Demonstration of Geometric Effects and Resonant Scattering in the X-Ray Spectra of High-Energy-Density Plasmas.

In a plasma of sufficient size and density, photons emitted within the system have a probability of being reabsorbed and reemitted multiple times-a phenomenon known in astrophysics as resonant scattering. This effect alters the ratio of optically thick to optically thin lines, depending on the plasma geometry and viewing angle, and has significant implications for the spectra observed in a number of astrophysical scenarios, but has not previously been studied in a controlled laboratory plasma. We demonstrate the effect in the x-ray spectra emitted by cylindrical plasmas generated by high power laser irradiation, and the results confirm the geometrical interpretation of resonant scattering.

Observation of Hydrodynamic Flows in Imploding Fusion Plasmas on the National Ignition Facility.

Inertial confinement fusion implosions designed to have minimal fluid motion at peak compression often show significant linear flows in the laboratory, attributable per simulations to percent-level imbalances in the laser drive illumination symmetry. We present experimental results which intentionally varied the mode 1 drive imbalance by up to 4% to test hydrodynamic predictions of flows and the resultant imploded core asymmetries and performance, as measured by a combination of DT neutron spectroscopy and high-resolution x-ray core imaging. Neutron yields decrease by up to 50%, and anisotropic neutron Doppler broadening increases by 20%, in agreement with simulations. Furthermore, a tracer jet from the capsule fill-tube perturbation that is entrained by the hot-spot flow confirms the average flow speeds deduced from neutron spectroscopy.

Thermal Temperature Measurements of Inertial Fusion Implosions.

Accurate measurement of the thermal temperature in inertially confined fusion plasmas is essential for characterizing ignition performance and validating the basic physics understanding of the stagnation conditions. We present experimental results from cryogenic deuterium-tritium implosions on the National Ignition Facility using a differential filter spectrometer designed to measure the thermal electron temperature from x-ray continuum emission from the stagnated plasma. Furthermore, electron temperature measurements, used in conjunction with the Doppler-broadened DT neutron spectra, allow one to infer the partition of energy in the hot spot between internal energy and unconverted kinetic energy.

Publisher's Note: Development of improved radiation drive environment for high foot implosions at the National Ignition Facility [Phys. Rev. Lett. 117, 225002 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.117.225002.

Development of Improved Radiation Drive Environment for High Foot Implosions at the National Ignition Facility.

Analyses of high foot implosions show that performance is limited by the radiation drive environment, i.e., the hohlraum. Reported here are significant improvements in the radiation environment, which result in an enhancement in implosion performance. Using a longer, larger case-to-capsule ratio hohlraum at lower gas fill density improves the symmetry control of a high foot implosion. Moreover, for the first time, these hohlraums produce reduced levels of hot electrons, generated by laser-plasma interactions, which are at levels comparable to near-vacuum hohlraums, and well within specifications. Further, there is a noteworthy increase in laser energy coupling to the hohlraum, and discrepancies with simulated radiation production are markedly reduced. At fixed laser energy, high foot implosions driven with this improved hohlraum have achieved a 1.4×increase in stagnation pressure, with an accompanying relative increase in fusion yield of 50% as compared to a reference experiment with the same laser energy.

Dynamics of relativistic laser-plasma interaction on solid targets.

A novel time-resolved diagnostic is used to record the critical surface motion during picosecond-scale relativistic laser interaction with a solid target. Single-shot measurements of the specular light show a redshift decreasing with time during the interaction, corresponding to a slowing-down of the hole boring process into overdense plasma. On-shot full characterization of the laser pulse enables simulations of the experiment without any free parameters. Two-dimensional particle-in-cell simulations yield redshifts that agree with the data, and support a simple explanation of the slowing-down of the critical surface based on momentum conservation between ions and reflected laser light.

Using L-shell x-ray spectra to determine conditions of non-local thermal dynamic equilibrium plasmas.

K-shell x-ray spectra of Li- to H-like ions have long been used to determine plasma conditions. The ratio of integrated line intensities is used to determine the temperature. At the density of non-local thermal dynamic equilibrium (NLTE) plasmas (n e ≈ 1021 cm-3), the K-shell spectrum is not very sensitive to density. We propose using the L-shell emission of open L-shell ions (C- to Li-like) as an alternative to determine both temperature and density of NLTE plasmas. First, the L-shell models of a mid-Z material need to be verified against the temperatures obtained using a K-shell spectrum of a low-Z material. A buried layer platform is being developed at the OMEGA laser to study the open L-shell spectra of NLTE plasmas of mid-Z materials. Studies have been done using a 250 µm diameter dot composed of a layer of 1200 Å thick Zn between two 600 Å thick layers of Ti, in the center of a 1000 µm diameter, 13 µm thick beryllium tamper. Lasers heat the target from both sides for up to 3 ns. The size of the emitting volume vs time was measured with x-ray imaging (face-on and side-on) to determine the density. The temperature was measured from the Ti K-shell spectra. The use of this platform for the verification of atomic L-shell models is discussed.

Implementing time resolved electron temperature capability at the NIF using a streak camera.

A new capability at the National Ignition Facility (NIF) has been implemented to measure the temperature of x-ray emitting sources. Although it is designed primarily for Inertial Confinement Fusion (ICF), it can be used for any hot emitting source that is well modeled. The electron temperature (Te) of the hot spot within the core of imploded ICF capsules is an effective indicator of implosion performance. Currently, there are spatially and temporally integrated Te inferences using image plates. A temporally resolved measurement of Te will help elucidate the mechanisms for hot spot heating and cooling such as conduction to fuel, alpha-heating, mix, and radiative losses. To determine the temporally resolved Te of hot spots, specific filters are added to an existing x-ray streak camera "streaked polar instrumentation for diagnosing energetic radiation" to probe the emission spectrum during the x-ray burn history of implosions at the NIF. One of the difficulties in inferring the hot spot temperature is the attenuation of the emission due to opacity from the shell and fuel. Therefore, a series of increasingly thick titanium filters were implemented to isolate the emission in specific energy regions that are sensitive to temperatures above 3 keV while not significantly influenced by the shell/fuel attenuation. Additionally, a relatively thin zinc filter was used to measure the contribution of colder emission sources. Since the signal levels of the emission through the thicker filters are relatively poor, a dual slit (aperture) was designed to increase the detected signal at the higher end of the spectrum. Herein, the design of the filters and slit is described, an overview of the solving technique is provided, and the initial electron temperature results are reported.

Calibration and characterization of a highly efficient spectrometer in von Hamos geometry for 7-10 keV x-rays.

We have built an absolutely calibrated, highly efficient, Bragg crystal spectrometer in von Hamos geometry. This zinc von Hamos spectrometer uses a crystal made from highly oriented pyrolytic graphite that is cylindrically bent along the non-dispersive axis. It is tuned to measure x-ray spectra in the 7-10 keV range and has been designed to be used on a Ten Inch Manipulator for the Omega and OmegaEP target chambers at the Laboratory for Laser Energetics in Rochester, USA. Significant shielding strategies and fluorescence mitigation have been implemented in addition to an imaging plate detector making it well suited for experiments in high-intensity environments. Here we present the design and absolute calibration as well as mosaicity and integrated reflectivity measurements.

Hotspot electron temperature from x-ray continuum measurements on the NIF.

We report on measurements of the electron temperature in the hotspot of inertially confined, layered, spherical implosions on the National Ignition Facility using a differential filtering diagnostic. Measurements of the DT and DD ion temperatures using neutron time-of-flight detectors are complicated by the contribution of hot spot motion to the peak width, which produce an apparent temperature higher than the thermal temperature. The electron temperature is not sensitive to this non-thermal velocity and is thus a valuable input to interpreting the stagnated hot spot conditions. Here we show that the current differential filtering diagnostic provides insufficient temperature resolution for the hot spot temperatures of interest. We then propose a new differential filter configuration utilizing larger pinhole size to increase spectral fluence, as well as thicker filtration. This new configuration will improve measurement uncertainty by more than a factor of three, allowing for a more accurate hotspot temperature.

Development of a krypton-doped gas symmetry capsule platform for x-ray spectroscopy of implosion cores on the NIF.

The electron temperature at stagnation of an ICF implosion can be measured from the emission spectrum of high-energy x-rays that pass through the cold material surrounding the hot stagnating core. Here we describe a platform developed on the National Ignition Facility where trace levels of a mid-Z dopant (krypton) are added to the fuel gas of a symcap (symmetry surrogate) implosion to allow for the use of x-ray spectroscopy of the krypton line emission.

High-contrast laser acceleration of relativistic electrons in solid cone-wire targets.

The consequences of small scale-length precursor plasmas on high-intensity laser-driven relativistic electrons are studied via experiments and simulations. Longer scale-length plasmas are shown to dramatically increase the efficiency of electron acceleration, yet, if too long, they reduce the coupling of these electrons into the solid target. Evidence for the existence of an optimal plasma scale-length is presented and estimated to be from 1 to 5µm. Experiments on the Trident laser (I=5×10(19)W/cm(2)) diagnosed via Kα emission from Cu wires attached to Au cones are quantitively reproduced using 2D particle-in-cell simulations that capture the full temporal and spatial scale of the nonlinear laser interaction and electron transport. The simulations indicate that 32%±8%(6.5%±2%) of the laser energy is coupled into electrons of all energies (1-3 MeV) reaching the inner cone tip and that, with an optimized scale-length, this could increase to 35% (9%).

Time-resolved compression of a capsule with a cone to high density for fast-ignition laser fusion.

The advent of high-intensity lasers enables us to recreate and study the behaviour of matter under the extreme densities and pressures that exist in many astrophysical objects. It may also enable us to develop a power source based on laser-driven nuclear fusion. Achieving such conditions usually requires a target that is highly uniform and spherically symmetric. Here we show that it is possible to generate high densities in a so-called fast-ignition target that consists of a thin shell whose spherical symmetry is interrupted by the inclusion of a metal cone. Using picosecond-time-resolved X-ray radiography, we show that we can achieve areal densities in excess of 300 mg cm(-2) with a nanosecond-duration compression pulse--the highest areal density ever reported for a cone-in-shell target. Such densities are high enough to stop MeV electrons, which is necessary for igniting the fuel with a subsequent picosecond pulse focused into the resulting plasma.

An evaluation of high energy bremsstrahlung background in point-projection x-ray radiography experiments.

Backlit pinhole x-ray radiography has provided high-resolution images in many recent high-energy-density laser experiments. Its aim is to image the object of interest with a roughly monochromatic Kα source. However, despite the high intrinsic brightness achieved by the technique, data on x-ray film have shown a signal to background ratio near one, with data on image plates producing a higher background. This has been attributed, without direct evidence, to the interaction of suprathermal electrons with the (high Z) pinhole substrate. We present here the first direct measurement of the hard x-rays produced by such a backlighter target and a test of an approach to reducing the background. Specifically, a thick, low-Z layer was added on the side of the substrate toward the detector, intended to stop the energetic electrons and produce smaller emissions. Results from the Omega-60 laser experiment showed that the oft-seen background signal is in the range of 60-80 keV, a plausible energy range for energetic electrons produced in the laser-irradiated plasma. It also showed a comparable level of background signal in both types of targets. The work presented here includes target design and motivating theory, as well as the unexpected findings about x-ray background production.