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...
Applying a short electric field pulse to an ultracold plasma induces an electron plasma oscillation. This manifests itself as an oscillation of the electron center of mass around the ion center of mass in the ultracold plasma. In general, the oscillation can damp due to either collisionless or collisional mechanisms, or a combination of the both. To investigate the nature of oscillation damping in ultracold plasmas, we developed a molecular dynamics model of the ultracold plasma electrons. Through this model, we found that depending on the neutrality of the ultracold plasma and the size of an applied DC electric field, there are some parameter ranges where the damping is primarily collisional and some primarily collisionless. We conducted experiments to compare the measured damping rate with theory predictions and found them to be in good agreement. Extension of our measurements to different parameter ranges should enable studies for strong-coupling influence on electron-ion collision rates.
Ultracold plasmas (UCPs) are created under conditions of near but not perfect neutrality. In the limit of zero electron temperature, electron screening results in non-neutrality manifesting itself as an interior region of the UCP with both electrons and ions and an exterior region composed primarily of ions. The interior region is the region of the most scientific interest for 2-component ultracold plasma physics. This work presents a theoretical model through which the time evolution of non-neutral UCPs is calculated. Despite Debye screening lengths much smaller than the characteristic plasma spatial size, model calculations predict that the expansion rate and the electron temperature of the UCP interior is sensitive to the neutrality of the UCP. The predicted UCP dependence on neutrality has implications for the correct measurement of several UCP properties, such as electron temperature, and a proper understanding of evaporative cooling of the electrons in the UCP.
Electron evaporation plays an important role in the electron temperature evolution and thus expansion rate in low-density ultracold plasmas. In addition, evaporation is useful as a potential tool for obtaining colder electron temperatures and characterizing plasma parameters. Evaporation theory has been developed for atomic gases and has been applied to a one-component plasma system. We numerically investigate whether such an adapted theory is applicable to ultracold neutral plasmas. We find that it is not due to the violation of fundamental assumptions of the model. The details of our calculations are presented as well as a discussion of the implications for a simple description of the electron evaporation rate in ultracold plasmas.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.