Powder diffraction from solids in the terapascal regime

Abstract

A method of obtaining powder diffraction data on dynamically compressed solids has been implemented at the Jupiter and OMEGA laser facilities. Thin powdered samples are sandwiched between diamond plates and ramp compressed in the solid phase using a gradual increase in the drive-laser intensity. The pressure history in the sample is determined by back-propagation of the measured diamond free-surface velocity. A pulse of x rays is produced at the time of peak pressure by laser illumination of a thin Cu or Fe foil and collimated at the sample plane by a pinhole cut in a Ta substrate. The diffracted signal is recorded on x-ray sensitive material, with a typical d-spacing uncertainty of ~0.01 Å. This diagnostic has been used up to 0.9 TPa (9 Mbar) to verify the solidity, measure the density, constrain the crystal structure, and evaluate the strain-induced texturing of a variety of compressed samples spanning atomic numbers from 6 (carbon) to 82 (lead). Further refinement of the technique will soon enable diffraction measurements in solid samples at pressures exceeding 1 TPa.

X-ray diffraction of molybdenum under shock compression to 450 GPa

“…The foil was mounted 30 mm from the target at an angle of 45 o . Irradiation of the foil generated a source of quasi-monochromatic He-α X-rays at 8.38 keV with bandwidth of ~2% 43 . Satellite peaks in the emitted x-ray spectrum are suppressed using filtering 43 .…”

“…Irradiation of the foil generated a source of quasi-monochromatic He-α X-rays at 8.38 keV with bandwidth of ~2% 43 . Satellite peaks in the emitted x-ray spectrum are suppressed using filtering 43 . The X-rays passed through the Mo sample and were collimated by the Ta pinhole.…”

“…The error in pressure was determined from propagation of uncertainties in the measured shock and interface velocities and the equation of state parameters. Powder X-ray diffraction was used to probe the structure and volume compression of shocked molybdenum 43,44,54 . The sample assembly was attached to a 300-µm-diameter Ta pinhole and mounted on the front of an X-ray enclosure (Fig.…”

“…Typical diffraction peak widths were ~1° and diffraction angles could be resolved to ~0.1°. 43 Computer simulations were carried out using the hydrocode HYADES 56 in order to determine the laser pulse shapes and power levels required to achieve the desired pressures in the Mo sample. The simulations were also used to determine the time delay required between the laser pulse that compresses the sample and the pulse that generates the X-ray source.…”

“…In laser-shock experiments, the application of pulsed high-powered laser energy ablates the surface of a sample creating a hot, expanding plasma which exerts pressure on the surrounding material resulting in the propagation of a strong shock wave through the sample. The development of nanosecond X-ray diffraction diagnostics provides a new means to probe the lattice compression and structural state of materials under these extreme conditions [41][42][43][44] . Here we use powder X-ray diffraction on laser-shock-compressed molybdenum to directly probe the phases on the Hugoniot up to 450 GPa.…”