Session GO4: Opacity, X-ray Spectroscopy and Radiation Shocks and Flow

Iron Opacity Measurements on the National Ignition Facility (NIF)

The opacity of iron helps regulate the transfer of energy out from the center of the sun, particularly at the radiative-convective zone boundary at temperatures between 150 and 200 eV. Recent experiments at Sandia National Laboratory's Z machine have shown as much as a factor of two discrepancy between calculated and measured iron opacities. To reconcile this discrepancy a platform for measuring iron opacities at solar conditions has been developed on the NIF. Initial measurements on iron opacities have been obtained at temperatures of about 150 eV. The temperature and density of the iron plasma have also been measured. The accuracy of the measurements and a comparison to results from Z will be presented. Plans for extending the measurements to higher temperatures will also be discussed.   

High-Resolving-Power, Streaked X-Ray Spectroscopy on the OMEGA EP Laser System

A high-resolving-power, streaked x-ray spectrometer is being developed and tested on the OMEGA EP Laser System to study temperature-equilibration dynamics in rapidly heated metal. The instrument is based on two diagnostic channels, each with a spherical Bragg crystal. Channel 1 couples a spherical Si220 crystal to an x-ray streak camera. Channel 2 couples a second, identical crystal to an x-ray charge-coupled device, allowing for photometric calibration of the time-resolved spectrum. The instrument covers the spectral range of 7.97 to 8.11 keV, centered on the Cu Kα1 line at 8.05 keV. The time-resolved spectrometer is designed to achieve a resolving power of 2000 and a temporal resolution of 2 ps. The instrument capabilities are demonstrated by resolving the Cu Kα1,2 doublet on high-power shots. Time-resolved Cu Kα spectra will be presented for a wide range of high-power laser and target interactions, where heating and Kα emission is generated by hot-electron-energy deposition.    

Inverse Problem Instabilities in Large-Scale Plasma Modelling

Our understanding of physical systems generally depends on our ability to match complex computational modelling with measured experimental outcomes. However, simulations with large parameter spaces suffer from inverse problem instabilities, where similar simulated outputs can map back to very different sets of input parameters. While of fundamental importance, such instabilities are seldom resolved due to the intractably large number of simulations required to comprehensively explore parameter space. Here we show how Bayesian machine learning can be used to address inverse problem instabilities, and apply it to two popular experimental diagnostics in plasma physics: x-ray Thomson scattering and x-ray emission spectroscopy. We find that the extraction of information from measurements simply on the basis of agreement with simulations is unreliable, and leads to a significant underestimation of uncertainties. We describe how to statistically quantify the effect of unstable inverse models, and describe an approach to experimental design that mitigates its impact.
  

Observation of Stagnation in Counter-Streaming Laser-Created Plasmas with Optical Thomson Scattering

We present optical Thomson scattering data from counter-streaming plasma jet experiments and argue that the observed slowing is due to stagnation.  The jets were created on the OMEGA laser (Laboratory for Laser Energetics, Rochester, NY) by rear-irradiating thin acrylic cones.  The resulting jets were probed with a 526.5 nm Thomson beam in both single jet and head-on configurations.  From the single jet Thomson measurements, we determined that over the timeframe of 12 to 18 ns, mass density increased from 10^-7 to 10^-4 gm/cc while velocity fell from 300 to 100 km/s.  Using these plasma parameters, we find that the interpenetration-to-stagnation transition occurred when the ion-ion mean free path was roughly 1 mm.
  

Measurement of Plasma Conditions at Shock Collapse on OMEGA

A platform on the OMEGA laser at the Laboratory for Laser Energetics is used to measure the plasma conditions at shock collapse in a solid deuterated plastic (CD) ball. A single spherically symmetric shock is driven using the OMEGA laser while observing the self-emission from the collapse. The observations are used to deduce temperatures and densities that are predicted to be of the order of 10^7 K and several ten’s of g/cc, respectively, by simulations. The primary observations include temporally, spatially, and spectrally resolved x-ray self-emission measurements and measurements of the D–D fusion neutrons. These measurements are used in conjunction to understand the conditions in the dense fusing plasma created around the time of shock collapse.