Stagnating Plasma-Piston Ramp Compression: A New Way to Measure Thermal Conductivity of Materials under Extreme Conditions

The thermal conductivity of iron at core pressure-temperature conditions (135-360 GPa, 2500-5000 K) is a key parameter for quantifying heat transport within the Earth's interior. An accurate measurement of this value has direct relevance for our understanding of multiple planetary processes, such as differentiation and generation of a magnetic field. However, both theoretical and experimental studies on the thermal conductivity of iron at core conditions are limited and not in agreement. At the OMEGA laser at the Laboratory for Laser Energetics (LLE), we use a recently developed stagnating plasma piston compression technique to smoothly and quasi-isentropically compress an iron sample to outer core conditions while simultaneously sending a thermal pulse through the iron. The sample consists of three planar targets of different thicknesses arranged in a stair-step pattern. By using a streaked optical pyrometer (SOP), we obtain time-resolved thermal emission curves from each step thickness on the side opposite to the heat source. These emission curves can be compared to the outputs of a finite element heat diffusion code in order to constrain thermal conductivity. Initial results suggest a moderately high value for thermal conductivity compared to most other experimental studies.