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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, B.J. Bender, and M.A. Phillips, "Simulations of an ultracold, neutral plasma with equal mass for every charge," J. Phys. B 47, 245701 (2014). PDF (283 kB)

This paper gives the results of calculations for a system suggested to me by Phil Gould. The idea is to investigate the behavior of an ultracold plasma but for the case where the mass of the positive and the negative charges are the same.



This image shows the time dependence of the fraction of ions in bound states when the ions are initially created with ~0 K of thermal energy. The two curves are for different number of ions at the same density (showing the convergence of the results with respect to the number of ions in the calculation). Note that almost 1/4 of the ions form bound states on a time scale of ~100 plasma oscillation periods.



This image shows the speed distribution of the ions after the plasma has expanded for two different initial temperatures of the ions. All calculations are for the same initial density. The number of ions in the plasma is 80,000 for the dash dot lines and increases by a factor of 4 for each line type. As the number of ions increase, the final distribution becomes closer to a Maxwell-Boltzmann distribution because the initial cloud size is larger and there are more opportunities for collisions that will thermalize the ions.




Five Recent Publications

F. Robicheaux, B.J. Bender, and M.A. Phillips, "Simulations of an ultracold, neutral plasma with equal mass for every charge," J. Phys. B 47, 245701 (2014). PDF (283 kB)

K Niffenegger, K A Gilmore and F Robicheaux, "Early time properties of ultracold neutral plasmas," J. Phys. B 44, 145701 (2011). PDF (441 kB)

S.D. Bergeson, A. Denning, M. Lyon, and F. Robicheaux, "Density and temperature scaling of disorder-induced heating in ultracold plasmas," Phys. Rev. A 83, 023409 (2011). PDF (310 kB)

F. Robicheaux, S. D. Loch, M. S. Pindzola, and C. P. Ballance, "Contribution of near threshold states to recombination in plasmas," Phys. Rev. Lett 105, 233201 (2010). PDF (173 kB)

A. Denning, S.D. Bergeson, and F. Robicheaux, "Measurement and simulation of laser-induced fluorescence from nonequilibrium ultracold neutral plasmas," Phys. Rev. A 80, 033415 (2009). PDF (121 kB)

Francis Image

robichf[at]purdue.edu
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