A review. Ultrafast x-ray techniques using diffraction and absorption are discussed with an emphasis on the absorption techniques. Ultrafast x-ray sources and detectors for use in x-ray absorption spectroscopy are also reviewed. [on SciFinder (R)]

Ultrafast X-ray absorption spectroscopy

Laboratory measurements of iron opacity made under conditions similar to those inside the Sun reveal much higher opacity than predicted, helping to resolve inconsistencies within stellar models of the internal temperatures of stars. Internal temperature profiles of the Sun and other stars are controlled in large part by the rate at which radiation is absorbed by stellar matter. Until now it has not been possible to determine the opacity of matter in star-like conditions in the laboratory, but James Bailey et al. have now achieved that feat using the Sandia National Laboratories' Z facility, the world's most powerful X-ray generator. The experiments reveal a wavelength-resolved iron opacity that is 30 to 400 times greater than predicted in conditions very similar to those at the radiation/convection zone boundary in the Sun. Previous measurements of stellar interiors have been based on observations of surface waves, and there were serious discrepancies between theoretical predictions and observations. The new measurements account for about half of adjustment in opacity figures required to restore agreement between standard solar models and observations. Nearly a century ago it was recognized1 that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models2,3,4,5. A particular problem arose2,3,6,7,8 when refined photosphere spectral analysis9,10 led to reductions of 30–50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models11 using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted2,3,6,7,8, because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity2,12 at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9–2.3 million kelvin and electron densities of (0.7–4.0) × 1022 per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30–400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.

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A higher-than-predicted measurement of iron opacity at solar interior temperatures.

Laser-wakefield accelerators (LWFAs) were proposed more than three decades ago, and while they promise to deliver compact, high energy particle accelerators, they will also provide the scientific community with novel light sources. In a LWFA, where an intense laser pulse focused onto a plasma forms an electromagnetic wave in its wake, electrons can be trapped and are now routinely accelerated to GeV energies. From terahertz radiation to gamma-rays, this article reviews light sources from relativistic electrons produced by LWFAs, and discusses their potential applications. Betatron motion, Compton scattering and undulators respectively produce x-rays or gamma-rays by oscillating relativistic electrons in the wakefield behind the laser pulse, a counter-propagating laser field, or a magnetic undulator. Other LWFA-based light sources include bremsstrahlung and terahertz radiation. We first evaluate the performance of each of these light sources, and compare them with more conventional approaches, including radio frequency accelerators or other laser-driven sources. We have then identified applications, which we discuss in details, in a broad range of fields: medical and biological applications, military, defense and industrial applications, and condensed matter and high energy density science.

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Applications of laser wakefield accelerator-based light sources

Measurements of the opacity of aluminum in a well characterized, hot, dense, laser produced plasma are reported. Measurements of the absorption of X-rays by 1 to 2 transitions in Al XII through Al VIII have been made in a laser-heated slab plasma at the measured temperature and density of 58 ^ 4e V and 0.020 ^ 0.007 g cm~3. Separate measurements of the temperature and density were made. The con- ditions in the plasma were determined to be reproducible, spatially uniform, and in nearly complete local thermodynamic equilibrium. The absorption spectra and the temperature-density data obtained provide an improved means for comparison with detailed atomic physics and opacity calculations. Subject headings: atomic datamethods: laboratoryplasmasX-rays: general

Opacity measurements in a hot dense medium

Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.

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Review of pulsed power-driven high energy density physics research on Z at Sandia

We report the application of a technique for performing opacity measurements on hot, dense plasmas. Plasmas were generated by using the x rays emitted from a laser‐produced gold plasma to heat an aluminum sample. Observations were made using the technique of point projection spectroscopy. The distribution of ion stages observed is compared with predictions based on Saha distributions, at densities and temperatures deduced from hydrodynamic simulations.

K. A. Warburton

Papers

Investigation of the opacity of hot, dense aluminum in the region of its K edge