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Quantum Electronics

Moderation of recombination in an ultracold laser-produced plasma

Aleksei N Tkachev, Sergei I Yakovlenko

Published Dec 31, 2000• 23 citations
Publisher

Abstract

It is shown that the recent production [T C Killian et al. Phys. Rev. Lett . 83 4776 (1999)] of an ultracold laser plasma ( N e ~ 2 × 10 9 cm -3 , T e ~ 0.1 K, T i ~ 10 μK) can be considered as the first experimental demonstration of the metastable state of a supercooled plasma which we predicted theoretically. Our theory provides an explanation for the moderation of three-body recombination observed in the above-cited paper. We present the calculations that simulate the conditions of the ultracold-plasma experiments.

DOI
10.1070/qe2000v030n12abeh001869

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Physics of Plasmas•Aug 30, 2002•101 citations
It is shown that the recent production [T C Killian et al. Phys. Rev. Lett . 83 4776 (1999)] of an ultracold laser plasma ( N e ~ 2 × 10 9 cm -3 , T e ~ 0.1 K, T i ~ 10 μK) can be considered as the first experimental demonstration of the metastable state of a supercooled plasma which we predicted theoretically. Our theory provides an explanation for the moderation of three-body recombination observed in the above-cited paper. We present the calculations that simulate the conditions of the ultracold-plasma experiments.
Recent experiments by T.C. Killian et al. [Phys. Rev. Lett. 83 , 4776 (1999)], in which an ultracold plasma ( N e ∼2×10 9 cm −3 , T e ∼0.1K, and T i ∼10 μK) with anomalously long lifetime of ∼100 μs was obtained, are explained based on a previously developed theory. The results of computer simulations of the plasma transition into a metastable state and initial heating of electrons up to several K are presented. An expression earlier obtained for the rate of the metastable plasma recombination agrees with the measured anomalously long lifetime. A conclusion is drawn that the previously predicted new physical object—a metastable overcooled plasma—is realized experimentally.
We present the first experiment to observe an ultracold neutral plasma. The plasma, which was created by photoionization of laser cooled atoms, has charge densities as high as 2×109 cm−3, and the temperatures of electrons and ions are as low as 100 mK and 10 μK, respectively. The plasma has a lifetime of about 100 μs, much longer than predicted by recombination rates. When the laser that excites the atoms is tuned below the ionization limit we create a sample of very highly excited cold Rydberg atoms. At our highest densities and during a time of a few microseconds, in which the Rydberg atoms are essentially stationary, the ensemble evolves towards an unbound plasma-like state.
Summary form only given. We have observed recombination into a large range of Rydberg levels in an ultracold plasma. The plasma is created by two-photon ionization of laser-trapped and -cooled metastable xenon atoms. By changing the intensity and detuning of the ionizing laser pulse, the initial temperature and density of the plasma can be varied. This allows us to access plasma coupling parameters (???) between 0.1 and 5. In this range of values, we previously reported unexplained heating of the expanding cloud, between 10 and 100 ???s after creation of the plasma. The recently observed recombination appears to provide much of the extra energy responsible for this behavior. The recombined atoms, up to 20% of the created plasma, populate energy levels down to n=40. They survive for /spl sim/1 ms indicating that high angular momentum states are present. However, standard scaling laws for three-body recombination do not agree with the observed dynamics. Furthermore, Rydberg atom formation is incomplete at the time when the plasma expansion velocity has reached its maximum value. Various effects are under consideration as possible remedies to these inconsistencies.
Photoionizing laser-cooled atoms produces ultracold neutral plasmas with initial temperatures of 1–1000 K and densities as high as 10 10 cm −3 . Applied radio frequency fields can excite plasma oscillations that are used to monitor the expansion of the unconfined plasma. Significant three-body recombination of electrons and ions into Rydberg atoms takes place during the plasma expansion. Previous experiments have been done with xenon, but a new experiment is planned with laser-cooled strontium. The strontium ion has an optically allowed transition at a convenient blue wavelength. This will allow direct imaging of the plasma through fluorescence or absorption, and may enable laser cooling and trapping of the plasma.
In recent experiments ultracold plasmas were produced by photoionizing small clouds of laser cooled atoms. This paper presents the results of molecular dynamic simulations for the early time evolution of such plasmas. Contrary to earlier speculation, no evidence of strong electron–electron correlations is observed in the simulations even if the initial value of the coupling parameter (Γe=e2/akTe) is much larger than unity. As electron–electron correlations begin to develop, the correlation energy is released to heat the electrons, raising the electron temperature to the point where Γe∼1 and limiting further development of correlation. Further heating of the electrons occurs as a by-product of three-body recombination. When a model of laser cooling is added to the simulation, the formation of strong ion–ion correlation is observed. Contrary to earlier suggestion, the rate of three-body recombination is observed to be in reasonable agreement with the traditional formula, R=3.9×10−9 s−1[n(cm−3)]2[Te(K)]−9/2, but care must be taken to use the correct temporally evolving temperature, Te. The simulations are challenging because it is necessary to follow three-body recombination into weakly bound (high n quasiclassical) Rydberg states, and the time scale for such states is short compared to that for the plasma dynamics. This kind of problem was faced earlier in computational astrophysics when studying binary star formation in globular clusters and the simulation method used here is adapted from such studies.