Bent crystal spectrometer for both frequency and wavenumber resolved x-ray scattering at a seeded free-electron laser

Abstract

We present a cylindrically curved GaAs x-ray spectrometer with energy resolution ∆E/E = 1.1 • 10 −4 and wave-number resolution of ∆k/k = 3 • 10 −3 , allowing plasmon scattering at the resolution limits of the Linac Coherent Light Source (LCLS) x-ray free-electron laser. It spans scattering wavenumbers of 3.6 to 5.2/ Å in 100 separate bins, with only 0.34% wavenumber blurring. The dispersion of 0.418 eV/13.5 µm agrees with predictions within 1.3%. The reflection homogeneity over the entire wavenumber range was measured and used to normalize the amplitude of scattering spectra. The proposed spectrometer is superior to a mosaic HAPG spectrometer when the energy resolution needs to be comparable to the LCLS seeded bandwidth of 1 eV and a significant range of wavenumbers must be covered in one exposure.

Free-Electron X-Ray Laser Measurements of Collisional-Damped Plasmons in Isochorically Heated Warm Dense Matter

“…• provides a highly resolved measure of the source function using a GaAs crystal [44]. (b) Pulse shape and calculated electron temperature as function of time from the collisional-radiative code SCFLY [45] using focal spot sizes of 1 µm (10 µm) predicting final temperatures of 5.95 eV (0.06 eV).…”

Resolving electrical conductivities from collisionally damped plasmons in isochorically heated warm dense aluminum

“…1) were measured in forward direction at θ = 18 • (k = 1.26Å −1 ) and θ = 24 • (k = 1.68Å −1 ) by a Highly Annealed Pyrolytic Graphite (HAPG) crystal in a von Hámos configuration [45]. In addition, scattered photons are collected at an angle of θ = 60 • (k = 4.04Å −1 ) by a GaAs crystal [46]. The comparison of both spectrometer signals allows an accurate monitoring of the source function for each shot (shown later).…”

Design and performance characterisation of the HAPG von Hámos Spectrometer at the High Energy Density Instrument of the European XFEL

“…In other words, the crystal is suspended free from a protective housing and a PMMA shield can be added for plasma experiments with an expected debris spread. The crystal's width is reduced to 30 mm to minimise k-vector blurring and focal aberrations without detriment to its collection efficiency [9]. The detector mounting can be exchanged to accept either an ePix100 detector, or Jungfrau detector (in an airbox housing for use in vacuum) and incorporates a light-tight cap over the sensor -4 - with a wide slit aperture to accept rays reflected from the crystal, attenuated with black kapton to block optical light (other attenuating foils, depending on background or source brightness, are available).…”

“…Cylindrically bent crystals, exploiting mosaic focusing in von Hámos geometry [6], therefore offer a compelling choice for experiments that require high reflectivity and modest resolving power and have been previously characterised at length for variants of Highly Oriented Pyrolitic Graphite (HOPG) [7] and Highly Annealed Pyrolitic Graphite (HAPG) [8]. Alternative concepts without mosaic focusing include perfect, bent crystals of GaAs [9], and segmented Si crystals [10] operated at high Bragg angles. These crystals have been used to enable measurements of Thomson scattering [11], and thereby elucidate plasmon dispersions [2,3,12,13] and further measurements of transport [5] and optical properties [14] of warm-dense matter, amongst others.…”