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Journal of Experimental and Theoretical Physics Letters

Anomalous slowdown of relaxation in an ultracold plasma

А. Н. Ткачев, Sergey I. Yakovlenko

Published Jan 1, 2001• 4 citations
Publisher

Abstract

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.

DOI
10.1134/1.1358421

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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.
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.
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.
In the recent work Fletcher et al. [Phys. Rev. Lett., v.99, p.145001 (2007)] reported on the novel experimental technique, enabling to measure the temperature of the expanding ultracold plasmas over a considerable time interval (up to 60-70 μs). It was unexpectedly found that the electron temperature dropped with time as T_e(t) ~ t^{-α} with α= 1.2 +/- 0.1 \approx 1 instead of α= 2, which would be expected for the adiabatic cooling of electrons in the cloud expanding linearly in time. The above-cited authors supposed that 'the difference is likely due to the significant heating effects from 3-body recombination' (i.e. the inelastic processes), but they did not provide sufficient quantitative estimates supporting such a conclusion. The aim of the present comment is to mention that the experimentally revealed t^{-1}-dependence can be explained under quite general assumptions by the purely elastic processes in the ultracold plasma, as it was done a few years ago in our work [Yu.V. Dumin, J. Low Temp. Phys., v.119, p.377 (2000)]. Therefore, t^{-1}-dependence revealed in the above-cited experiment should be primarily a manifestation of "virialization" of the electron velocities in the regime of strong electron-ion correlations, while the heat release by inelastic processes should be of secondary importance (it might be responsible, particularly, for changing the exponent from -1 to -1.2). This is confirmed also by our ab initio molecular-dynamic simulations taking into account the strong electron-ion correlations (i.e. not based on the PIC method, Vlasov approximation for electrons, etc., which are applicable only to small-angular scattering).
Ultracold plasma provides a possible route to approach the strongly-coupled regime under laboratory conditions.Normally,the lifetime of ultracold plasma is very limited due to plasma heating and expansion mechanisms.We present a new method to generate long lifetime ultracold plasmas consisting mainly of cations and anions.This plasma demonstrates a capability of traversing a DC barrier of up to 5(or-3) V.The lifetime of this plasma is expected to be more than 250 μs.Finally,molecular dynamics(MD) simulation is used to explain how anions slow the expansion rate and prolong the lifetime of this plasma.
Ultracold plasmas are a promising candidate for the creation of strongly coupled Coulomb systems. Unfortunately, the values of the coupling parameter Γe actually achieved after photoionization of the neutral atoms remain relatively small because of the considerable intrinsic heating of the electrons. A conceivable way to get around this obstacle might be to utilize a spontaneous ionization of the ultracold Rydberg gas, where the initial kinetic energies could be much less. However, the spontaneous avalanche ionization will result in a very inhomogeneous distribution (clusterization) of the ions, which can change the efficiency of the electron relaxation in the vicinity of such clusters substantially. In the present work, this hypothesis is tested by an extensive set of numerical simulations. As a result, it is found that despite a less initial kinetic energy, the subsequent relaxation of the electron velocities in the clusterized plasmas proceeds much more violently than in the case of the statistically uniform ionic distribution. The electron temperature, first, experiences a sharp initial jump (presumably, caused by the “virialization” of energies of the charged particles) and, second, exhibits a gradual subsequent increase (presumably, associated with a multi-particle recombination of the electrons at the ionic clusters). As a possible tool to reduce the anomalous temperature increase, we also considered a two-step plasma formation, involving the blockaded Rydberg states. This leads to a suppression of the clusterization due to a quasi-regular distribution of ions. In such a case, according to the numerical simulations, the subsequent evolution of the electron temperature proceeds more gently, approximately with the same rate as in the statistically uniform ionic distribution.