Ion Transport in Strongly Coupled Plasmas

Brigham Young University, Provo UT

Investigators

Abstract

This award supports an experimental study of energy transport in collision-dominated ultracold plasmas. Ultracold plasma can serve as a model system for extremely dense and hot plasmas generated in laser-driven nuclear fusion experiments, which create matter that is hotter than the Sun and more dense than solid metal. Improved understanding of such plasmas would enable faster progress towards development of nuclear fusion energy sources, as well as address national nuclear security priorities. This research project will test portions of the detailed computer models used to predict transport processes in hot, dense plasmas by conducting precision laser measurements to create small-scale ultracold plasmas and measure everything about them – how the ions collide, how energy is transferred, and how the plasma approaches equilibrium. These small-scale plasmas are prepared with adjustable shapes and with different kinds of atoms, sometimes in combination with strong magnetic fields, so that transport processes in them will mimic the transport processes that occur in hot, dense plasmas. Testing the computer models and highlighting ways they can reach higher fidelity will help advance plasma science in the national interest. Radiation-hydrodynamic codes successfully capture the temperature, density, and neutron yield of high energy laser experiments that are designed to produce plasmas close to the hydrodynamic limit. However, many current and planned experiments are far from this limit. Codes to model these high energy density plasmas (HEDPs) must include kinetic effects, which are challenging to validate because many diagnostics yield integrated quantities such as effective temperatures and average or line-integrated densities. Experimental access to the underlying distribution functions in HEDPs is nearly impossible. Instead, a new method for measuring the ion distribution functions in model systems, called ultracold neutral plasmas (UNPs), has been developed at Brigham Young University. With proper energy scaling, UNPs are thermodynamically equivalent to HEDPs. The new experiments supported under this project will probe gaps in existing theoretical understanding and computer models of ion jetting, interfacial mixing, electron-ion thermalization, and ion stopping power. These are all critical "kinetic" processes that occur in technologically-relevant HEDP plasmas. Using precision laser spectroscopy and advanced analysis techniques, the project team will measure how the ion distribution functions evolve. By making precision measurements in the UNP environment and then comparing them to predictions from kinetic codes, the codes will be tested while avoiding the complications of quantum potentials, electron degeneracy, high optical opacity, and impossibly short time scales. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.