Creation of an Ultracold Neutral Plasma

Abstract

We report the creation of an ultracold neutral plasma by photoionization of laser-cooled xenon atoms. The charge carrier density is as high as 2 × 10 9 cm −3 , and the temperatures of electrons and ions are as low as 100 mK and 10 µK, respectively. Plasma behavior is evident in the trapping of electrons by the positive ion cloud when the Debye screening length becomes smaller than the size of the sample. We produce plasmas with parameters such that both electrons and ions are strongly coupled.The study of ionized gases in neutral plasma physics spans temperatures ranging from 10 16 K in the magnetosphere of a pulsar to 300 K in the earth's ionosphere [1]. At lower temperatures the properties of plasmas are expected to differ significantly. For instance, three-body recombination which is prevalent in high temperature plasmas, should be suppressed [2]. If the thermal energy of the particles is less than the Coulomb interaction energy, the plasma becomes strongly coupled, and the usual hydrodynamic equations of motion and collective mode dispersion relations are no longer valid [3]. Strongly coupled plasmas are difficult to produce in the laboratory and only a handful of examples exist [4], but such plasmas do occur naturally in astrophysical systems.In this work we create an ultracold neutral plasma with an electron temperature as low as T e = 100 mK, an ion temperature as low as T i = 10 µK, and densities as high as n = 2×10 9 cm −3 . We obtain this novel plasma by photoionization of laser-cooled xenon atoms. Within the experimentally accessible ranges of temperatures and densities both components can be simultaneously strongly coupled. A simple model describes the evolution of the plasma in terms of the competition between the kinetic energy of the electrons and the Coulomb attraction between electrons and ions. A numerical calculation accurately reproduces the data.Photoionization and laser-cooling have been used before in plasma experiments. Photoionization in a 600 K Cs vapor cell produced a plasma with T e ≥ 2000 K [5], and a strongly coupled non-neutral plasma was created by laser-cooling magnetically trapped Be + ions [6]. A plasma is often defined as an ionized gas in which the charged particles exhibit collective effects [7]. The length scale which divides individual particle behavior and collective behavior is the Debye screening length λ D . It is the distance over which an electric field is screened by redistribution of electrons in the plasma, and is given by λ D = ǫ 0 k B T e /e 2 n. Here, ǫ 0 is the electric permittivity of vacuum, k B is the Boltzmann constant, and e is the elementary charge. An ionized gas is not a plasma unless the Debye length is smaller than the size of the system [7]. In our experiment, the Debye length can be as low as 500 nm, while the size of the sample is σ ≈ 200 µm. The condition λ D < σ for creating a plasma is thus easily fulfilled.The atomic system we use is metastable xenon in the 6s [3/2] 2 state. This state has a lifetime of 43 s [8] and can be treated as the groun...

Ultrafast electronic spectroscopy for chemical analysis near liquid water interfaces: concepts and applications

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“…The length scale, which divides individual particle behavior and collective behavior, is the Debye screening length λ D . It is the distance over which an electric field is screened by redistribution of electrons in the plasma, and is given by [69] …”

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