A dual high-energy radiography platform with 15 µm resolution at the National Ignition Facility.

To study matter at extreme densities and pressures, we need mega laser facilities such as the National Ignition Facility as well as creative methods to make observations during timescales of a billionth of a second. To facilitate this, we developed a platform and diagnostic to characterize a new point-projection radiography configuration using two micro-wires irradiated by a short pulse laser system that provides a large field of view with up to 3.6 ns separation between images. We used tungsten-carbide solid spheres as reference objects and inferred characteristics of the back-lighter source using a forward-fitting algorithm. The resolution of the system is inferred to be 15 µm (using 12.5 µm diameter wires). The bremsstrahlung temperature of the source is 70-300 keV, depending on laser energy and coupling efficiency. By adding the images recorded on multiple stacked image plates, the signal-to-noise of the system is nearly doubled. The imaging characterization technique described here can be adapted to most point-projection platforms where the resolution, spectral contrast, and signal-to-noise are important.

Single line-of-sight dual energy backlighter for mix width experiments.

We present a diagnostic technique used to spatially multiplex two x-ray radiographs of an object onto a detector along a single line-of-sight. This technique uses a thin, <2 µm, cosputtered backlighter target to simultaneously produce both Ni and Zn Heα emission. A Ni picket fence filter, 500 µm wide bars and troughs, is then placed in front of the detector to pass only the Ni Heα emission in the bar region and both energies in the trough region thereby spatially multiplexing the two radiographs on a single image. Initial experimental results testing the backlighter spectrum are presented along with simulated images showing the calculated radiographic images though the nickel picket fence filter which are used to measure the mix width in an accelerated nickel foam.

Long duration backlighter experiments at Omega.

We have successfully demonstrated a 7.5 ns duration pinhole-apertured backlighter at the Omega laser facility. Pinhole-apertured point-projection backlighting for 8 ns will be useful for imaging evolving features in experiments at the National Ignition Facility. The backlighter consisted of a 20 microm diameter pinhole in a 75 microm thick Ta substrate separated from a Zn emitter (9 keV) by a 400 microm thick high-density carbon piece. The carbon prevented the shock from the laser-driven surface from reaching the substrate before 8 ns and helped minimize x-ray ablation of the pinhole substrate. Grid wires in x-ray framing camera images of a gold grid have a source-limited resolution significantly smaller than the pinhole diameter due to the high aspect ratio of the pinhole, but do not become much smaller at late times.

Demonstration of the density dependence of x-ray flux in a laser-driven hohlraum.

Experiments have been conducted using laser-driven cylindrical hohlraums whose walls are machined from Ta2O5 foams of 100 mg/cc and 4 g/cc densities. Measurements of the radiation temperature demonstrate that the lower density walls produce higher radiation temperatures than the high density walls. This is the first experimental demonstration of the prediction that this would occur [M. D. Rosen and J. H. Hammer, Phys. Rev. E 72, 056403 (2005)10.1103/PhysRevE.72.056403]. For high density walls, the radiation front propagates subsonically, and part of the absorbed energy is wasted by the flow kinetic energy. For the lower wall density, the front velocity is supersonic and can devote almost all of the absorbed energy to heating the wall.

Dynamic Hohlraums as x-ray sources in high-energy density science.

The first demonstration of laser driven dynamic Hohlraums (LDDH) as a spectrally smooth backlighter source for opacity and temperature measurements through absorption spectrometry of materials in local thermodynamic equilibrium at temperatures >150 eV has been made. This is a crucial temperature regime for future astrophysics and ignition fusion experiments at the nearly completed National Ignition Facility (NIF) [E. I. Moses and C. R. Wuest, Fusion Sci. Technol. 47, 314 (2005)] at the Lawrence Livermore National Laboratory. The new backlighter consists of a LDDH filled with either krypton or argon that implodes to create an x-ray flash. The properties of this x-ray flash have been measured in experiments at the Omega laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the Laboratory for Laser Energetics in Rochester, New York, satisfying all requirements imposed by future experiments: (1) the emission spectrum extends to at least 5.5 keV, well above the maximum x-ray energy ( approximately 3.5 keV) obtained from the previously "best" opacity backlighters (uranium M-shell emission backlighters); (2) the spectrum is smooth and featureless (intensity variation <6% rms), allowing absorption spectrometry through experimental samples; (3) the emission source size is sufficiently small (<50 microm) for projection backlighting through future samples; (4) the emission is bright enough (and twice as bright as imploding hydrogen-filled capsules) for gated spectrometer measurements; (5) the emission duration is optimized ( approximately 100 ps) for the current and future generations of spectrometers; and (6) by using only a small number of beams with limited energy and symmetry for the backlighter (10 out of 60 beams in the Omega experiments), the majority of laser beams are left available for heating sample materials to >150 eV.

Experimental investigation of high-mach-number 3D hydrodynamic jets at the national ignition facility.

The first hydrodynamic experiments were performed on the National Ignition Facility. A supersonic jet was formed via the interaction of a laser driven shock ( approximately 40 Mbar) with 2D and 3D density perturbations. The temporal evolution of the jet's spatial scales and ejected mass were measured with point-projection x-ray radiography. Measurements of the large-scale features and mass are in good agreement with 2D and 3D numerical simulations. These experiments provide quantitative data on the evolution of 3D supersonic jets and provide insight into their 3D behavior.

Efficient, 1-100-keV x-ray radiography with high spatial and temporal resolution.

Transient x-ray radiography using laser-plasma emission is a powerful tool for diagnosing a large variety of high-energy-density phenomena. Traditional area- and point-backlighting techniques used at inertial confinement fusion facilities such as Nova and Omega cannot be extended efficiently to the future 100-times-larger National Ignition Facility. We have developed an x-ray backlighting technique that uses a backlit pinhole as a source for point-projection radiography. This method incorporates the principal advantages of point projection over traditional area backlighting in that it requires far less backlighter energy and produces data that are free from residual backlighter plasma structure. Moreover, the use of pinholes overcomes the usual disadvantages of point projection from pin targets, namely, degradation of spatial resolution and cooling due to plasma expansion.

High-Energy X-ray Microscopy Techniques for Laser-Fusion Plasma Research at the National Ignition Facility.

Multi-kilo-electron-volt x-ray microscopy will be an important laser-produced plasma diagnostic at future megajoule facilities such as the National Ignition Facility (NIF). However, laser energies and plasma characteristics imply that x-ray microscopy will be more challenging at NIF than at existing facilities. We use analytical estimates and numerical ray tracing to investigate several instrumentation options in detail, and we conclude that near-normal-incidence single spherical or toroidal crystals may offer the best general solution for high-energy x-ray microscopy at NIF and similar large facilities. Apertured Kirkpatrick-Baez microscopes using multilayer mirrors may also be good options, particularly for applications requiring one-dimensional imaging over narrow fields of view.