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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Mono-energetic ion beam acceleration in solitary waves during relativistic transparency using high-contrast circularly polarized short-pulse laser and nanoscale targets

In recent experiments at the Trident laser facility, quasi-monoenergetic ion beams have been obtained from the interaction of an ultraintense, circularly polarized laser with a diamond-like carbon target of nm-scale thickness under conditions of ultrahigh laser pulse contrast. Kinetic simulations of this experiment under realistic laser and plasma conditions show that relativistic transparency occurs before significant radiation pressure acceleration and that the main ion acceleration occurs after the onset of relativistic transparency. Associated with this transition are a period of intense ion acceleration and the generation of a new class of ion solitons that naturally give rise to quasi-monoenergetic ion beams. An analytic theory has been derived for the properties of these solitons that reproduces the behavior observed in kinetic simulations and the experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

High resolution, large field of view x-ray differential phase contrast imaging on a compact setup

X-ray grating interferometry is a well established technique to perform differential phase contrast imaging on conventional x-ray tubes. So far, the application of this technique in commercial micro computed tomography scanners has remained a major challenge due to the compact setup geometry. In this letter, we report on the design of a compact imaging setup using a microfocus source. Due to the extreme wave front curvature, the gratings are fabricated on a flexible substrate, enabling precise cylindrical shaping. A laboratory setup and a modified SCANCO {mu}CT100 scanner have been built, allowing high resolution and large field of view imaging.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Hard X-ray Phase-Contrast Tomographic Nanoimaging

Synchrotron-based full-field tomographic microscopy established itself as a tool for noninvasive investigations. Many beamlines worldwide routinely achieve micrometer spatial resolution while the isotropic 100-nm barrier is reached and trespassed only by few instruments, mainly in the soft x-ray regime. We present an x-ray, full-field microscope with tomographic capabilities operating at 10 keV and with a 3D isotropic resolution of 144 nm recently installed at the TOMCAT beamline of the Swiss Light Source. Custom optical components, including a beam-shaping condenser and phase-shifting dot arrays, were used to obtain an ideal, aperture-matched sample illumination and very sensitive phase-contrast imaging. The instrument has been successfully used for the nondestructive, volumetric investigation of single, unstained cells.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Fast ion acceleration from thin foils irradiated by ultra-high intensity, ultra-high contrast laser pulses

Ion acceleration resulting from the interaction of ultra-high intensity (2 x 10{sup 20 }W/cm{sup 2}) and ultra-high contrast ({approx}10{sup 10}) laser pulses with 0.05-10 {mu}m thick Al foils at normal (0 deg.) and 35 deg. laser incidence is investigated. When decreasing the target thickness from 10 {mu}m down to 0.05 {mu}m, the accelerated ions become less divergent and the ion flux increases, particularly at normal (0 deg.) laser incidence on the target. A laser energy conversion into protons of {approx}6.5% is estimated at 35 deg. laser incidence. Experimental results are in reasonable agreement with theoretical estimates and can be a benchmark for further theoretical and computational work.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Simultaneous generation of ions and high-order harmonics from thin conjugated polymer foil irradiated with ultrahigh contrast laser

We report the manufacturing of an (ultra-)thin foil target made of conjugated polymer, poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT), and the simultaneous observation of laser-accelerated ions and second harmonic radiation, when irradiated with ultrahigh-contrast laser pulse at a maximum intensity of 4 x 10{sup 19 }W/cm{sup 2}. Maximum proton energy of 8 MeV is achieved along the target normal direction. Strong second harmonic with over 6% energy ratio compared to fundamental is emitted along the specular direction. Two-dimensional particle-in-cell simulations confirm the simultaneous generation of protons and high-order harmonics, which demonstrates the feasibility of applications requiring particle and radiation sources at once, effectively using the same laser and target.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

ICRF Mode Conversion Studies with Phase Contrast Imaging and Comparisons with Full-Wave Simulations

Waves in the ion cyclotron range of frequencies (ICRF) are widely used to heat toka-mak plasmas. In a multi-ion-species plasma, the FW converts to ion cyclotron waves (ICW) and ion Bernstein waves (IBW) around the ion-ion hybrid resonance (mode conversion). The mode converted wave is of interest as an actuator to optimise plasma performance through flow drive and current drive. Numerical simulations are essential to describe these processes accurately, and it is important that these simulation codes be validated. On Alcator C-Mod, direct measurements of the mode converted waves have been performed using Phase Contrast Imaging (PCI), which measures the line-integrated electron density fluctuations. The results were compared to full-wave simulations AORSA and TORIC. AORSA is coupled to a Fokker-Planck code CQL3D for self-consistent simulation of the wave electric field and the minority distribution function. The simulation results are compared to PCI measurements using synthetic diagnostic. The experiments were performed in D-H and D-{sup 3}He plasmas over a wide range of ion species concentrations. The simulations agreed well with the measurements in the strong absorption regime. However, the measured fluctuation intensity was smaller by 1-2 orders of magnitudes in the weakly abosorbing regime, and a realistic description of the plasma edge including dissipation and antenna geometry may be required in these cases.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Investigation of a high spatial resolution method based on polar coordinate maximum entropy method for analyzing electron density fluctuation data measured by laser phase contrast

Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Fabrication and characterization of analyzer gratings with high aspect ratios for phase contrast imaging using a Talbot interferometer

In recent years, X-ray imaging based on the differential phase contrast gained more and more attention to be used in X-ray imaging. Among other techniques like crystal-based diffraction and propagation methods, the grating based Talbot interferometry offers an approach measuring phase modulations of X-rays while passing low absorbing objects. The Talbot interferometer yields for highly efficient X-ray imaging signals for hard X-rays with energies above 10 keV. One factor with high impact on the imaging performance of such grating interferometers is the gratings quality. We introduce a fabrication process allowing both, the fabrication of phase modulating and analyzer gratings with high aspect ratios, up to 100. Structural deviations from the optimal geometry of the gratings are investigated and their influence on the obtained image quality is discussed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Gas gun shock experiments with single-pulse x-ray phase contrast imaging and diffraction at the Advanced Photon Source

The highly transient nature of shock loading and pronounced microstructure effects on dynamic materials response call for in situ, temporally and spatially resolved, x-ray-based diagnostics. Third-generation synchrotron x-ray sources are advantageous for x-ray phase contrast imaging (PCI) and diffraction under dynamic loading, due to their high photon fluxes, high coherency, and high pulse repetition rates. The feasibility of bulk-scale gas gun shock experiments with dynamic x-ray PCI and diffraction measurements was investigated at the beamline 32ID-B of the Advanced Photon Source. The x-ray beam characteristics, experimental setup, x-ray diagnostics, and static and dynamic test results are described. We demonstrate ultrafast, multiframe, single-pulse PCI measurements with unprecedented temporal (<100 ps) and spatial ({approx}2 {mu}m) resolutions for bulk-scale shock experiments, as well as single-pulse dynamic Laue diffraction. The results not only substantiate the potential of synchrotron-based experiments for addressing a variety of shock physics problems, but also allow us to identify the technical challenges related to image detection, x-ray source, and dynamic loading.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

The spectra of the multicharged argon hollow ions: Observation, modeling and using for diagnostics of the early stage of the heating of clusters by a super high contrast femtosecond laser pulses

A study is made of the ultra-short laser pulse irradiation of Ar cluster targets. Experiments have been performed with large cluster sizes and using very high laser contrasts, which have allowed clear and unambiguous observation of exotic inner-shell transitions in near-neutral Ar ions. The interaction of the main laser pulse with the unperturbed target is a necessary requirement for observing these lines. Our measurements are supported by kinetics calculations in which a very detailed atomic model is used. The calculations predict all of the spectral features found experimentally, and support the notion that the X-ray emission arises from many ion stages of the Ar plasma, from near-neutral through He-like ions, and from a range of plasma temperatures and densities. Differences between X-ray argon clusters excited at the laser-cluster and laser-droplet interactions have been analyzed. X-ray spectral methods have been proposed to determine the parameters of the plasma formed at the early stages of its evolution. It has been shown that the spectra of hollow ions are the most informative in the first moments of the heating of a cluster, whereas the diagnostics of the late stages can be performed using the conventional lines of multicharged ions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Towards nanometer period gratings for hard x-ray phase-contrast imaging

Transmission grating-based x-ray interferometers have been developed into wide-field imaging devices that are sensitive to x-ray refraction and diffraction in the sample. While current grating designs rely on UV and x-ray lithography processes to produce periodic vertical structures, it becomes prohibitively difficult to make the grating periods below 1 - 2 microns due to the high aspect ratios of the structures. Since the phase-contrast sensitivity is inversely related to the grating period, we describe a new grating design for sub-micron to nanometer grating periods. In this design, multiple bi-layers of two alternating materials are deposited on a stair like substrate, and mostly on the floor surfaces of the steps only. The incident x-ray beam is parallel to the planes of the layers (side illumination). Thus, the multilayer structure on each step serves as a micro grating whose grating period is the thickness of a bi-layer. The array of micro gratings over the whole length of the stair can act as a single continuous grating, when certain continuity conditions between neighboring steps are met. Since the layer thickness can be as small as tens of nanometers, as has been demonstrated in multilayer x-ray zone plates, this design allows nanometer grating periods over large grating areas. Here we describe a prototype intensity grating of 440 nm period. We show x-ray projection images of the grating which were obtained by contact lithography.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Measurements of ion cyclotron range of frequencies mode converted wave intensity with phase contrast imaging in Alcator C-Mod and comparison with full-wave simulations

Radio frequency waves in the ion cyclotron range of frequencies (ICRF) are widely used to heat tokamak plasmas. In ICRF heating schemes involving multiple ion species, the launched fast waves convert to ion cyclotron waves or ion Bernstein waves at the two-ion hybrid resonances. Mode converted waves are of interest as actuators to optimise plasma performance through current drive and flow drive. In order to describe these processes accurately in a realistic tokamak geometry, numerical simulations are essential, and it is important that these codes be validated against experiment. In this study, the mode converted waves were measured using a phase contrast imaging technique in D-H and D-{sup 3}He plasmas. The measured mode converted wave intensity in the D-{sup 3}He mode conversion regime was found to be a factor of {approx}50 weaker than the full-wave predictions. The discrepancy was reduced in the hydrogen minority heating regime, where mode conversion is weaker.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Ionization effects in the generation of wake-fields by ultra-high contrast femtosecond laser pulses in argon gas

Difference in mechanisms of wake-field generation and electron self-injection by high contrast femtosecond laser pulses in an initially neutral Argon gas and in pre-ionized plasma without ionization is studied via 2D particle-in-cell simulations including optical ionization of the media. For shorter laser pulses, 40 fs, ionization results only in an increase of the charge of accelerated electrons by factor of {approx}3 with qualitatively the same energy distribution. For longer pulses, 80 fs, a more stable wake field structure is observed in the neutral gas with the maximal energy of the accelerated electrons exceeding that in the fixed density plasma. In higher density Argon, an ionizing laser pulse converts itself to a complex system of solitons at a self-induced, critical density ramp.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Source effects in analyzer-based X-ray phase contrast imaging with conventional sources

Several recent papers have shown the implementation of analyzer based X-ray phase contrast imaging (ABI) with conventional X-ray sources. The high flux is always a requirement to make the technique useful for bio-medical applications. Here, we present and discuss three important parameters, which need to be taken into account, when searching for the high flux ABI: anisotropic magnification, double image, and source size spread due to intrinsic dispersive diffraction by asymmetrically cut crystals. These parameters, if not well optimized, may cause important features in the acquired images which can mislead the interpretation. A few ways to minimize these effects are implemented and discussed, including some experimental results.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

High speed synchrotron x-ray phase contrast imaging of dynamic material response to split Hopkinson bar loading

The successful process of amalgamating both the time-resolved imaging capabilities present at the Advanced Photon Source beamline 32ID-B and the proficiency of high-rate loading offered by the split Hopkinson or Kolsky compression/tension bar apparatus is discussed and verification of system effectiveness is expressed via dynamic experiments on various material systems. Single particle sand interaction along with glass cracking during dynamic compression, and fiber-epoxy interfacial failure, ligament-bone debonding, and single-crystal silicon fragmentation due to dynamic tension, were imaged with 0.5 {mu}s temporal resolution and {mu}m-level spatial resolution. Synchrotron x-ray phase contrast imaging of said material systems being loaded with the Kolsky bar apparatus demonstratively depicts the effectiveness of the novel union between these two powerful techniques, thereby allowing for in situ analysis of the interior of the material system during high-rate loading for a variety of applications.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

High-resolution and high-sensitivity phase-contrast imaging by focused hard x-ray ptychography with a spatial filter

We demonstrate high-resolution and high-sensitivity x-ray phase-contrast imaging of a weakly scattering extended object by scanning coherent diffractive imaging, i.e., ptychography, using a focused x-ray beam with a spatial filter. We develop the x-ray illumination optics installed with the spatial filter to collect coherent diffraction patterns with a high signal-to-noise ratio. We quantitatively visualize the object with a slight phase shift ({approx}{lambda}/320) at spatial resolution better than 17 nm in a field of view larger than {approx}2 Multiplication-Sign 2{mu}m{sup 2}. The present coherent method has a marked potential for high-resolution and wide-field-of-view observation of weakly scattering objects such as biological soft tissues.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Electron channeling contrast imaging studies of nonpolar nitrides using a scanning electron microscope

Threading dislocations, stacking faults, and associated partial dislocations significantly degrade the optical and electrical properties of materials such as non-polar III-nitride semiconductor thin films. Stacking faults are generally difficult to detect and quantify with existing characterization techniques. We demonstrate the use of electron channeling contrast imaging in the scanning electron microscope to non-destructively reveal basal plane stacking faults terminated by partial dislocations in m-plane GaN and InGaN/GaN multiple quantum well structures grown on {gamma}-LiAlO{sub 2} by metal organic vapor phase epitaxy.

36 MATERIALS SCIENCE↗
DOE OSTI2013

High contrast ion acceleration at intensities exceeding 10{sup 21} Wcm{sup −2}

Ion acceleration from short pulse laser interactions at intensities of 2×10{sup 21}Wcm{sup −2} was studied experimentally under a wide variety of parameters, including laser contrast, incidence angle, and target thickness. Trends in maximum proton energy were observed, as well as evidence of improvement in the acceleration gradients by using dual plasma mirrors over traditional pulse cleaning techniques. Extremely high efficiency acceleration gradients were produced, accelerating both the contaminant layer and high charge state ions from the bulk of the target. Two dimensional particle-in-cell simulations enabled the study of the influence of scale length on submicron targets, where hydrodynamic expansion affects the rear surface as well as the front. Experimental evidence of larger electric fields for sharp density plasmas is observed in simulation results as well for such targets, where target ions are accelerated without the need for contaminant removal.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗