Time-Resolved Fuel Density Profiles of the Stagnation Phase of Indirect-Drive Inertial Confinement Implosions.

The implosion efficiency in inertial confinement fusion depends on the degree of stagnated fuel compression, density uniformity, sphericity, and minimum residual kinetic energy achieved. Compton scattering-mediated 50-200 keV x-ray radiographs of indirect-drive cryogenic implosions at the National Ignition Facility capture the dynamic evolution of the fuel as it goes through peak compression, revealing low-mode 3D nonuniformities and thicker fuel with lower peak density than simulated. By differencing two radiographs taken at different times during the same implosion, we also measure the residual kinetic energy not transferred to the hot spot and quantify its impact on the implosion performance.

Neutron Time-of-Flight Measurements of Charged-Particle Energy Loss in Inertial Confinement Fusion Plasmas.

Neutron spectra from secondary ^{3}H(d,n)α reactions produced by an implosion of a deuterium-gas capsule at the National Ignition Facility have been measured with order-of-magnitude improvements in statistics and resolution over past experiments. These new data and their sensitivity to the energy loss of fast tritons emitted from thermal ^{2}H(d,p)^{3}H reactions enable the first statistically significant investigation of charged-particle stopping via the emitted neutron spectrum. Radiation-hydrodynamic simulations, constrained to match a number of observables from the implosion, were used to predict the neutron spectra while employing two different energy loss models. This analysis represents the first test of stopping models under inertial confinement fusion conditions, covering plasma temperatures of k_{B}T≈1-4 keV and particle densities of n≈(12-2)×10^{24} cm^{-3}. Under these conditions, we find significant deviations of our data from a theory employing classical collisions whereas the theory including quantum diffraction agrees with our data.

Performance of high-convergence, layered DT implosions with extended-duration pulses at the National Ignition Facility.

Radiation-driven, low-adiabat, cryogenic DT layered plastic capsule implosions were carried out on the National Ignition Facility (NIF) to study the sensitivity of performance to peak power and drive duration. An implosion with extended drive and at reduced peak power of 350 TW achieved the highest compression with fuel areal density of ~1.3±0.1 g/cm2, representing a significant step from previously measured ~1.0 g/cm2 toward a goal of 1.5 g/cm2. Future experiments will focus on understanding and mitigating hydrodynamic instabilities and mix, and improving symmetry required to reach the threshold for thermonuclear ignition on NIF.

Measurements of the T(t,2n)4He neutron spectrum at low reactant energies from inertial confinement implosions.

Measurements of the neutron spectrum from the T(t,2n)4He (tt) reaction have been conducted using inertial confinement fusion implosions at the OMEGA laser facility. In these experiments, deuterium-tritium (DT) gas-filled capsules were imploded to study the tt reaction in thermonuclear plasmas at low reactant center-of-mass (c.m.) energies. In contrast to accelerator experiments at higher c.m. energies (above 100 keV), these results indicate a negligible n + 5He reaction channel at a c.m. energy of 23 keV.

Plasma devices to guide and collimate a high density of MeV electrons.

The development of ultra-intense lasers has facilitated new studies in laboratory astrophysics and high-density nuclear science, including laser fusion. Such research relies on the efficient generation of enormous numbers of high-energy charged particles. For example, laser-matter interactions at petawatt (10(15) W) power levels can create pulses of MeV electrons with current densities as large as 10(12) A cm(-2). However, the divergence of these particle beams usually reduces the current density to a few times 10(6) A cm(-2) at distances of the order of centimetres from the source. The invention of devices that can direct such intense, pulsed energetic beams will revolutionize their applications. Here we report high-conductivity devices consisting of transient plasmas that increase the energy density of MeV electrons generated in laser-matter interactions by more than one order of magnitude. A plasma fibre created on a hollow-cone target guides and collimates electrons in a manner akin to the control of light by an optical fibre and collimator. Such plasma devices hold promise for applications using high energy-density particles and should trigger growth in charged particle optics.

Photonuclear fission from high energy electrons from ultraintense laser-solid interactions

A new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic, with energies extending well above the threshold for nuclear processes, is studied using a petawatt laser system. In solid target experiments with focused intensities exceeding 10(20) W/cm(2), high energy electron generation, hard bremsstrahlung, and nuclear phenomena have been observed. We report here a quantitative comparison of the high energy electrons and the bremsstrahlung spectrum, as measured by photonuclear reaction yields, including the photoinduced fission of 238U.

Experimental measurements of deep directional columnar heating by laser-generated relativistic electrons at near-solid density.

In our experiments, we irradiated solid CH targets with a 400 J, 5 ps, 3 x 10(19) W/cm(2) laser, and we used x-ray imaging and spectroscopic diagnostics to monitor the keV x-ray emission from thin Al or Au tracer layers buried within the targets. The experiments were designed to quantify the spatial distribution of the thermal electron temperature and density as a function of buried layer depth; these data provide insights into the behavior of relativistic electron currents which flow within the solid target and are directly and indirectly responsible for the heating. We measured approximately 200-350 eV temperatures and near-solid densities at depths ranging from 5 to 100 microm beneath the target surface. Time-resolved x-ray spectra from Al tracers indicate that the tracers emit thermal x rays and cool slowly compared to the time scale of the laser pulse. Most intriguingly, we consistently observe annular x-ray images in all buried tracer-layer experiments, and these data show that the temperature distribution is columnar, with enhanced heating along the edges of the column. The ring diameters are much greater than the laser focal spot diameter and do not vary significantly with the depth of the tracer layer for depths greater than 30 microm. The local temperatures are 200-350 eV for all tracer depths. We discuss recent simulations of the evolution of electron currents deep within solid targets irradiated by ultra-high-intensity lasers, and we discuss how modeling and analytical results suggest that the annular patterns we observe may be related to locally strong growth of the Weibel instability. We also suggest avenues for future research in order to further illuminate the complex physics of relativistic electron transport and energy deposition inside ultra-high-intensity laser-irradiated solid targets.

Ion acceleration from the shock front induced by hole boring in ultraintense laser-plasma interactions.

Ion-acceleration processes have been studied in ultraintense laser plasma interactions for normal incidence irradiation of solid deuterated targets via neutron spectroscopy. The experimental neutron spectra strongly suggest that the ions are preferentially accelerated radially, rather than into the bulk of the material from three-dimensional Monte Carlo fitting of the neutron spectra. Although the laser system has a 10(-7) contrast ratio, a two-dimensional magnetic hydrodynamics simulation shows that the laser pedestal generates a 10 mum scale length in the coronal plasma with a 3 mum scale-length plasma near the critical density. Two-dimensional particle-in-cell simulations, incorporating this realistic density profile, indicate that the acceleration of the ions is caused by a collisionless shock formation. This has implications for modeling energy transport in solid density plasmas as well as cone-focused fast ignition using the next generation PW lasers currently under construction.

K(alpha) fluorescence measurement of relativistic electron transport in the context of fast ignition.

Electron transport within solid targets, irradiated by a high-intensity short-pulse laser, has been measured by imaging K(alpha) radiation from high- Z layers (Cu, Ti) buried in low- Z (CH, Al) foils. Although the laser spot is approximately 10 microm [full width at half maximum (FWHM)], the electron beam spreads to > or =70 microm FWHM within <20 microm of penetration into an Al target then, at depths >100 microm, diverges with a 40 degree spreading angle. Monte Carlo and analytic models are compared to our data. We find that a Monte Carlo model with a heuristic model for the electron injection gives a reasonable fit with our data.

Sample handling and storage for the quantitative analysis of volatile compounds in blood: the determination of toluene by headspace gas chromatography.

The problems associated with the handling and storage of blood samples for the analysis of toluene in toxicological casework using headspace gas chromatography have been investigated. Significant losses of toluene and the internal standard, ethylbenzene, from solution may be observed during handling. Toluene may be lost into the headspace above the blood in a storage container while such headspaces can also be detrimental, leading to the formation of n-hexanal, a possible interference with the assay. Large differences in the stability of toluene were seen for blood samples in different types of storage container. Storage containers in which the sample was only in contact with glass, teflon, or aluminum foil were most satisfactory. It is recommended that blood samples should be stored in appropriate containers with minimum headspace under cold conditions (4 degrees C). Transfers of blood should also be made at cold temperatures. The applicability of these guidelines to the analysis of other volatile compounds is discussed.

The response of evidential breath alcohol testing instruments with subjects exposed to organic solvents and gases. I. Toluene, 1,1,1-trichloroethane and butane.

Experimental work has been undertaken to investigate the potential interference of toluene, 1,1,1-trichloroethane and butane with the evidential breath alcohol testing instruments used in Great Britain (Lion Intoximeter 3000 and Camic Breath Analyser). Volunteers inhaled the volatile substances in an exposure chamber for up to 4 hours, at concentrations of 100, 350 and 600ppm respectively. Subsequently breath was tested on leaving the chamber. No interference was observed with the breath alcohol instruments when the subjects were exposed to toluene and 1,1,1-trichloroethane. A short-term response immediately after exposure was observed for subjects exposed to butane. Further analytical work involving blood and breath samples demonstrated that all three volatile substances were absorbed during exposure and were detectable in blood for at least 3 hours post-exposure. Their elimination post-exposure followed an exponential decay.

Measuring the absolute deuterium-tritium neutron yield using the magnetic recoil spectrometer at OMEGA and the NIF.

A magnetic recoil spectrometer (MRS) has been installed and extensively used on OMEGA and the National Ignition Facility (NIF) for measurements of the absolute neutron spectrum from inertial confinement fusion implosions. From the neutron spectrum measured with the MRS, many critical implosion parameters are determined including the primary DT neutron yield, the ion temperature, and the down-scattered neutron yield. As the MRS detection efficiency is determined from first principles, the absolute DT neutron yield is obtained without cross-calibration to other techniques. The MRS primary DT neutron measurements at OMEGA and the NIF are shown to be in excellent agreement with previously established yield diagnostics on OMEGA, and with the newly commissioned nuclear activation diagnostics on the NIF.

Neutron spectrometry--an essential tool for diagnosing implosions at the National Ignition Facility (invited).

DT neutron yield (Y(n)), ion temperature (T(i)), and down-scatter ratio (dsr) determined from measured neutron spectra are essential metrics for diagnosing the performance of inertial confinement fusion (ICF) implosions at the National Ignition Facility (NIF). A suite of neutron-time-of-flight (nTOF) spectrometers and a magnetic recoil spectrometer (MRS) have been implemented in different locations around the NIF target chamber, providing good implosion coverage and the complementarity required for reliable measurements of Y(n), T(i), and dsr. From the measured dsr value, an areal density (ρR) is determined through the relationship ρR(tot) (g∕cm(2)) = (20.4 ± 0.6) × dsr(10-12 MeV). The proportionality constant is determined considering implosion geometry, neutron attenuation, and energy range used for the dsr measurement. To ensure high accuracy in the measurements, a series of commissioning experiments using exploding pushers have been used for in situ calibration of the as-built spectrometers, which are now performing to the required accuracy. Recent data obtained with the MRS and nTOFs indicate that the implosion performance of cryogenically layered DT implosions, characterized by the experimental ignition threshold factor (ITFx), which is a function of dsr (or fuel ρR) and Y(n), has improved almost two orders of magnitude since the first shot in September, 2010.

Electron heated target temperature measurements in petawatt laser experiments based on extreme ultraviolet imaging and spectroscopy.

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.

First measurements of the absolute neutron spectrum using the magnetic recoil spectrometer at OMEGA (invited).

A neutron spectrometer, called a magnetic recoil spectrometer (MRS), has been built and implemented at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] for absolute measurements of the neutron spectrum in the range of 6-30 MeV, from which fuel areal density (rhoR), ion temperature (T(i)), and yield (Y(n)) can be determined. The results from the first MRS measurements of the absolute neutron spectrum are presented. In addition, measuring rhoR at the National Ignition Facility (NIF) [G. H. Miller et al., Nucl. Fusion 44, S228 (2004)] will be essential for assessing implosion performance during all stages of development from surrogate implosions to cryogenic fizzles to ignited implosions. To accomplish this, we are also developing an MRS for the NIF. As much of the research and development and instrument optimization of the MRS at OMEGA are directly applicable to the MRS at the NIF, a description of the design and characterization of the MRS on the NIF is discussed as well.

Development of backlighting sources for a Compton radiography diagnostic of inertial confinement fusion targets (invited).

We present scaled demonstrations of backlighter sources, emitting bremsstrahlung x rays with photon energies above 75 keV, that we will use to record x-ray Compton radiographic snapshots of cold dense DT fuel in inertial confinement fusion implosions at the National Ignition Facility (NIF). In experiments performed at the Titan laser facility at Lawrence Livermore National Laboratory, we measured the source size and the bremsstrahlung spectrum as a function of laser intensity and pulse length from solid targets irradiated at 2x10(17)-5x10(18) W/cm(2) using 2-40 ps pulses. Using Au planar foils we achieved source sizes down to 5.5 microm and conversion efficiencies of about 1x10(-13) J/J into x-ray photons with energies in the 75-100 keV spectral range. We can now use these results to design NIF backlighter targets and shielding and to predict Compton radiography performance as a function of the NIF implosion yield and associated background.

Determination of electron-heated temperatures of petawatt laser-irradiated foil targets with 256 and 68 eV extreme ultraviolet imaging.

Measurements of plasma temperature at the rear surface of foil targets due to heating by hot electrons, which were produced in short pulse high intensity laser matter interactions using the 150 J, 0.5 ps Titan laser, are reported. Extreme ultraviolet (XUV) imaging at 256 and 68 eV energies is used to determine spatially resolved target rear surface temperature patterns by comparing absolute intensities to radiation hydrodynamic modeling. XUV mirrors at these two energies were absolutely calibrated at the Advanced Light Source at the Lawrence Berkeley Laboratory. Temperatures deduced from both imagers are validated against each other within the range of 75-225 eV.

Laser heating of solid matter by light-pressure-driven shocks at ultrarelativistic intensities.

The heating of solid targets irradiated by 5 x 10(20) W cm(-2), 0.8 ps, 1.05 microm wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to approximately 5 keV with an axial temperature gradient of 0.6 microm scale length. Images of Ni Ly(alpha) show the hot region has 100 G bar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer.