RUI: Rydberg Atoms and their Effect on Ultra-Cold Plasma Dynamics

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Research in the PI's laboratory at Colby College is directed towards characterizing and controlling ultra-cold neutral plasmas (UNPs). In particular, we are investigating non-optical measurements of the electron temperature in UNPs, and techniques that offer the ability to control the electron temperature during the plasma evolution process. Our UNPs are made by pulsed-laser photoionization of laser-cooled, magnetically trapped rubidium atoms. Such a plasma is initially 95% neutral, and the initial electron temperature and ion density are easily controllable functions of the pulsed laser intensity and frequency. Atomic processes, principally involving Rydberg states, are critical to the plasma evolution process, and generally heat the plasma in its initial stage. Nevertheless, the UNP may exist for longer than 100 microseconds. As it expands, the electron temperature decreases, and electrons evaporate from the plasma. In UNPs made from alkaline-earth atoms, optically accessible ionic transitions are used to extract precise values for the electron temperature from the plasma ion expansion velocity. However, this is not feasible in alkali atoms such as rubidium. We are pursuing several electron temperature measurement techniques in alkali plasmas that potentially offer precision equal to the optical technique in alkaline earth UNPs. In addition, preliminary experiments in the PI's lab, in which Rydberg atoms are embedded in a UNP, indicate that it may be possible to control electron-atom collisions to counteract heating of the UNP caused by three-body recombination, and push the UNP into the strongly-coupled regime.Plasmas are ubiquitous: they illuminate our lives in the form of fluorescent lights,; a long-existing goal of physicists is to use plasmas to perform controlled thermonuclear fusion; and they occur in many other manifestations in the Universe and our everyday lives. UNPs are of fundamental interest because they can be made with uniquely low electron (0-1000 K) and ion (1 K) temperatures, and because they approach the strongly-coupled regime in which the potential energy of interaction between particles becomes comparable to their kinetic energies. They therefore bridge the gap between atomic systems and the correlations found in the solid or liquid state. In addition, UNPs are an exceptionally useful environment for testing theoretical modeling techniques used in plasma physics. Experiments can be carried out in a very reproducible manner, and one has the ability to set the initial conditions with a high degree of precision compared with other kinds of plasma experiment. At a more practical level, the PI's research on UNPs is carried out at an undergraduate-only institution, and undergraduates have been critically involved at all levels of the research, from equipment construction and data-acquisition programming, through to performing the experiments.