The Dynamics of Thermal Expansion in Single Crystal Beryllium from Nanosecond X-Ray Irradiation

Single crystals of beryllium were illuminated with nanosecond X-ray pulses generated from laser irradiated (~1.5 × 10¹⁴ W/cm²) gold targets. The characteristic gold M-band centered at 2.5 keV was measured by time-integrated transmission grating spectroscopy and a time-resolved (spectrally integrated) X-ray photodiode through beryllium targets of various thickness. Approximately decaying exponential temperature profiles were predicted to be induced in 100- and 160-m-thick single crystal targets producing nearly instant surface motion as measured by free surface velocity interferometry. This temperature profile gave rise to free surface (opposite to drive laser surface) velocity histories in a c-axis single crystal and a (10[overbar]10) single crystal in which large initial acceleration gave way to lower (ramped) acceleration due to the internal temperature gradient. A smooth rise to the peak velocity was then followed by a sharp release originating from the free surface nearest to the laser drive. Differences between the velocities in each of these regions were found between the two single crystals investigated, which were due to the thermal expansion properties as a function of direction (including plasticity). These results can be used to predict the behavior of preheated polycrystalline targets relevant to instability seeding in inertial confinement fusion ablators.