Direct Measurement of Ice-Ablator Interface Motion for Instability Mitigation in Indirect Drive ICF Implosions.

In indirect drive inertial confinement fusion (ICF) implosions hydrodynamic instability growth at the imploding capsule ablator-DT fuel interface can reduce fuel compressibility and inject ablator into the hot spot hence reducing hot spot pressure and temperature. As a mitigation strategy, a gentle acceleration of this interface is predicted by simulations and theory to significantly reduce this instability growth in the early stage of the implosion. We have performed high-contrast, time-resolved x-ray refraction enhanced radiography (RER) to accurately measure the level of acceleration as a function of the initial laser drive time history for indirect-drive implosions on the National Ignition Facility. We demonstrate a transition from no acceleration to 20±1.8 µm ns^{-2} acceleration by tweaking the drive that should reduce the initial instabilities by an order of magnitude at high modes.

Performance of a hardened x-ray streak camera at Lawrence Livermore National Laboratory's National Ignition Facility.

Electron tubes continue to provide the highest speeds possible for recording dynamics of hot high-energy density plasmas. Standard streak camera drive electronics and CCD readout are not compatible with the radiation environment associated with high DT fusion yield inertial confinement fusion experiments >1013 14 MeV DT neutrons or >109 n cm-2 ns-1. We describe a hardened x-ray streak camera developed for the National Ignition Facility and present preliminary results from the first experiment on which it has participated, recording the time-resolved bremsstrahlung spectrum from the core of an inertial confinement fusion implosion at more than 40× the operational neutron yield limit of the previous National Ignition Facility x-ray streak cameras.

Simultaneous compression and opacity data from time-series radiography with a Lagrangian marker.

Time-resolved radiography can be used to obtain absolute shock Hugoniot states by simultaneously measuring at least two mechanical parameters of the shock, and this technique is particularly suitable for one-dimensional converging shocks where a single experiment probes a range of pressures as the converging shock strengthens. However, at sufficiently high pressures, the shocked material becomes hot enough that the x-ray opacity falls significantly. If the system includes a Lagrangian marker such that the mass within the marker is known, this additional information can be used to constrain the opacity as well as the Hugoniot state. In the limit that the opacity changes only on shock heating, and not significantly on subsequent isentropic compression, the opacity of the shocked material can be determined uniquely. More generally, it is necessary to assume the form of the variation of opacity with isentropic compression or to introduce multiple marker layers. Alternatively, assuming either the equation of state or the opacity, the presence of a marker layer in such experiments enables the non-assumed property to be deduced more accurately than from the radiographic density reconstruction alone. An example analysis is shown for measurements of a converging shock wave in polystyrene at the National Ignition Facility.

Absolute Hugoniot measurements from a spherically convergent shock using x-ray radiography.

The canonical high pressure equation of state measurement is to induce a shock wave in the sample material and measure two mechanical properties of the shocked material or shock wave. For accurate measurements, the experiment is normally designed to generate a planar shock which is as steady as possible in space and time, and a single state is measured. A converging shock strengthens as it propagates, so a range of shock pressures is induced in a single experiment. However, equation of state measurements must then account for spatial and temporal gradients. We have used x-ray radiography of spherically converging shocks to determine states along the shock Hugoniot. The radius-time history of the shock, and thus its speed, was measured by radiographing the position of the shock front as a function of time using an x-ray streak camera. The density profile of the shock was then inferred from the x-ray transmission at each instant of time. Simultaneous measurement of the density at the shock front and the shock speed determines an absolute mechanical Hugoniot state. The density profile was reconstructed using the known, unshocked density which strongly constrains the density jump at the shock front. The radiographic configuration and streak camera behavior were treated in detail to reduce systematic errors. Measurements were performed on the Omega and National Ignition Facility lasers, using a hohlraum to induce a spatially uniform drive over the outside of a solid, spherical sample and a laser-heated thermal plasma as an x-ray source for radiography. Absolute shock Hugoniot measurements were demonstrated for carbon-containing samples of different composition and initial density, up to temperatures at which K-shell ionization reduced the opacity behind the shock. Here we present the experimental method using measurements of polystyrene as an example.

Characterization of the preformed plasma for high-intensity laser-plasma interaction.

The interaction of a very intense, very short laser pulse is modified by the presence of a preformed plasma prior to the main short pulse. The preformed plasma is created by a small prepulse interacting with the target prior to the main pulse. The prepulse has been monitored using a water-cell-protected fast photodiode allowing on every shot a high dynamic measurement of the pulse profile. Simultaneously we have used time-resolved interferometry to look at the preformed plasma on a 300 TW, 700 fs laser. The two-dimensional density maps obtained have been compared with two-dimensional hydrodynamic simulations.

Density measurement of shock compressed foam using two-dimensional x-ray radiography.

We have used spherically bent quartz crystal to image a laser-generated shock in a foam medium. The foam targets had a density of 0.16 g/cm(3) and thickness of 150 microm, an aluminum/copper pusher drove the shock. The experiment was performed at the Titan facility at Lawrence Livermore National Laboratory using a 2 ns, 250 J laser pulse to compress the foam target, and a short pulse (10 ps, 350 J) to generate a bright Ti K alpha x-ray source at 4.5 keV to radiograph the shocked target. The crystal used gives a high resolution (approximately 20 microm) monochromatic image of the shock compressed foam.

Absolute calibration of image plates for electrons at energy between 100 keV and 4 MeV.

We measured the absolute response of image plate (Fuji BAS SR2040) for electrons at energies between 100 keV and 4 MeV using an electron spectrometer. The electron source was produced from a short pulse laser irradiated on solid density targets. This paper presents the calibration results of image plate photon stimulated luminescence per electron at this energy range. The Monte Carlo radiation transport code MCNPX results are also presented for three representative incident angles onto the image plates and corresponding electron energy depositions at these angles. These provide a complete set of tools that allows extraction of our absolute calibration to other spectrometer setting at this electron energy range.

X-ray imaging of shock waves generated by high-pressure fuel sprays.

Synchrotron x-radiography and a fast x-ray detector were used to record the time evolution of the transient fuel sprays from a high-pressure injector. A succession of 5.1-microsecond radiographs captured the propagation of the spray-induced shock waves in a gaseous medium and revealed the complex nature of the spray hydrodynamics. The monochromatic x-radiographs also allow quantitative analysis of the shock waves that has been difficult if not impossible with optical imaging. Under injection conditions similar to those found in operating engines, the fuel jets can exceed supersonic speeds and result in gaseous shock waves.