Engility (United States)

We investigate the effect of nanosecond pulsed electric fields on optical breakdown thresholds for picosecond optical pulses (6ps pulse duration at wavelength of 1064nm) in aqueous solutions. Optical breakdown thresholds are determined both with and without the application of the nanosecond pulsed electric fields in water, phosphate-buffered saline solution, and a typical buffer solution used for live cell imaging. As optical breakdown at this time-scale mostly on a combination of multi-photon ionization as well as free electrons in the focal volume to achieve an avalanche or cascade ionization phenomena, we hypothesize that the application of a strong external electric field will have a significant impact on the breakdown probability at the lower-end of pulse energy levels, effectively modifying the pulse-energy breakdown threshold of the materials. Single optical pulses were triggered at specific times with respect to the leadingedge of the applied electric field to determine if there were any time-dependent effects on breakdown thresholds. Results of the experiments are examined and the physical phenomena as well as the concept “electrostatic purification” are discussed.

Understanding the optical properties of water is critical to both laser-tissue interactions as well as setting ocular laser safety standards. The nonlinear properties of water are responsible for supercontinuum generation; however, these effects are poorly understood for wavelengths longer than 1064 nm. A previous study suggested that the supercontinuum generation may convert retinal-safe femtosecond near-infrared pulses with wavelengths longer than 1064 nm into visible wavelength pulses that are above the maximum permissible exposure limit as defined by ANSI Z136.1-2014. To address this knowledge gap, we extend the Z-scan technique in distilled water to wavelengths between 1150 nm to 1400 nm, where linear absorption is strong. Utilizing wavelength tunable, nominally 100 fs laser pulses, we observe wavelength dependence of the nonlinear optical properties of water. The nonlinear refractive index at 1150 nm was consistent with measurements taken at 532 nm in previous studies, and was observed to increase at longer wavelengths. The nonlinear absorption was positive for wavelengths between 1150 nm and 1350 nm and reversed to saturable absorption at 1400 nm. Saturable absorption poses a previously unanticipated eye safety risk as current ocular laser safety standards assume strong absorption at 1400 nm. These results expand our current understanding of the nonlinear optical properties of water to wavelengths in the 1150 nm to 1400 nm region, and inform efforts to revise national and international exposure limits to account for retinal hazards due to nonlinear effects.

Abstract Currently, Technology Readiness Assessments (TRAs) are used in determining the maturity of the Critical Technology Elements (CTEs) of a system as it moves forward in the system development life cycle. The TRA method uses Technology Readiness Levels (TRLs) as the decision metric. TRL values are assessed and determined by Subject Matter Experts (SMEs). Since expert evaluators often differ in their judgment when scoring a system element against the TRL scale criteria, this paper argues for the use of a Bayesian network model to provide a mathematical method to consistently combine and validate the judgment of these SMEs and increase the confidence in the determination of the readiness of system components and their technologies.

Pulsed infrared light can excite action potentials in neurons; yet, the fundamental mechanism underlying this phenomenon is unknown. Previous work has observed a rise in intracellular calcium concentration following infrared exposure, but the source of the calcium and mechanism of release is unknown. Here, we used fluorescence lifetime imaging of Oregon Green BAPTA-1 to study intracellular calcium dynamics in primary rat hippocampal neurons in response to infrared light exposure. The fluorescence lifetime of Oregon Green BAPTA-1 is longer when bound to calcium, and allows robust measurement of intracellular free calcium concentrations. First, a fluorescence lifetime calcium calibration curve for Oregon Green BAPTA-1 was determined in solutions. The normalized amplitude of the short and long lifetimes was calibrated to calcium concentration. Then, neurons were incubated in Oregon Green BAPTA-1 and exposed to pulses of infrared light (0-1 J/cm2; 0-5 ms; 1869 nm). Fluorescence lifetime images were acquired prior to, during, and after the infrared exposure. Fluorescence lifetime images, 64x64 pixels, were acquired at 12 or 24 ms for frame rates of 83 and 42 Hz, respectively. Accurate α₁approximations were achieved in images with low photon counts by computing an α₁ index value from the relative probability of the observed decay events. Results show infrared light exposure increases intracellular calcium in neurons. Altogether, this study demonstrates accurate fluorescence lifetime component analysis from low-photon count data for improved imaging speed.

Abstract. A climate model represents a multitude of processes on a variety of timescales and space scales: a canonical example of multi-physics multi-scale modeling. The underlying climate system is physically characterized by sensitive dependence on initial conditions, and natural stochastic variability, so very long integrations are needed to extract signals of climate change. Algorithms generally possess weak scaling and can be I/O and/or memory-bound. Such weak-scaling, I/O, and memory-bound multi-physics codes present particular challenges to computational performance. Traditional metrics of computational efficiency such as performance counters and scaling curves do not tell us enough about real sustained performance from climate models on different machines. They also do not provide a satisfactory basis for comparative information across models. codes present particular challenges to computational performance. We introduce a set of metrics that can be used for the study of computational performance of climate (and Earth system) models. These measures do not require specialized software or specific hardware counters, and should be accessible to anyone. They are independent of platform and underlying parallel programming models. We show how these metrics can be used to measure actually attained performance of Earth system models on different machines, and identify the most fruitful areas of research and development for performance engineering. codes present particular challenges to computational performance. We present results for these measures for a diverse suite of models from several modeling centers, and propose to use these measures as a basis for a CPMIP, a computational performance model intercomparison project (MIP).

Radiation-magnetohydrodynamic simulations using the non-local thermodynamic equilibrium Mach2-Tabular Collisional-Radiative Equilibrium code in (r, z) geometry are performed for two pairs of recent Ar gas-puff Z-pinch experiments on the refurbished Z generator with an 8 cm diameter nozzle. One pair of shots had an outer-to-inner shell mass ratio of 1:1.6 and a second pair had a ratio of 1:1. In each pair, one of the shots had a central jet. The experimental trends in the Ar K-shell yield and power are reproduced in the calculations. However, the K-shell yield and power are significantly lower than the other three shots for the case of a double-shell puff of 1:1 mass ratio and no central jet configuration. Further simulations of a hypothetical experiment with the same relative density profile of this configuration, but higher total mass, show that the coupled energy from the generator and the K-shell yield can be increased to levels achieved in the other three configurations, but not the K-shell power. Based on various measures of effective plasma radius, the compression in the 1:1 mass ratio and no central jet case is found to be less because the plasma inside the magnetic piston is hotter and of lower density. Because of the reduced density, and the reduced radiation cooling (which is proportional to the square of the density), the core plasma is hotter. Consequently, for the 1:1 outer-to-inner shell mass ratio, the load mass controls the yield and the center jet controls the power.