Subnanosecond x-ray diffraction from laser-shocked crystals

Multikilobar shocks were launched into single crystals of (111) silicon with a 1-ns pulse (full width at half maximum) of 1.06-μm light at an irradiance of ${10}^9$–${10}^{10}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ using the JANUS research laser at Lawrence Livermore National Laboratory. Transient strains on the order of several percent were thus introduced into the crystal. During the compression of the crystal a short (100 ps) intense burst of x-ray line radiation was produced by focusing a second laser beam, synchronous but delayed with respect to the shock-driving beam, onto a solid target. The x rays were Bragg diffracted from the surface of the shocked crystal, and recorded on x-ray film. The spectral brightness of the x rays was sufficient to allow data to be recorded on a single laser shot. The shift in the Bragg angle with compression allows the interatomic spacings to be directly measured in the shocked region. A sequence of shots at various delay times and laser irradiances was recorded, mapping the interatomic spacing as a function of time. Compression above the Hugoniot elastic limit was achieved, with evidence of single-crystal nature being preserved. The compression results are in good agreement with calculations based on hydrodynamic-code pressure simulations and dynamical diffraction theory. The relevance of the technique to some of the fundamental problems of shock-wave physics and phase transitions is discussed.