Collaborative Research: Plasma Physics At Small Coulomb Logarithms

New Mexico Consortium, Los Alamos NM

Investigators

Abstract

This collaborative research project will advance fundamental understanding of how groups of high speed ions crash into and interact with each other. One can measure just how violent a collision is by comparing the energy of the crashing high-speed ions with the electrical force between them. The most violent ion collisions, the ones that are most important both for extending our scientific knowledge and for developing technological applications, are very difficult to measure or calculate. They occur in extremely hot and very dense gases of charged particles called plasmas. In this project, new ideas will be used to measure and understand these collisions. Lasers will be used to slow atoms from speeds of nearly 1000 meters per second to a crawl of about an inch per second; and then to turn these slow atoms into ions. Additional lasers will then be used to measure how these ions crash into each other. The ions in these slow-motion collisions have the same amount of crash energy compared to the ion-ion electrical force, which means that the collision results can be directly compared to similar collisions at any energy. This project will use state-of-the-art large-scale computer simulations to make movies of the ion-ion collisions and compare these to the experimental measurements. When the computations are proven to be sufficiently accurate, approximations will be gradually introduced and tested in order to speed up the computations. These results will then set the standard for accurate and fast computations of ion collisions in plasmas. Several students will work on this project: Undergraduate and graduate students and post-doctoral scientists will work closely with expert scientists at Willamette University (Oregon), Brigham Young University (Utah), and the New Mexico Consortium (New Mexico). The proposed collaborative research project will investigate energy relaxation in a system in which the value of the Coulomb logarithm is small. This is typical of high-energy-density systems, where violent small-impact-parameter collisions result in large particle deflections. Understanding these collisions is a priority for advancing fundamental plasma physics and for accurately modeling small impact parameter collisions in high energy density plasmas. The proposed work will generate high quality data in plasma regimes where traditional diagnostics are limited. The proposed work will combine data from a new dual-species ultracold neutral plasma experiment and state-of-the-art simulations to study temperature equilibration in moderately coupled plasmas, in which classic plasma assumptions are invalid. The dual-species plasma will be generated by resonantly photo-ionizing laser-cooled Yb and Ca atoms in the same magneto-optical trap. Laser-induced fluorescence measurements will be used to measure the time-evolving ion velocity distribution for each ion species simultaneously. By delaying the ionization of one species relative to the other, the time scale for full energy relaxation can be determined. State-of-the-art molecular dynamics simulations will be performed that match the density, stoichiometry, and geometry of the experiments. The calculations will provide a first-principles description of collision processes by directly integrating many-body trajectories. Arbitrarily complicated orbits will be computed self-consistently with dynamical many-body screening. The many-body phase dynamics will be inverted to yield highly accurate effective Coulomb logarithms, providing important information back to the high energy density community. This project will support one graduate student per year for three years at BYU, two undergraduate students per year at BYU, two undergraduate students per year at WU, and one post-doc per year for two years at NMC.