Topography of bent crystals with urad resolution in one dimension.
Abstract not provided.
Harding, Eric↗
Engineering topics
Pereira, N. R.
Publications and source records attributed to Pereira, N. R..
Detailed diffraction imaging of x-ray optics crystals with synchrotron radiation
Rocking curve topography at the Advanced Photon Source’s beamline 1-BM measures the x-ray reflection from large (many cm 2 ) flat crystals on a sub-mm scale with microradian angular resolution. The $(01\bar{1}1)$ reflection at 8 keV is uniform across the crystal and close to theory for three thick quartz wafers well-polished with increasingly finer grit. However, the reflection is non-uniform for some ~ 0.1mmthin, bendable crystals that are made flat by optical contact with a flat substrate. Here, these thin crystals are bent to serve in certain x-ray diagnostics of plasmas, and similar non-uniformities could then occur in bent crystals as well. The same detail in x-ray reflection in bent crystals is unachievable with the existing topography setup: one way to get the desired resolution is with a standard microfocusing approach.
47 OTHER INSTRUMENTATION↗
Characterization of shaped Bragg crystal assemblies for narrowband x-ray imaging
X-ray imaging using shaped crystals in Bragg reflection is a powerful technique used in high-energy-density physics experiments. The characterization of these crystal assemblies with conventional x-ray sources is very difficult because of the required angular resolution of the order of ~10 μrad and the narrow bandwidth of the crystal. The 10-J, 1-ps Multi-Terawatt (MTW) laser at the Laboratory for Laser Energetics was used to characterize a set of Bragg crystal assemblies. The small spot size (of the order of 5 μm) and the high power (>10 18 W/cm 2 ) of this laser make it possible to measure the spatial resolution at the intended photon energy. A set of six crystals from two different vendors was checked on MTW, showing an unexpectedly large variation in spatial resolution of up to a factor of 4.
47 OTHER INSTRUMENTATION↗
On evaluating x-ray imaging crystals with synchrotron radiation
Bent crystals are central to 1-D resolved x-ray spectroscopy and two-dimensional x-ray imaging of plasmas. Nominally identical crystals may differ in their performance with x-rays even though their visible-light images are close to identical. The detailed information needed to understand those differences, viz., the crystal’s local diffraction properties over the area that contributes to the image, is averaged out in the imaging tests that ideally precede the actual crystal’s use. Here the local diffraction of spherically bent crystals made from quartz is examined in the x-ray topography setup at the x-ray optics testing beamline 1-BM at the Advanced Photon Source, with radiation obtained from a flat quartz conditioning crystal made for the purpose.
Pereira, N. R.↗
Note: Narrow x-ray reflections are easier to locate with sandpaper
Here, synchrotrons can provide almost perfectly unidirectional and monochromatic x-rays. Such x-rays reflect from ideal crystals only over a minute part of the angular range that must be searched for the reflection. Spoiling the incoming x-rays' directionality with sandpaper makes it easier to find the reflection.
47 OTHER INSTRUMENTATION↗
Spatially resolved single crystal x-ray spectropolarimetry of wire array z-pinch plasmas
This journal article describes the design, development, and implementation of a single-crystal x-ray spectropolarimeter. The article also reports polarization sensitive spectra for He-like Al x-ray lines emitted from hot dense plasma formed from wire-array loads driven by the z-pinch pulsed power machine at the University of Nevada, Reno Nevada Terawatt Facility.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Spatial resolution of a spherical x-ray crystal spectrometer at various magnifications
A high spatial resolution of a few μm is often required for probing small-scale high-energy-density plasmas using high resolution x-ray imaging spectroscopy. This resolution can be achieved by adjusting system magnification to overcome the inherent limitation of the detector pixel size. Laboratory experiments on investigating the relation between spatial resolution and system magnification for a spherical crystal spectrometer are presented. Tungsten Lβ{sub 2} rays from a tungsten-target micro-focus x-ray tube were diffracted by a Ge 440 crystal, which was spherically bent to a radius of 223 mm, and imaged onto an x-ray CCD with 13-μm pixel size. The source-to-crystal (p) and crystal-to-detector (q) distances were varied to produce spatial magnifications (M = q/p) ranging from 2 to 10. The inferred instrumental spatial width reduces with increasing system magnification M. However, the experimental measurement at each M is larger than the theoretical value of pixel size divided by M. Future work will focus on investigating possible broadening mechanisms that limit the spatial resolution.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Spherical quartz crystals investigated with synchrotron radiation
The quality of x-ray spectra and images obtained from plasmas with spherically bent crystals depends in part on the crystal's x-ray diffraction across the entire crystal surface. We employ the energy selectivity and high intensity of synchrotron radiation to examine typical spherical crystals from alpha-quartz for their diffraction quality, in a perpendicular geometry that is particularly convenient to examine sagittal focusing. The crystal's local diffraction is not ideal: the most noticeable problems come from isolated regions that so far have failed to correlate with visible imperfections. In conclusion, excluding diffraction from such problem spots has little effect on the focus beyond a decrease in background.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Measurements of the energy spectrum of electrons emanating from solid materials irradiated by a picosecond laser
In this work, we present the results of experiments observing the properties of the electron stream generated laterally when a laser irradiates a metal. We find that the directionality of the electrons is dependent upon their energies, with the higher-energy tail of the spectrum (∼1 MeV and higher) being more narrowly focused. This behavior is likely due to the coupling of the electrons to the electric field of the laser. The experiments are performed by using the Titan laser to irradiate a metal wire, creating the electron stream of interest. These electrons propagate to nearby spectator wires of differing metals, causing them to fluoresce at their characteristic K-shell energies. This fluorescence is recorded by a crystal spectrometer. By varying the distances between the wires, we are able to probe the divergence of the electron stream, while by varying the medium through which the electrons propagate (and hence the energy-dependence of electron attenuation), we are able to probe the energy spectrum of the stream.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
X-ray polarization splitting by a single crystal evaluated with synchrotron x-rays
In hexagonal crystals such as quartz, an asymmetric Bragg reflection from two equivalent internal crystal planes can separate unpolarized x-rays into two linearly polarized components. The perfectly polarized and tunable x-rays from a synchrotron are ideal to evaluate polarization spitting in detail. One unanticipated feature is that additional reflections from the crystal affect the diffraction intensity of the two polarized components, an effect that is unlikely to matter in polarization spectroscopy of radiating plasmas for which the crystal is intended.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Ionization energy shift of characteristic K x-ray lines from high-Z materials for plasma diagnostics
The energy of the characteristic x-rays emitted by high atomic number atoms in a plasma that contains energetic electrons depends on the atom's ionization. For tungsten, the ionization energy shift of the L-lines has recently been used to diagnose the plasma's ionization; the change in energy of a K-line has been measured for iridium and observed for ytterbium. Here, we present detailed computations of the ionization energy shift to K-lines of these and an additional element, dysprosium; for these atoms, some K-lines nearly coincide in energy with K-edges of slightly lower Z atoms so that a change in transmission behind a K-edge filter betrays a change in energy. The ionization energy shift of such high-energy K-lines may enable a unique diagnostic when the plasma is inside an otherwise opaque enclosure such as hohlraums used on the National Ignition Facility.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Radiation damping of long, finite-amplitude internal waves
Numerical solutions of a damped, nonlinear wave equation are presented. The equation describes the propagation of waves in a narrow thermocline or inversion which lose energy by exciting internal waves in the weakly stratified ambient environment. The results provide estimates for the persistence of finite-amplitude internal waves propagating in a thermoclinic waveguide.
Pereira, N. R.↗
Internal wave solitons
Attention is given to the Benjamin-Ono equation for waves within a stratified fluid, i.e., internal waves. Numerical computations indicate soliton-like behavior since solitary waves pass through each other upon collision. In addition, two and three Lorentzian solitons are noted to pass through one another. An initial Lorentzian having an amplitude larger than soliton amplitude is observed to decay into solitons. The velocities of these solitons may be predicted by conservation laws. Future work will be directed toward determining exact solutions.
Meiss, J. D.↗