NIF Research Highlighted at Plasma Physics Meeting
Experiments Show Molybdenum Remains Solid under High Pressures
Molybdenum (Mo) is a strong, widely used metal—for example, as an alloy to strengthen steel—with a high melting point at normal pressure. Its stability under extremely high pressures, however, has been the subject of lively scientific debate in recent years, with some experiments indicating that Mo undergoes a phase transition at 210 GPa, or a little more than two megabars (two million atmospheres) of pressure, and other studies suggesting a much steeper melting curve. Experimental setup for x-ray diffraction of shock-compressed molybdenum: The locations of the target package (white square), drive beams, x-ray source, and VISAR laser are indicated. The x-ray source is generated by laser ablation of a copper foil shown in the upper left. The x-ray diagnostic box lined with image plate detectors is shown here unfolded illustrating representative diffraction images. White arrows point to diffraction lines, and the orange dashed-dotted line traces a representative diffraction peak across a series of image plates at a constant diffraction angle 2θ. The panels are labeled L, R, U, D, and B corresponding to the left, right, up, down, and back image plates.The discrepancies translate to differences in melting temperature of thousands of Kelvin at megabar pressures.
In a Physical Review B paper published on Nov. 20, LLNL researchers and colleagues report on laser-shock experiments at the OMEGA Laser Facility at the University of Rochester that subjected Mo samples to shock loading to 450 GPa of pressure. 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. Nanosecond x-ray diffraction diagnostics are used to probe the lattice compression and structural state of the materials under these extreme conditions.
The OMEGA experiments used powder x-ray diffraction on laser-shock-compressed molybdenum to directly probe the phases on the Hugoniot (the curve of the material’s pressure, density, and energy when subjected to shocks of varying strengths) up to 450 GPa. “Shock-compressed molybdenum shows no evidence of a…phase transition along the Hugoniot,” the researchers reported. “The bcc (body-centered-cubic) structure remains stable until shock melting begins at about 390 GPa. Previous suggestions of a low melting temperature for Mo are not supported by our data. Our finding that Mo remains in the bcc structure to 380 GPa is consistent with both the most advanced theoretical calculations and the latest sound velocity measurements.”
Lead author Jue Wang of Princeton University was joined on the paper by Princeton colleague Thomas Duffy; Federica Coppari, Ray Smith, Jon Eggert, Amy Lazicki, Dayne Fratanduono, Ryan Rygg, and Rip Collins of LLNL; and Thomas Boehly of the Laboratory for Laser Energetics at the University of Rochester.
NIF Research Highlighted at Plasma Physics Meeting
Seven invited talks on NIF research were presented at the Nov. 16-20 meeting of the American Physical Society’s Division of Plasma Physics in Savannah, Georgia. Talks included research on implosions, hohlraum physics, integrated modeling, and hydrodynamic instabilities. Some highlights:
- In a session on ICF preheat and drive chaired by NIF physicist Felicie Albert, Sebastien le Pape described the near-vacuum hohlraum campaign at NIF. Le Pape said hohlraums filled with 0.1 mg/cc of helium may provide a viable alternative to traditional gas-filled hohlraums, producing negligible laser-plasma instabilities and high laser-to-hohlraum coupling.
- In the same session, Maria Alejandra Barrios discussed the use of x-ray spectroscopy to characterize NIF hohlraum plasma conditions. She said characterizing the plasma conditions, particularly the plasma electron temperature, is critical to understanding mechanisms that affect energy coupling, such as laser-plasma interactions, hohlraum x-ray conversion efficiency, and dynamic drive symmetry.
- In a session on ICF instabilities and astrophysics, David Martinez described NIF Discovery Science experiments investigating the ablative Rayleigh-Taylor (RT) instability in the transition from linear to highly nonlinear regimes. Martinez said the experiment provides critical data needed to validate current theories on the ablative RT instability for indirect-drive implosions that relies on the ablative stabilization of short-scale modulations for ICF ignition.
- The results of NIF implosions designed to simultaneously achieve both high stability and high areal density (ρR) were described by Harry Robey in a session on ICF stagnation and burn. These implosions employ adiabat-shaping, where the driving laser pulse is high in the initial picket similar to NIF “high-foot” implosions to retain their favorable stability properties at the ablation front. The remainder of the foot is similar to that of the “low-foot,” driving a lower-velocity shock into the deuterium-tritium fuel to keep the adiabat low and compression high.
- In the same session, Dan Clark discussed the current state of progress of 3-D, high-resolution, capsule-only simulations of NIF implosions aimed at accurately describing the performance of specific NIF experiments. Current simulations include the effects of hohlraum radiation asymmetries, capsule surface defects, and the capsule support tent and fill tube, and use a grid resolution shown to be converged in companion two-dimensional simulations.