RUI: Using Atomic Physics to Achieve Strong Electron Coupling in Ultracold Plasmas

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General audience abstract:The plasma state is ubiquitous in the universe - it is estimated that 99% of the atomic matter in the universe is in the plasma state. Ultracold neutral plasmas (UNPs) are plasmas in which the initial electron and ion temperatures are significantly lower than any other known systems yet have densities high enough for their electrostatic interaction energies to be comparable with their kinetic energies (the so-called “strongly coupled” regime). In this regime, states of matter with long range structure, similar to crystallization, may occur. UNPs also have many similarities with plasmas formed when intense lasers are focused into solids, for instance in laser-induced fusion. The PI will carry out experiments on UNPs in his lab at Colby College with the assistance of undergraduate students, and simultaneously pursue numerical simulations. In the experiments, laser-cooled rubidium atoms in a magneto-optical trap (MOT) will be photoionized using pulsed lasers, and the plasma evolution will be observed by detecting the electrons and ions using time-of-flight techniques. In addition to new knowledge gained from the experiments, there is a significant undergraduate research training component to this project. Furthermore, the PI has a strong record of incorporating instrumentation from his research program in the physics teaching curriculum. This project is jointly funded by the Atomic, Molecular, and Optical Experimental Physics program, the Established Program to Stimulate Competitive Research (EPSCoR), and the Plasma Physics program.Technical audience abstract:There has been significant recent success in experiments that have revealed the evolution of ion temperature and coupling strength using optical probes in ultracold neutral plasmas (UNPs) made using cold alkaline earth atoms. Additionally, lasers have been used to cool the ions in such plasmas, dramatically increasing the ionic coupling parameter. In contrast, electrons have several additional heating mechanisms to ions, primarily due to heating when electrons recombine with ions, that limit the minimum temperature (and maximum coupling strength) that can be achieved. The PI and his undergraduate students will use embedded Rydberg atoms to see if cooling mechanisms already identified in his lab can be tailored to overcome these heating mechanisms, and push the electron coupling parameter as high as 0.5. A second goal of the research will be to explore robust methods for measuring electron temperatures in UNPs. Specifically, spatial and temporal mapping of plasma electric fields will be carried out using mm-wave spectroscopy of embedded Rydberg atom sensors, and this method will be compared with direct spatial measurements of the plasma expansion by quenching the UNP with a fast electric field pulse and observing the resulting ion time-of-flight spectra.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.