In plasmas, charged particles move together in concert to produce density or velocity patterns that oscillate in time in what are called collective modes. Collective modes often dominate the properties of plasmas, such as their stability, so it is crucial to understand them. This work will study ion collective modes in ultracold neutral plasmas for the first time. Ultracold neutral plasmas are created by laser ionization of clouds of atoms that have been cooled to temperatures as low as 1/1000th of a degree above absolute zero. In addition to expanding our fundamental understanding of an exotic regime of nature, these experiments may shed light on the physics of dense plasmas in thermonuclear devices or the cores of gas giant planets.
The broader impact of this proposal starts with the training of graduate students for careers in science and technology. It is augmented by the diversity of Rice's physics program and strength of Rice's geographical location for attracting minority students to the program. One third of the students who have worked on these projects in the past have been women, and one third have been under-represented minorities. An additional goal of the project is to attract and retain more students in science and engineering through involvement of undergraduate students. These experiments are ideal for this purpose because concepts are accessible and lasers capture students' imaginations.
Plasma is the state of matter in which neutral particles like atoms and molecules are broken apart into ions and electrons, which interact strongly with each other through electrical forces. Plasmas are the dominant form of visible matter in the universe, for example in stars and the ionosphere around the Earth. They are also important for many industrial applications, like in fluorescent light bulbs, the manufacturing of computer chips, and schemes to produce safe and clean energy sources harnessing nuclear fusion. The strong interactions between particles in a plasma gives rise to complex dynamics that are often very hard to understand, which complicates applications or leave gaps in our understanding of fundamental processes of nature. Most plasmas are hot, like the surface of the sun, or a flame. The experiments in this grant from the National Science Foundation and the Department of Energy studied an unusual form of plasma called an ultracold neutral plasma, formed with powerful techniques that use lasers to cool atoms to near absolute zero. Ultracold neutral plasmas are ideal platforms with which to test our understanding of basic plasma physics because all the parameters of the plasma are well-characterized and controllable, and powerful imaging techniques can be used to measure plasma temperature and density evolution. These measurements have helped benchmark models used to describe plasmas on the surface of the sun and can shed light on the dynamics of other laser-produced plasmas like in laser-driven fusion experiments. A major theme of the research was creation of plasmas that stream into each other, which allowed study of the crossover from when a plasma behaves like a thick viscous fluid to when a plasma behaves like a thin, diffuse gas. These regimes are known as the hydrodynamic and kinetic regimes respectively. Hydrodynamic descriptions of plasmas are relatively simple and are commonly used to approximate their behavior. But kinetic effects, such as transport of energy and mass over long distances in plasmas, often have important effects that are hard to capture in the simple description. Experiments performed as part of this research identified hallmarks of the onset of kinetic behavior. The work was used to test numerical codes that are used to describe plasmas on the surface of the sun. The attached figure shows pictures of ions in an ultracold plasma streaming towards each other and forming a wave-like disturbance that propagates away from the region where the ions collide. Further information/interesting details: Ultracold plasma are created using techniques of laser cooling, which was the subject of the Nobel Prize in Physics in 1997 (www.nobelprize.org/nobel_prizes/physics/laureates/1997/). Laser-cooled atoms are turned into a plasma by using additional lasers to knock an electron off of each atom. When done carefully, the resulting plasmas are orders of magnitude colder than any other neutral plasma every created. More information on the creation and study of ultracold plasmas can be found at http://ultracold.rice.edu/.
