On the anomalous moderation of relaxation in an ultracold plasma

Jacob Roberts, Chad Fertig, S. L. Rolston

Trevor Strickler, G. Bannasch, Jose Castro +4 more

Edward R. Grant, Ruoxi Wang, Kevin Madera +6 more

А. Н. Ткачев, Sergey I. Yakovlenko

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.

Aleksei N Tkachev, Sergei I Yakovlenko

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.

Yurii V. Dumin, A. T. Lukashenko

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.