Photoionization of Cold Atoms and Non-Neutral Plasma Dynamics

Abstract

The development of atom trapping and cooling techniques has been a boon for the field of low temperature plasma physics. By forming plasmas through photoionization (PI) of cold atoms, researchers are able to create cold, dense plasmas that often lie in the strong-coupling regime. This is especially remarkable considering that many naturally-occurring plasmas in the strong-coupling regime are in inaccessible environments (e.g. the insides of stars and gas planets), whereas these laboratory-based plasmas prepared in cold-atom systems are relatively easy to produce and analyze with standard AMO techniques.

In this thesis, I present my work from several projects on photoionization, plasma formation and dynamics, and plasma trap development. I first discuss an experiment that I spearheaded on expansion dynamics in cold, non-neutral plasmas. In this experiment, cold plasmas are formed by photoionizing rubidium atoms in a MOT, and the resulting plasma ions are allowed to expand for a variable wait time before being imaged with a single-ion imaging system. This ion imaging system, along with the rest of the experimental setup, was originally used to study Rydberg atoms. I have adapted the setup for plasmas by installing a new laser and changing the experimental timings. I report on plasma features seen in this experimental setup, such as shock shells and time-evolved ion pair correlations.

I also present findings from a computational study of wavelength-dependent PI of Rydberg atoms, namely Rydberg-state rubidium and cesium. The main goal of the study is to find PI minima, wavelengths where the atom’s PI cross section decreases dramatically. These minima can be caused by Cooper minima or shape resonances, which each have their own telltale signs. This computational study paves the way for an informed experimental measurement of the locations of some of the identified PI minima, and comparing their experimental locations with the computational results. This is, in part, interesting because the latter depend on whether length or velocity gauge is used. An experimental measurement of PI cross sections could be a good way to explore the significance of gauge dependence in l-dependent potentials.

I conclude with a discussion of a project centered on creating a small MOT with diverging cooling beams inside of a metal enclosure. This project was a collaboration with researchers at Eastern Michigan University. The idea to make a MOT with these design features was born out of plans for cryostatic plasma experiments. Now, it has potential applications in plasma experiments, precision measurements, and quantum information. Design-wise, the diverging beams are formed by sending narrow laser beams into 1.5 mm ball lenses, which cause the beams to diverge rapidly. Six ball lenses, one for each cooling beam, are mounted on a metal structure called the ball lens optical box (BLOB), which encases the trapped atom cloud. This enclosure provides immediate benefits for plasma experiments, as it offers Faraday shielding from stray electric fields. The first build successfully produced and maintained a MOT, making it the first known case of a MOT made with diverging beams from ball lenses. No further experiments are planned for this first build, but the concept of a ball lens MOT will certainly be implemented into future experimental efforts. Subsequent designs may include more design features, such as interior electrodes for custom field configurations.