Bonding properties and phase transitions in hydrogen under extreme conditions are a central focus of EFree. Hydrogen at high pressure may be a superconductor or a super fluid, and is a grand challenge problem within EFree. Furthermore, the development of enabling technologies for the study of hydrogen rich-systems under extreme conditions is also an ongoing goal of the Center. EFree's part of the project explores the potential to generate and characterize extreme conditions in hydrogen using a groundbreaking new dynamic compression method.
Dynamic compression experiments often use laser pulses to generate highly compressed states of matter, where the amount of compression increases with the laser energy. Traditionally, such experiments have applied laser pulses of greater than 1 nanosecond duration. In contrast to such "long time scale" compression experiments, a group of researchers from Lawrence Livermore and Carnegie, including EFree Chief Scientist Malcolm Guthrie, saught to compress hydrogen on a time scale that is 10 times shorter, some hundreds of picoseconds. In a paper recently published in Applied Physics Letters, they showed theoretically that for rapidly equilibrating materials, the laser energy required to obtain a given compressed state strongly depends on the time scale of the compression. For a 10x shorter compression time, the laser energy required to obtain a given compressed state should be 1000x smaller. As such, the energy required to obtain extreme conditions with fast compression might be greatly reduced compared to compression over longer time scales. Such a large reduction in energy would be a tremendous advantage for dynamic compression, since it enables higher throughput, more extreme conditions for a given laser energy, and inexpensive implementation [M. Armstrong et al., Appl. Phys. Lett. 101, 101904 (2012)].
Figure captions: (top) An ultrafast laser pulse ablates aluminum, launching a shock wave (Kiyong Kim photo). (bottom) The results of a 2D hydrodynamics simulation of the wave evolution for multiple shock wave compression of cryogenic liquid deuterium from 20 K initial temperature with a cylindrically symmetric Gaussian particle speed distribution with a full width half maximum of 90 microns. Particle speed distributions for different times (as labeled) along the axis of symmetry are shown. Vertical black lines in the main plot show the piston position and are labeled by the time for each plot. Density is labeled for each step on the right side axis. The shock fronts are designed to converge at a single depth. The piston speed as a function of time along the axis of symmetry is shown in the inset. The final temperature from the simulations is ~3600 K, whereas the temperature of shock loaded deuterium at the same final pressure (~135 GPa, also from simulations) is >30 000 K.