Broadband X-ray Source Research Articles

Electronic Structure Investigation of Highly Compressed Aluminum withKEdge Absorption Spectroscopy

The electronic structure evolution of highly compressed aluminum has been investigated using time resolved K edge x-ray absorption spectroscopy. A long laser pulse (500 ps, I(L)≈8×10(13) W/cm(2)) was used to create a uniform shock. A second ps pulse (I(L)≈10(17) W/cm(2)) generated an ultrashort broadband x-ray source near the Al K edge. The main target was designed to probe aluminum at reshocked conditions up to now unexplored (3 times the solid density and temperatures around 8 eV). The hydrodynamical conditions were obtained using rear side visible diagnostics. Data were compared to ab initio and dense plasma calculations, indicating potential improvements in either description. This comparison shows that x-ray-absorption near-edge structure measurements provide a unique capability to probe matter at these extreme conditions and severally constrains theoretical approaches currently used.

Lensless imaging using broadband X-ray sources

High-resolution X-ray imaging techniques using optical elements such as zone plates are widely used for viewing the internal structure of samples in exquisite detail. The resolution attainable is ultimately limited by the manufacturing tolerances for the optics. Combining ideas from crystallography and holography, this limit may be surpassed by the method of coherent diffractive imaging (CDI) 1 . Although CDI shows particular promise in applications involving X-ray free-electron lasers 2 , it is also emerging as an important new technique for imaging at third-generation synchrotrons. The limited coherent output of these sources, however, is a significant barrier to obtaining shorter exposure times. A fundamental assumption of coherent diffractive imaging is that the incident light is well-approximated by a single optical frequency. In this Letter, we demonstrate the first experimental realization of ‘polyCDI’, using a broadband source to achieve a factor of 60 reduction in the exposure time over quasi-monochromatic coherent diffractive imaging. Over the past decade, the extension of crystallographic techniques to the high-resolution imaging of non-crystalline objects 3 has proved enormously successful in a wide variety of applications 4 . Iterative retrieval of the phase of the far-field diffraction pattern permits reconstruction of the diffracting object with a spatial resolution that is limited, in principle, only by the wavelength of the incident illumination. In crystallography, the periodic arrangement of atoms amplifies the signal diffracted from the unit cell, making detection of the Bragg peaks straightforward. A main goal for coherent diffractive imaging (CDI) is to develop it into a form of highspatial-resolution microscopy that can be routinely applied using synchrotron sources. In this endeavour it must compete with the very successful results obtained using zone-plate-based full-field transmission X-ray microscopy 5 . A fundamental issue for both forms of imaging is that higher spatial resolution requires that the sample be illuminated with a greater number of photons per resolution element, scaling approximately as the fourth power of the desired spatial resolution 6 . Although it has been asserted that CDI is able to achieve equivalent resolution to zone plate microscopy, but with fewer photons 7 , the two techniques are limited to precisely the same degree by the spectral bandwidth (longitudinal coherence length) of the incident light. Significant increases in the spatial resolution require brighter sources, much longer exposure times or the capacity to increase the acceptable limits on the bandwidth. A pathway to achromatic zone plates has been proposed 8 that

Absolute measurements of x-ray backlighter sources at energies above 10 keV

Line emission and broadband x-ray sources with x-ray energies above 10 keV have been investigated using a range of calibrated x-ray detectors for use as x-ray backlighters in high energy density (HED) experiments. The conversion efficiency of short- and long-pulse driven Mo and Ag line-emission backlighters at 17 and 22 keV was measured to investigate the crossover region between short- and long-pulse conversion efficiency. It was found that significant 17 and 22 keV line emissions were observed using a 3 ω, 1 ns long-pulse drive for Mo and Ag targets and a comparison between the measured Mo x-ray spectrum and calculations using an atomic physics code suggests that the line emission is due to thermal emission from N-like Mo atoms. Electron temperatures derived from fits to the continuum region of the x-ray spectra agree well with the Thot scaling as 100×(Iλ2)1/3. The continuum emissions from empty and 1 atm Kr-filled imploded CH shell targets were also measured for the use as broadband backlighters.

Picosecond Short-Range Disordering in Isochorically Heated Aluminum at Solid Density

Using ultrafast x-ray probing, we experimentally observed a progressive loss of ordering within solid-density aluminum as the temperature raises from 300 K to >10{4} K. The Al sample was isochorically heated by a short ( approximately ps), laser-accelerated proton beam and probed by a short broadband x-ray source around the Al K edge. The loss of short-range ordering is detected through the progressive smoothing of the time-resolved x-ray absorption near-edge spectroscopy (XANES) structure. The results are compared with two different theoretical models of warm dense matter and allow us to put an upper bound on the onset of ion lattice disorder within the heated solid-density medium of approximately 10 ps.

High-power 1 kHz laser-plasma x-ray source for ultrafast x-ray absorption near-edge spectroscopy in the keV range

A high average power broadband x-ray source is developed in the multi-keV range, based on the thermal emission of plasmas produced with a 1 kHz fs laser focused on high Z element target. This compact ultrafast x-ray source is used to measure the x-ray absorption near-edge spectroscopy of aluminum K-edge (1.559 keV) with noise lower than 1% of the absorption edge when accumulating laser shots over a few tens of seconds. That demonstrates its suitability to study atomic and electronic structures of matter during ultrafast phase transitions among solid, liquid, or higher energy density states.

High repetition rate laser produced soft x-ray source for ultrafast x-ray absorption near edge structure measurements

Recent progress in high intensity ultrafast laser systems provides the opportunity to produce laser plasma x-ray sources exhibiting broad spectrum and high average x-ray flux that are well adapted to x-ray absorption measurements. In this paper, the development of a laser based x-ray absorption near edge structure (XANES) beamline exhibiting high repetition rate by using the Advanced Laser Light Source (ALLS) facility 100 Hz laser system (100 mJ, 35 fs at 800 nm) is presented. This system is based on a broadband tantalum solid target soft x-ray source and a grazing incidence grating spectrometer in the 1-5 nm wavelength range. To demonstrate the high potential of this laser based XANES technique in condensed matter physics, material science, or biology, measurements realized with several samples are presented: VO2 vanadium L edge, Si3N4 nitrogen K edge, and BPDA/PPD polyimide carbon K edge. The characteristics of this laser based beamline are discussed in terms of brightness, signal to noise ratio, and compared to conventional synchrotron broadband x-ray sources which allow achieving similar measurements. Apart from the very compact size and the relative low cost, the main advantages of such a laser based soft x-ray source are the picosecond pulse duration and the perfect synchronization between this x-ray probe and a laser pulse excitation which open the way to the realization of time resolved x-ray absorption measurements with picosecond range time resolution to study the dynamics of ultrafast processes and phase transition.