Eckhart Foerster

Characterization of the diffraction properties of quantum-dot-array diffraction grating

A new dispersive element named as quantum-dot-array diffraction grating [L. F. Cao, China patent No. 200410081499 (August 10, 2004)] for visible light has been developed and characterized experimentally. A large number of quantum dots distributed on a substrate as sinusoidal function can be used to diffract x rays without higher-order diffraction. The experimental patterns show that the higher-order diffractions which inevitably exist in the spectrum recorded using traditional diffraction gratings can be eliminated effectively by this newly designed element. It indicates that quantum-dot-array diffraction grating could be an attractive alternative of presently used diffraction grating in soft x-ray spectroscopy application to get rid of the higher-order diffraction distortions.

Laser-driven x-ray source for diagnostic radiology

Prospects for utilization of laser produced plasma x-ray source in diagnostic radiology are discussed with special emphasis on application in non-invasive coronary artery angiography and low dose high resolution mammography.

Line X-ray emission from Al targets irradiated by high-intensity, variable-length laser pulses

The absolutely calibrated K-shell spectra emitted form short-living aluminum plasma at laser intensities of 5 by 1015- 4 by 1018 W/cm2 are reported. The experiments performed with the constant energy, variable-length laser pulse are modeled by the 1D hydrodynamics code including non-linear resonance absorption of the laser radiation, fast electron acceleration and energy transfer into the target. The characteristic features of the measured and the post- processed spectra are outlined; the scaling rules for the conversion efficiency of the laser radiation into the line x-ray emission are discussed.

Vertical dispersion methods in x-ray spectroscopy of high-temperature plasmas

General formulae for the applying the vertical dispersion principle in x-ray spectroscopy of multiply charged ions are summarized, the characteristics of the experimental schemes based on flat and bent crystals are discussed. The unique properties of the novel spectroscopic methods, i.e. their extremely high dispersion, high spectral and 1D spatial resolution, and good collection efficiency, make them very attractive for ultrahigh-resolution spectroscopy. The examples of successful use of the vertical dispersion modifications of the double-crystal and the Johann sepctrometer in diagnostics of several types of laser-generated plasma are presented.

Limits of high-resolution x-ray spectroscopy of laser-produced plasma

Characteristics of several crystal spectrographs are analyzed on the basis of high-resolution spectra from laser-produced plasma. Using a tray tracing procedure, potential methods to optimize the experimental setup are indicated. Non-conventional applications of the double crystal spectrometer and focusing Johann spectrometer are discussed from the point of view of the highest spectral and spatial resolution, luminosity, and spectral range attainable.

Femtosecond time-resolved x-ray diffraction with a laser-produced plasma x-ray source

Optical pump, x-ray diffraction probe experiments have been used to study the lattice dynamics of organic materials using a laser-produced plasma x-ray source. The x-ray source is generated from a 10 Hz, 26 mJ, 120 fs laser beam focused on a silicon wafer target. The emitted K(alpha ) x-ray radiation is used to probe a cadmium arachidate Langmuir-Blodgett film and a TlAP crystal optically perturbed at laser fluences from 1.8 J/cm2 to 27 J/cm2. Ultrafast disordering inside the lattice -- within a time scale below 600 fs to few tens of picoseconds -- is clearly observed and produce a drop of the probe x-ray diffracted signal.

Monochromatic x-ray imaging with streak and framing cameras

This article describes the first experiment of our groups to combine monochromatic x-ray imaging with a time-resolving detector i.e., a streak camera and a 120 ps gated framing camera. The aim of setting such a time-resolved diagnostic is to image the x-ray emission from colliding plasmas with high spatial resolution in a very narrow spectral window. Both camera types were tested and the adjustment procedure for the crystal was tested with film as a detector. The obtained spectral and spatial resolution of the x-ray microscope was measured.

High-precision x-ray diagnostics of laser-produced plasmas

Diagnostic information obtained from x-ray emission accompanying the laser-matter interaction represents a primary tool for identification and exploitation of phenomena occurring in hot dense plasmas. To fully utilize the potential contained in the shapes and shifts of the spectral lines, sophisticated high-resolution instruments have been developed. The basic spectroscopic conceptions for K-shell plasma diagnosis are outlined. The main characteristics of toroidally bent crystal spectrometers and vertical-dispersion instruments are briefly reviewed. The applications of high-precision x-ray spectrometers in investigation of strongly correlated plasmas are demonstrated on a detailed analysis of the spectral line emission from two types of laser-produced plasmas. The redults of experiments performed at 0.4 ns laser system ASTERIX and 150 fs system ATLAS are presented, the diverse character of the observed line profiles and the dense-plasma line shifts to red is discussed. The conclusions for the line shift-based plasma diagnosis are drawn.

Time-resolved x-ray diffraction with subpicosecond x-ray pulses

The emission from plasmas created with fs-lasers provides sub-picosecond x-ray pulses in the keV-range. Intense emission of K(alpha) lines as well as quasi continuum x-rays can be used for time-resolved diffraction and spectroscopy, i.e. to study lattice or atomic dynamics with sub-picosecond resolution by using a laser pump x-ray probe technique. The x-ray yield and x-ray pulse duration of the laser plasma source depend on the laser parameters and the target design, such as intensity, laser wavelength, pulse duration and prepulse level. To accumulate as many photons as possible of the isotropic source an efficient large aperture optic has to be used to select an x-ray line or a wavelength range and focus the radiation onto the sample. It is shown that the use of toroidally bent crystals provides the possibility to refocus 10-4 of the photons emitted in the whole solid angel to spot size of around 80 micrometers with a temporal broadening below 100 fs. Combinations of bent focusing crystals with a flat sample crystal for fast x-ray diffraction application are discussed. Experiments showing the temporal response of laser heated crystals are presented and compared with theoretical simulations based on Takagi-Taupin theory.

Measurements of the integrated reflectivity of a mica crystal for different orders of reflection

Simultaneous measurements of the integrated reflectivity of a mica crystal for different orders of reflection have been performed at a predefined Bragg angle of 45 degree(s) with use of a new method. The method is less time consuming than previous techniques and provides data with small statistical errors. It can be readily used for the calibration of x-ray crystal spectrometers. The paper presents experimental results for Bragg reflections up to the 22nd order. The obtained experimental results are compared with theoretical predictions.