Stagnating Plasma Piston: A New Method to Measure Thermal Conductivity at Planetary Core 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 the stagnating plasma-piston compression technique to smoothly and quasi-isentropically compress iron samples up to 190 GPa while simultaneously sending a >20,000 K thermal pulse through the sample. Each sample consists of three planar targets of different thicknesses arranged in a stair-step shape. 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. To analyze the data, we compare the results to the output from a finite element code (FEniCS) that models the thermal pulse as a square wave temperature boundary condition and models the compression with uniform mesh shortening and density increase. The finite element model is put through a differential evolution minimization algorithm to find the best thermal conductivity parameters and the timing and temperature of the thermal boundary condition. Hydrocode simulations of the plasma-piston setup help constrain this boundary condition. Initial results suggest a moderately high value for thermal conductivity compared to most other experimental studies.