R. S. Thoe

Accurate determination of the charge state distribution in a well characterized highly ionized Au plasma

Abstract The density, temperature and charge state distribution are accurately determined in a highly ionized non-LTE Au sample. Laser heated Au microdots buried in a thin Be foil, reach temperatures of 2 keV and ionize into the M-shell. During expansion, the tamped Au samples remain uniform and in near steady-state ionization equilibrium. The electron temperature is measured with time and space resolved Thomson scattering while the density is determined from time-gated X-ray imaging the expanded Au sample. The charge state distribution is obtained from analysis of emission measurements of Au 5f–3d transition arrays in the wavelength range 3.3–3.9 A, allowing the average charge to be determined to within ∼1% accuracy.

Plasma diagnostics for x-ray driven foils at Z

We report the development of techniques to diagnose plasmas produced by x-ray photoionization of thin foils placed near the Z-pinch on the Sandia Z Machine. The development of 100+ TW x-ray sources enables access to novel plasma regimes, such as the photoionization equilibrium. To diagnose these plasmas one must simultaneously characterize both the foil and the driving pinch. The desired photoionized plasma equilibrium is only reached transiently for a 2-ns window, placing stringent requirements on diagnostic synchronization. We have adapted existing Sandia diagnostics and fielded an additional gated three-crystal Johann spectrometer with dual lines of sight to meet these requirements. We present sample data from experiments using 1-cm, 180-eV tungsten pinches to photoionize foils made of 200 A Fe and 300 A NaF co-mixed and sandwiched between 1000 A layers of Lexan (C16H14O3), and discuss the application of this work to benchmarking astrophysical models.

Towards an experimental benchmark for aluminum X-ray spectra

Abstract To test the validity of kinetics for laser-produced plasmas, one would like to measure the X-ray spectrum emitted from a plasma volume whose characteristics are determined by diagnostics that do not rely on interpreting the X-ray spectrum itself. An experimental test bed has been developed at the Janus laser at Lawrence Livermore National Laboratory to simultaneously characterize the electron temperature, the electron density and the X-ray emission from laser-irradiated aluminum dot targets. Thomson scattering, interferometry and pinhole imaging are implemented to achieve this. Further, the X-ray spectrum from 1–2 keV is spatially integrated and is obtained using a flat crystal (PET) and an X-ray streak camera for time resolution. Spectra have been calculated using the HULLAC and FLY atomic physics codes which use the measured density and temperature as input, and DCA which calculates X-ray spectra from the 2-D expanding plasma calculated by a hydrodynamics code. Comparisons of the models with the data will be discussed and future directions will be indicated.

Performance of a variable-line-spaced master reflection grating for use in the reflection grating spectrometer on the x-ray multimirror mission

The X Ray Multimirror Mission will include a spectrometer consisting of two arrays of variable line-spaced reflection gratings for use in the 350 eV to 2.5 keV energy range. Approximately 720 replica gratings will be needed for two flight grating arrays and one spare. Evaluation of potential master gratings to be used in the replication process has begun. Both reflectivity and scattering x-ray measurements for three mechanically ruled prototype master gratings have been reported.

Single-sided tomography of extremely large dense objects

One can envision many circumstances where radiography could be valuable but is frustrated by the geometry of the object to be radiographed. For example, extremely large objects, the separation of rocket propellants from the skin of solid fuel rocket motor, the structural integrity of an underground tank or hull of a ship, the location of buried objects, inspection of large castings etc. In our laboratory I have been investigating ways to do this type of radiography and as a result have developed a technique which can be used to obtain three dimensional radiographs using Compton scattered radiation from a monochromatic source and a high efficiency, high resolution germanium spectrometer. This paper will give specific details of the reconstruction technique and present the results of numerous numerical simulations and compare these simulations to spectra obtained in our laboratory. In addition I will present the results of calculations made for the development of an alternative single sided radiography technique which will permit inspection of the interior of large objects.

First measurement of backscatter dependence on ion acoustic damping in a high density helium/hydrogen laser-plasma

The dependence of stimulated backward and forward scattered light on ion acoustic damping (νi) is measured for the first time in a long scale length He/H2 composition plasma at a density of 0.08 critical for 351-nm laser light. Both the stimulated Raman and Brillouin backscattering decrease with increasing ion acoustic damping. Modeling of the backward scattering agrees with the measurements when the Langmuir and ion acoustic fluctuations saturate at δn/n=0.01 and 0.001, respectively. These low saturation levels cannot be explained using standard nonlinear wave decay saturation mechanisms and may indicate that other saturation mechanisms are active in this plasma. Modeling of the forward scattering agrees qualitatively with the measurements and provides an estimate of the density fluctuations in the plasma.

Using high resolution x-ray spectroscopy of laser and EBIT plasma sources to test atomic models

Understanding of the formation and dynamics of non-LTE plasmas is crucial to inertial and magnetic fusion energy research as well as astrophysical research. The intrinsic complexity of non-LTE systems at high density and the remote nature of x-ray emitting astrophysical plasmas present stringent challenges to our models. Confidence in our atomic kinetics models is gained through comparisons with experimental data from plasmas that are well diagnosed by techniques that are independent of spectroscopic models. To this end experiments with Au foils buried in Be targets have been designed to achieve steady-state, gradient free conditions on the Nova laser. Simulations of the experiments that use detailed atomic kinetics models have shown good agreement with measured x-ray spectra. Large sets of data and many detailed processes are included in these models; issues involving satellite configurations, accuracy of dielectronic recombination rates and non-zero optical depths for resonant lines have to be addressed...

X-Ray-Spectroscopy of Astrophysically-Relevant Photoionized Iron Plasmas at Z

In order to provide benchmark data for models used to interpret X-ray astronomy data from newly-launched orbital telescopes such as Chandra, we have used 120 TW, 180 eV pinch plasmas on the Sandia Z facility to drive iron foils into X-ray photoionized equilibrium. The experiment was designed to achieve photoionization parameters characteristic of accretion-powered objects such as X-ray binaries (neutron stars) and active galactic nuclei (black holes). These objects comprise roughly half of observed X-ray sources, but the interpretation of their spectra is difficult: state-of-the-art models for photoionized iron plasmas do not yet agree on the expected ionization balance. In our initial experiments the foil samples consisted of 200 A of iron codeposited with 300 A of sodium fluoride and sandwiched between two 1000 A layers of Lexan (CH and O). We characterized the pinch spectrum, temperature, power and uniformity and qualified it as a photoionization driver. We obtained time-integrated absorption spectra f...

Mechanically simple flat-crystal x-ray source /monochromator.

The design of a new type of flat-crystal x-ray source/monochromator is discussed. This new design has many advantages over previous designs. It is extremely easy to construct, compact, and portable. It is easy to align and may be adapted to a wide variety of detectors. Its dispersion crystal is easily changed, allowing the same instrument to be used for a very wide range of wavelengths. For example, with a crystal such as LiF (422) its operating range would be in the tens of kilovolts, whereas with a phthalate crystal its range would be from ~900 eV to ~3 keV. Furthermore the same instrument can be used with a multilayer to extend its useful range almost down to the vacuum UV.

Recent progress in single sided gamma-ray tomography

The use of scattered radiation for radiography has many potential advantages over conventional projection techniques: for high energy photons the scattering process strongly dominates all other processes. The intensity of scattered radiation is due directly to the electron density and highly insensitive to chemical composition. Finally, the use of scattered radiation allows the investigator to position the radiation source on the same side of the object as the detector. In this paper I will present some recent results of a set of measurements made with our uncollimated Compton backscattering tomography apparatus. This technique uses the Compton energy shift of scattered gamma rays to determine the scattering site. By measuring the spectrum of these scattered gamma rays it is then possible to determine the electron density of the object being investigated. I will give a brief description of the apparatus and present the results of numerous measurements made on a brass phantom with voids placed at various depths. These results imply that for this crude apparatus occlusions as small as one cubic millimeter may be located to an accuracy of about one millimeter at depths of about 15 millimeters in solid brass.