Ultracold neutral plasmas provide an ideal environment in which to study fundamental processes such as the establishment of spatial correlations, phase transitions, heat transport, and non-equilibrium dynamics. In ultracold plasmas the relevant time scales are long, powerful optical diagnostics exist, and initial density profiles, energies, and ionization states are accurately known and controllable. All of which make it possible to study phenomena that are difficult to access in other experiments. This proposal centers around two diagnostics that are relatively straightforward but can be applied to ultracold neutral plasmas to study equilibration and correlations in unique ways. Fluorescence imaging will provide a spatially-resolved light-scattering spectrum that is Doppler broadened and shifted due to ion velocity. With proper choice of the excitation and detection geometry, ion velocity due to plasma expansion will be distinguishable from thermal ion velocity, which has not been possible before and is crucial for studying equilibration and thermal transport at later times after photoionization. Small-angle light scattering is a well established technique to study spatial correlations in liquids. Typically this is done with x-rays when the particle spacing is on the order of angstroms. But in ultracold neutral plasmas, the spacing is on the order of microns, allowing use of visible light that is resonant with the principle ion transition and takes advantage of the enormous resonant scattering cross-section. The angular dependence of forward scattered light will provide the static structure factor of the ions and a measurement of spatial correlations.
This proposal will have impact well beyond the field of ultracold neutral plasmas because of the connection to fast-pulse laser plasmas, and it will also foster substantial training and outreach efforts. graduate students working in the research group leverage Rice's geographical location and educational philosophy to contribute greatly to diversity in science. Support of undergraduate researchers, especially from historically under-represented groups, is also a priority. An optics summer school at Rice for REU students is being developed. There are also research opportunities for gifted high school students in a local school district.
Over 99% of the visible matter in the universe exists as plasma, in which neutral atoms have been ionized to produce free electrons and ions. Traditionally, neutral plasmas are relatively hot, such as the solar corona (1,000,000 K), a candle flame (1000 K), or the ionosphere around our planet (300 K). Using techniques of laser cooling, which originated in the atomic physics community, it is now possible to create ultracold neutral plasmas at temperatures as low as about 1 K. In a table-top apparatus, laser light traps and cools about 1 billion neutral atoms to a thousandth of a degree above absolute zero. A second laser illuminates the cloud with photons with barely enough energy to ionize the atoms and create the plasma. Little is known about plasmas in this new regime, and they are difficult to describe theoretically because they are strongly interacting, which means that interactions cannot be treated as a small perturbation. For more information, see the group website at www.owlnet.rice.edu/~killian/. During the course of this grant we studied equilibration of strongly coupled plasmas, which is important for modeling dense plasmas in thermonuclear devices and the cores of gas giant planets. We also developed a new technique for sculpting the density distribution of the plasma, which allowed us to excite ion acoustic waves. This represents a new direction in the study of ultracold neutral plasmas that will allow us to probe basic plasma physics phenomena with unprecedented precision, such as streaming plasmas and shock waves. (See attached figure.) Waves are present in all states of matter from the familiar waves of the ocean, to sound waves traveling through the air, and even seismic waves (earthquakes) traveling in the ground. In plasmas, the fourth state of matter, waves are also present, and they manifest the collective behavior of constituent charge particles. Studying waves in plasmas is central to understanding transport and thermodynamic properties in this state. We excited ion acoustic waves in ultracold neutral plasmas by implementing a new technique for sculpting the plasma density. Ion acoustic waves are long-wavelength, density waves in which ions provide the inertia and electrons provide the pressure. They are important in stellar and atmospheric plasmas, and in many plasma applications. Ultracold neutral plasmas (UNP) explore a new regime of plasma physics, orders of magnitude colder than traditional plasmas. They are so cold that electrical forces between the particles can lead to liquid-like behavior that is rare in plasmas but can be found in exotic environments like the interior of Jupiter or similar giant gaseous planets. With UNPs, we can now explore how ion collective modes behave at the boundary between traditional and liquid-like plasma conditions. Two graduate students worked on this project, and Jose Castro successfully defended his PhD thesis, "Collective effects in Ultracold Neutral Plasmas, " in August of 2011. Several undergraduate students also gained valuable research experience working in the laboratory. All of them went on to graduate school.
