Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

Abstract

Ultracold plasmas (UCPs) provide a well-controlled system for studying multiple aspects in plasma physics that include collisions and strong coupling effects. By applying a short electric field pulse to a UCP, a plasma electron center-of-mass (CM) oscillation can be initiated. For accessible parameter ranges, the damping rate of this oscillation is determined by the electron-ion collision rate. We performed measurements of the oscillation damping rate with such parameters and compared the measured rates to both a molecular dynamic (MD) simulation that includes strong coupling effects and a Monte-Carlo binary collision simulation designed to predict the damping rate including only weak coupling considerations. We found agreement between the experimentally measured damping rate and the MD result. This agreement did require including the influence of a previously unreported UCP heating mechanism whereby the presence of a DC electric field during ionization increased the electron temperature, but estimations and simulations indicate that such a heating mechanism should be present for our parameters. The measured damping rate at our coldest electron temperature conditions was much faster than the weak coupling prediction obtained from the Monte-Carlo binary collision simulation, which indicates the presence of a significant strong coupling influence. The density averaged electron strong coupling parameter Γ measured at our coldest electron temperature conditions was 0.35.Electron-ion collisions are a fundamental feature of plasmas that determine several plasma properties, such as electron-ion thermalization rates [1], transport coefficients (diffusion, electric conductivity) [2], and stopping power considerations that, for instance, influence achievable DT fusion [3,4]. For a weakly coupled plasma, the electron-ion collision rate is given by [5] where Z is the ion charge number, e is the elementary electron charge, n i is the ion density, ǫ 0 is the electric permittivity in vacuum, m e is the mass of an electron, v th = k b T e /m e , and ln Λ = ln (Cλ D /b 0 ) is called the Coulomb logarithm, where λ D is the Debye screening length, b 0 = e 2 /4πǫ 0 k b T is the characteristic large angle scattering impact parameter, where ǫ 0 is electric permittivity, and k b is Boltzmann constant, and C is a constant, suggested to be 0.765 in Ref. [1,6,7].The presence of the screening length in the collision rate shows collective effects are relevant in a plasma even for individual collisions. This comes about because of a logarithmic divergence in the computed collision rate arising from large impact parameter collisions. The screening in a plasma reduces the influence of such collisions by screening out the inter-particle Coulomb forces. When the screening length λ D is much larger than other scale lengths such as b 0 or the typical interparticle spacing given by the Wigner-Seitz radius a, the assumptions that go into the derivation of Eq. 1 are valid. For sufficiently cold and dense plasmas, however, λ D becomes on the order of...

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