Development of Laser Induced Fluorescence as a Diagnostic for Measuring Neutral, Ion, and Molecule Particle Fluxes in the PK-4 Experiment

California Institute Of Technology, Pasadena CA

Investigators

Abstract

This project will develop a way to measure the flows of ionized and neutral atoms in a dusty plasma, the material from which stars and planets begin to form. The so-called dusty, or complex, plasma consists of not only ions, electrons, and neutral atoms, but also has large numbers of tiny dust grains that are a few thousandths of a millimeter in diameter. Although small by human standards, these grains are many billions of times heavier than ions. The grains are continuously hit by ions and neutrals and these collisions can cause the grains to move in interesting ways. Because of the grains' size, they are affected by gravity and tend to fall in Earth-based experiments. The Plasma Krystal-4 (PK-4) experiment on board the International Space Station has the goal of studying the behavior of complex plasmas in zero gravity while subject to the forces from collisions from the ion and neutral flows. The ion and neutral flow patterns have been predicted by theoretical models but have never been actually measured. This Caltech research program will develop a diagnostic to measure these flows. The new diagnostic will be developed for use on a terrestrial replica of the PK-4 experiment located in Germany, and the resulting measurements will be used to predict the ion and neutral flows in the space-borne experiment. This work is fundamental science in the national interest because it reveals important behavior of the state of matter that occupies much of the visible Universe, but for which very little is now known. The laser induced fluorescence (LIF) diagnostic for the flow measurements will be developed using an existing complex plasma experiment at Caltech. The diagnostic will determine the ion and neutral velocities by detuning the laser to compensate for Doppler shifts and will use this to measure previously undetermined ion and neutral flow patterns. The Caltech experiment differs from the PK-4 experiment by having the dust grains consist of spontaneously-formed water ice grains rather than mechanically-introduced fixed-size plasma spheres, but this difference is of no significance for the purpose of LIF development. The LIF diagnostic will take advantage of substantial existing hardware at Caltech, expertise from previous LIF research, and new ultra-narrow-band tunable diode lasers that will provide excellent velocity resolution. Investigation of future microgravity experiments that could be carried out on the International Space Station will also be done. By providing a means for better understanding the underlying dynamics governing the PK-4 experiment, the proposed research is likely to reveal new PK-4 operational modes where there is better control of neutral, ion, or molecular flows or new and interesting regimes. In the course of being developed at Caltech, the LIF diagnostic will address the critical question of how ice grains grow in the Caltech experiment and why this growth saturates; answering this question will have substantial impact on astrophysical models of protoplanetary accretion disks. This award is made under a "NASA/NSF Partnership on Science of Dusty Plasmas: Utilizing the PK-4 Facility on board the International Space Station"; a complementary NASA award to support an undergraduate student is being made under the joint program.