Effects of Magnetic Fields on Transport in Laser-Driven Nonequilibrium Plasmas

Gregory Kemp (17-ERD-027)

Executive Summary

Our research focuses on understanding externally applied and self-generated magnetic field effects on heat, particle, and charge transport, as well as laser–plasma instabilities in regimes relevant to high-energy x-ray source development. Results will provide data for model validation in a regime where little to no data exist, impacting a range of national security and energy missions.

Project Description

Understanding energy transport in laser-driven nonequilibrium plasmas with mid- to high-atomic numbers and low density (such as those present in Inertial Confinement Fusion target capsules or in high-energy x-ray sources) is fundamentally important to high-energy-density physics. Plasma conditions determine laser propagation and absorption, backscatter, x-ray emission, and cavity-wall shape, which impact hohlraum target-capsule radiation drive and the resulting Inertial Confinement Fusion implosion symmetry. By developing a deeper understanding of magnetic field effects in these nonequilibrium plasmas, we can better assess their benefits and risks to such applications as magnetized Inertial Confinement Fusion hohlraums and targets, high-photon-energy x-ray sources for radiography, and the magnetized liner inertial fusion scheme, which uses the inward movement of the fusion fuel to reach fusion densities and temperatures. We plan to use the x-ray, optical Thomson scattering, laser backscatter data and the proton radiography data gathered on OMEGA at the University of Rochester's Laboratory for Laser Energetics and Livermore's National Ignition Facility (NIF) over a broad range of plasma conditions to validate models in a regime where little to no data currently exist. These data will help inform ongoing efforts to remove flux-limiters and other unresolved physics from models of high-energy-density plasmas.

We expect to quantify the effects of self-generated and externally applied magnetic fields on plasma hydrodynamics as well as particle, heat, and radiation transport in laser-driven plasmas. We intend to investigate the effects of magnetic fields on a broad range of related physical processes through experiment and modeling. We will also explore heat and particle-transport inhibition, the modification of laser coupling efficiency and growth rates of laser–plasma instabilities, and the processes of field generation and ensuing evolution. If preliminary simulations with existing unvalidated physics models prove to be reasonably correct, we anticipate that external magnetic fields will enhance coupling to and heating of the gold wall in Inertial Confinement Fusion hohlraum target capsules. Once models are validated, we expect to quantify the resulting changes in the x-ray drive, which could alter capsule implosion symmetry, resulting in enhanced fusion yield. We expect external magnetic fields will also enhance x-ray emission from laser-driven targets for increasingly penetrating x-ray radiography. Finally, we expect to quantify the relative influences of the Nernst and Ettingshausen effect, a thermal-magnetic phenomenon impacting the electric current in a conductor, believed to be important in the magnetized liner inertial fusion scheme.

Mission Relevance

Our research into magnetic fields on laser-driven plasmas supports the DOE's science and energy goal of delivering scientific discoveries that transform our understanding of nature and strengthen connections between fundamental science advances and technology innovation, as well as the Laboratory's high-energy-density science core competency.

FY17 Accomplishments and Results

We refocused the FY17 scope on reduced scale interactions at the Jupiter Laser Facility. In FY17 we (1) conducted experiments on Titan and Janus to quantify changes in plasma conditions, laser beam propagation, and instability-driven backscatter in titanium-doped silica aerogels with externally imposed magnetic fields; and (2) began exploring the possibility of using magnetic-dot probes and the Zeeman effect to measure magnetic fields of interest.

Two separate benchmarking experiments studying the influence of externally applied magnetic fields on thermal energy transport in mid-Z, non-LTE (non-local thermodynamic equilibrium), laser-driven plasmas have been performed in FY17 at the Jupiter Laser Facility on the Titan and Janus lasers, respectively. Single-digit milligram-per-cubic-centimeter density titanium-doped silica aerogel foams were irradiated by intense, nanosecond long laser pulses (illustrated on the left) to ~1 keV electron temperatures, preferentially radiating helium- and lithium-like titanium K-shell line-radiation. Axial magnetic fields (aligned antiparallel to the laser propagation direction) of strengths up to 40 T were (externally) applied and changes in plasma conditions (due to radial thermal heat flow inhibition in the presence of the external fields) were recorded with a variety of x-ray, laser-backscatter, and proton radiography diagnostics. An imaging x-ray spectrometer was the primary diagnostic of interest, which provided time-integrated, one-dimensional space-resolved (along the laser propagation axis), high-resolution titanium K-shell spectra. Shown on the right are representative spectra for 0, 5, and 10 T external field strengths. With increasing field strength, we observe longer plasma columns with increased emission coming from deeper inside the target (as anticipated by the preliminary modeling) along with spectral differences (currently under investigation). These experiments provided benchmarking data for verification and validation of our magnetized radiation-hydrodynamics modeling of laser-driven multi-keV x-ray sources currently under development on the Omega and National Ignition Facility laser facilities.

Publications and Presentations

Kemp, G. E., et al. 2017. "Enhancing Multi-keV Line-Radiation from Laser-Driven Non-Equilibrium Plasmas Through Application of External Magnetic Fields on the Omega Laser Facility." HEART. LLNL-ABS-739491.

——— 2017. "Study of Transport Phenomena in Laser-Driven, Non-LTE Plasmas in the Presence of External Magnetic Fields." APS DPP 2017, Milwaukee, WI, 23–27 October 2017. LLNL-PRES-740685.

——— 2017. "Study of Transport Phenomena in Laser-Driven, Non-Equilibrium Plasmas in the Presence of External Magnetic Fields." APS DPP 2017, Milwaukee, WI, 23–27 October 2017. LLNL-ABS-734489.

——— 2016. "Simulation Study of Enhancing Laser Driven Multi-keV Line-Radiation Through Application of External Magnetic Fields." Phys. Plasmas 23 (10):101204. doi: 10.1063/1.4965236. LLNL-JRNL-692158.

Klein, S. R., et al. 2017. "Solenoid for Laser Induced Plasma Experiments at Janus." APS DPP 2017, Milwaukee, WI, 23–27 October 2017. LLNL-ABS-734846.

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