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Ultracold Plasma

Ultracold plasmas review article
Ultracold plasmas can be created by photoionizing cold atoms so that electrons with low energy are created. If the density of the atoms is high enough and the electrons have low enough energy, the space charge of the ions will prevent the electrons from leaving the region of the ions. Below is a schematic of how the potential well for electrons deepens with time. Thus, a nearly neutral, but very cold plasma is created. The thermal pressure of the electrons cause the plasma to expend on a time scale of a few tens of microseconds.
(From T.C. Killian et al, Phys. Rev. Lett. 83, 4776 (1999))

In many of these plasmas, the electrons are cold enough to form a substantial population of Rydberg atoms. These atoms substantially change the evolution of the plasma. The interplay between the electrons, ions and Rydberg atoms leads to a rich variety of behavior. In several of the early experiments, it was not at all clear what was going on and many of the early interpretations of the experimental results were often flawed or very incomplete.

We performed calculations of this system using standard atomic and plasma processes. Our calculations agreed very well with all experiments and provided several predictions that were verified in later experiments. We could use our simulations to provide the needed interpretation of this system. Below is a brief description of results in two recent publications.

S.D. Bergeson and F. Robicheaux, “Recombination fluorescence in ultracold neutral plasmas,” Phys. Rev. Lett. 101, 073202 (2008). PDF (229 kB)

This was a collaboration with the experimental group of Scott Bergeson to study the result of fluorescence from an ultracold plasma. The fluorescence arises from a complex series of events that starts with three body recombination and the subsequent collisions between the electrons and the atoms that form in the plasma.
This image shows the measured fluorescence as a function of time. The rise at early times is because time is needed for the atoms to form and then get de-excited to the states that fluoresce. The drop at later times is due to the plasma expanding out of the focus and because less atoms form reach low states as the density drops.
This image shows the early time fluorescence at different temperatures; each temperature is for a different density with the fluorescence scaled by the density3 for the 30 and 57 K data. At low temperature, the scaling is different due to a level that perturbs the bound states of Ca which is the atom used in the experiment.

F. Robicheaux and J.D. Hanson, "Simulated expansion of an ultra-cold, neutral plasma", Phys. Plasmas 10, 2217 (2003). PDF (172 kB)

This paper covers many, many aspects of the formation and evolution of ultracold plasmas, including how the electrons thermalize, evaporate, recombine with the atoms, how the plasma expands, how Rydberg gases convert to plasmas, ...
This image shows the time dependence of the electron part of the plasma as a function of time at different parts of the plasma; notice that the innermost part of the plasma has a higher temperature than the edge and that it takes a long time (1 microsecond) for the temperature to equilibrate.
This image shows the time dependence of electrons leaving the plasma compared to experiment; at early times the electrons leave promptly because the space charge hasn't built up but at later times the electrons can still slowly leave due to evaporation.

The plasma expands due to the thermal pressure from the electrons on the ions.
This image shows the r dependence of the density at 3 different times. At early times, it is roughly a 3-dimensional gaussian. At later times, a step develops on the outer edge of the plasma because the outermost ions do not experience as strong a force as those just inside. This (plasmaexp.avi) shows a movie of the ion density as a function of time up to the point where the singularity in the density develops at the edge of the plasma.

Other Recent Publications

F. Robicheaux, "Ionization due to the interaction between two Rydberg atoms," J. Phys. B 38, S333 (2005). PDF (114 kB)

F. Robicheaux and J.D. Hanson, "Simulation of the Expansion of an Ultracold Neutral Plasma," Phys. Rev. Lett. 88, 055002 (2002). PDF (111 kB)

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Department of Physics
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Phyics Department

Auburn University

ALPHA Project