their beamlike qualities.

High-power, kilojoule class laser channeling in millimeter-scale underdense plasma.

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants.

High-resolution x-ray radiography with Fresnel zone plates on the University of Rochester's OMEGA Laser Systems.

Experiments performed at the Laboratory for Laser Energetics with a continuous-wave (cw) x-ray source and on the OMEGA and OMEGA EP Laser Systems [Boehly et al., Opt. Commun. 133, 495 (1997) and Waxer et al., Opt. Photonics News 16, 30 (2005)] have utilized a Fresnel zone plate (FZP) to obtain x-ray images with a spatial resolution as small as ∼1.5 µm. Such FZP images were obtained with a charge-coupled device or a framing camera at energies ranging from 4.5 keV to 6.7 keV using x-ray line emission from both the cw source and high-intensity, laser-beam-illuminated metal foils. In all cases, the resolution test results are determined from patterns and grids backlit by these sources. The resolutions obtained are shown to be due to a combination of the spectral content of the x-ray sources and detector resolution limited by the magnification of the images (14× to 22×). High-speed framing cameras were used to obtain FZP images with frame times as short as ∼30 ps. Double-shell implosions on OMEGA were backlit by laser-irradiated Fe foils, thus obtaining a framing-camera-limited, FZP-image resolution of ∼3 µm-4 µm.

Scaling hot-electron generation to high-power, kilojoule-class laser-solid interactions.

Thin-foil targets were irradiated with high-power (1 ≤ P(L) ≤ 210 TW), 10-ps pulses focused to intensities of I>10(18) W/cm(2) and studied with K-photon spectroscopy. Comparing the energy emitted in K photons to target-heating calculations shows a laser-energy-coupling efficiency to hot electrons of η(L-e) = 20 ± 10%. Time-resolved x-ray emission measurements suggest that laser energy is coupled to hot electrons over the entire duration of the incident laser drive. Comparison of the K-photon emission data to previous data at similar laser intensities shows that η(L-e) is independent of laser-pulse duration from 1 ≤ τ(p) ≤ 10 ps.

Fast advection of magnetic fields by hot electrons.

Experiments where a laser-generated proton beam is used to probe the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target are presented. At intensities of 10(15) W cm(-2) and under conditions of significant fast electron production and strong heat fluxes, the electron mean-free-path is long compared with the temperature gradient scale length and hence nonlocal transport is important for the dynamics of the magnetic field in the plasma. The hot electron flux transports self-generated magnetic fields away from the focal region through the Nernst effect [A. Nishiguchi, Phys. Rev. Lett. 53, 262 (1984)] at significantly higher velocities than the fluid velocity. Two-dimensional implicit Vlasov-Fokker-Planck modeling shows that the Nernst effect allows advection and self-generation transports magnetic fields at significantly faster than the ion fluid velocity, v(N)/c(s)≈10.

Interspecies radiative transition in warm and superdense plasma mixtures.

Superdense plasmas widely exist in planetary interiors and astrophysical objects such as brown-dwarf cores and white dwarfs. How atoms behave under such extreme-density conditions is not yet well understood, even in single-species plasmas. Here, we apply thermal density functional theory to investigate the radiation spectra of superdense iron-zinc plasma mixtures at mass densities of ρ = 250 to 2000 g cm-3 and temperatures of kT = 50 to 100 eV, accessible by double-shell-target implosions. Our ab initio calculations reveal two extreme atomic-physics phenomena-firstly, an interspecies radiative transition; and, secondly, the breaking down of the dipole-selection rule for radiative transitions in isolated atoms. Our first-principles calculations predict that for superdense plasma mixtures, both interatomic radiative transitions and dipole-forbidden transitions can become comparable to the normal intra-atomic Kα-emission signal. These physics phenomena were not previously considered in detail for extreme high-density plasma mixtures at super-high energy densities.

Constraining physical models at gigabar pressures.

High-energy-density (HED) experiments in convergent geometry are able to test physical models at pressures beyond hundreds of millions of atmospheres. The measurements from these experiments are generally highly integrated and require unique analysis techniques to procure quantitative information. This work describes a methodology to constrain the physics in convergent HED experiments by adapting the methods common to many other fields of physics. As an example, a mechanical model of an imploding shell is constrained by data from a thin-shelled direct-drive exploding-pusher experiment on the OMEGA laser system using Bayesian inference, resulting in the reconstruction of the shell dynamics and energy transfer during the implosion. The model is tested by analyzing synthetic data from a one-dimensional hydrodynamics code and is sampled using a Markov chain Monte Carlo to generate the posterior distributions of the model parameters. The goal of this work is to demonstrate a general methodology that can be used to draw conclusions from a wide variety of HED experiments.

Focussing Protons from a Kilojoule Laser for Intense Beam Heating using Proximal Target Structures.

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.

Generation of ultrahigh-velocity ionizing shocks with petawatt-class laser pulses.

Ultrahigh-velocity shock waves (approximately 10,000 km/s or 0.03c) are generated by focusing a 350-TW laser pulse into low-density helium gas. The collisionless ultrahigh-Mach-number electrostatic shock propagates from the plasma into the surrounding gas, ionizing gas as it becomes collisional. The shock undergoes a corrugation instability due to propagation of the ionizing shock within the gas (the Dyakov-Kontorovich instability). This system may be relevant to the study of very high-Mach-number ionizing shocks in astrophysical situations.