X-ray diffraction at the National Ignition Facility

Abstract

We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2–7 on the periodic table.

Structure and density of silicon carbide to 1.5 TPa and implications for extrasolar planets

“…Systematic corrections were applied to correct for the offset of the pinhole substrate from the pinhole center and the offset of the sample and the pinhole, as described in ref. 55 The uncertainties in the interplanar d -spacings include variation in the measured value of two-theta as a function of azimuthal angle ( ϕ ), uncertainty in fitting the sample peak positions to Gaussian profiles, and the uncertainty in the incident X-ray wavelength (<0.01 Å) 54 . The resultant uncertainties in d -spacing (~1%) are given in Supplementary Table 2 .…”

Femtosecond Diffraction and Dynamic High Pressure Science

“…The bremsstrahlung radiation from these hot electrons can produce an additional source of x-rays that is hard to filter out. As a result, considerable effort needs to be made to remove background effects from any recorded diffraction pattern 47 .…”

“…Despite the difficulties associated with laser-plasma xray sources, the fact that they can be produced by some of the largest and most powerful lasers on the planet, such as the National Ignition Facility (NIF) 47,48 at Lawrence Livermore National Laboratory, and the Omega Laser Facility at the University of Rochester 49 , has resulted in them being used to provide diffraction information on solid-state matter at extremely high pressure, well into the TPa regime, and far beyond what is achievable with DACs. As noted previously, keeping a material solid whilst compressing to such high pressures cannot be achieved via shock compression, but only if the material is loaded more slowly.…”

“…There is compelling evidence that underpins the above picture. Firstly, the diffraction experiments that have been performed using laser-plasma x-ray sources on the NIF and Omega facilities have readily demonstrated that, when ramp-compressed on the time-scale of several nanoseconds, the material remains solid into the TPa regime 1,47 . Secondly, MD simulations also indicate that ramp compression, even on rapid timescales (but slower than shock compression) keeps the material cool.…”

Molecular dynamics simulations of inelastic x-ray scattering from shocked copper

“…Shock compression is highly entropic, and as a result for most metals the Hugoniot (the locus of points that can be reached by shock compression) crosses the melt curve at pressures of order 100 -300 GPa [66][67][68] . However, with controlled laser pulses, shaped in time, or by compressing materials via multiple shocks (which can in principle be induced by appropriate target design 69 ), the application of high pressure can be achieved in a ramped manner 57,[70][71][72][73][74] , without the compression wave steepening into a shock. This so-called 'quasi-isentropic' (QI) compression has been shown to keep material solid well into the TPa regime [75][76][77][78][79][80] .…”