Plasma Transport at the Nexus of Measurement and Modeling

Project Overview

This project addressed the need for experiments and modeling to predict transport in warm dense matter (WDM). Very few experiments have been done in this regime and model predictions have considerable scatter, being largely based on extrapolation of plasma theory applicable to higher temperatures. We have developed novel experimental and modeling capabilities for WDM species diffusion: X-ray free electron laser (XFEL) experiments to probe interdiffusion and advanced simulations to predict species diffusivity from first principles. We have leveraged a novel X-ray scattering technique designed previously at Lawrence Livermore National Laboratory to probe molten metals using an XFEL for highly sensitive measurements of changes in interface structure on subnanosecond time scales. The technique has been extended to WDM using a redesigned delay line we call the pico-XTEL (the picosecond X-ray Tomographic dElay Line). A focused X-ray beam heats a solid multilayer sample rapidly to 1-10 eV temperatures where it is WDM, a state between liquid and plasma. Once heated, a few-ps delay probe beam scatters from the multilayer. The scattering is very sensitive to gradients at the interface, detecting diffusive intermixing. Simulations have been conducted on the same time and length scales as the experiment. The Multiscale Orbital-free Density Functional Theory (DFT) Molecular Dynamics (MOD-MD) technique has been adapted and improved to simulate this mixing. Based on a Thomas-Fermi-Poisson description of the electrons together with a consistent screened Coulomb molecular dynamics description of the ion motion, MOD-MD simulates ion diffusion in strongly coupled WDM.

Our experimental advances included the pico-XTEL design and construction. Multilayers were fabricated with a pitch of a few nanometers. At the Linac Coherent Light Source (LCLS), we conducted the first X-ray pump-probe experiments using the XTEL delay line. These experiments were held back several years by COVID restrictions. Testing showed the heating beam drove the multilayer sample to several thousand degrees Kelvin, and the proof of principle of the delay line was successful, but alignment and secondary scattering processes need further attention to get high quality scientific data. The modeling work considered thousands of possible combinations of materials for diffusion pairs and identified the most promising for the experiment. We developed a novel screening function and a new energy function to improve the stability of MOD-MD. First-principles DFT calculations of WDM interatomic forces and energy enabled improved force laws. Large-scale simulations of WDM interdiffusion were conducted to compare with experiment. The project has produced the first capability to measure WDM diffusive mixing and conduct simulations at the same length and time scales. It has also introduced a powerful new tomographic delay line for XFEL beam lines and a highly accelerated capability for WDM transport simulation.

Mission Impact

This project supports the Lab's Core Competency in High-Energy-Density Science as well as Advanced Materials. These capabilities are important for predictive plasma simulation such as for the National Ignition Facility. They extend large-scale simulation and model development in support of the Advanced Simulation and Computing integrated codes. The new delay line has great promise for a range of applications at X-ray free electron laser facilities. The new capabilities are tied into Advanced Materials and Manufacturing through the multilayers. The project has developed a unique capability to understand the principles of plasma transport in a challenging regime and to produce validated models to maintain and enhance the safety, security, and effectiveness of the U.S. nuclear weapons stockpile.

Publications, Presentations, and Patents

Haxhimali, T., et al. 2020. "Modelling of diffusive interface broadening between materials at warm dense conditions in support of XFEL experiments." In American Physical Society Division of Plasma Physics Meeting (APS DPP 2020). LLNL-ABS-812114.

Haxhimali, T., et al. 2021. "At the Nexus of Modelling and XFEL Experimental Design to Study Diffusion at Warm Dense Conditions." In American Physical Society Division of Plasma Physics Meeting (APS DPP 2021). LLNL-ABS-824616.

Haxhimali, T., et al. 2022. "At the Nexus of Modelling and XFEL Experimental Design to Study Diffusion at Warm Dense Conditions." In American Physical Society Division of Plasma Physics Meeting (APS DPP 2022). LLNL-ABS-836990.

Burcklen, C., et al. 2023. "Single-Pulse Multi-Frame X-Ray Imaging with a Crystal-Based X-Ray Split-and-Delay Line." (Invited talk) In Ultrafast Imaging and Tracking Instrumentation, Methods and Applications Conference (ULITIMA 2023). LLNL-ABS-833457.