Collisional recombination coefficient in an ultracold plasma: Calculation by the molecular dynamics method
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Collisional Recombination Coefficient in an Ultracold Plasma: Calculation by the Molecular Dynamics Method A. A. Bobrova, S. Ya. Bronina, B. B. Zelenera,c, B. V. Zelenera, E. A. Manykinb,c, and D. R. Khikhlukhaa,c,* a
Joint Institute for High Temperatures, Russian Academy of Sciences, ul. Izhorskaya 13/19, Moscow, 127412 Russia b“Kurchatov Institute” Russian Research Center, pl. Kurchatova 1, Moscow, 123182 Russia c “Moscow Engineering Physics Institute” National Research Nuclear University, Kashirskoe sh. 31, Moscow, 115409 Russia *email: [email protected] Received August 24, 2010
Abstract—New results of the calculations of the distribution function and the electron diffusion coefficient in energy space are presented. We analyze all of the data obtained and calculate the temperature dependence of the recombination coefficient. This dependence coincides with the Gurevich–Pitaevsky analytical for mula in the region of a weakly coupled plasma and begins to differ in the direction of a decline in the region of a strongly coupled plasma; the difference can reach several orders of magnitude. DOI: 10.1134/S1063776111020038
1. INTRODUCTION The influence of the Coulomb interaction between particles on the collisional recombination coefficient was considered in many works using both analytical [1–5] and numerical [6–9] methods. A nonequilib rium lowtemperature plasma with an electron tem perature Te higher than several thousand kelvins was traditionally considered in all works. The available experimental data on the recombination of such a plasma correspond to a coupling parameter γe < 0.2. The coupling parameter, which characterizes the ratio of the potential and kinetic energies for an electron in a Coulomb plasma, is defined by the formula 2 1/3
en γ e = e, Te
(1)
where e and ne are the electron charge and density, respectively. It should be noted that for the above mentioned plasma, distinguishing the contribution of collisional recombination to the general recombina tion process is a separate problem. Interest in the question about the influence of the interaction between particles on the recombination coefficient increased significantly when the results of experiments [10–12] in which a nonequilibrium ultra cold xenon plasma was produced at initial tempera tures Te = 1–30 K and densities ne = 109–1010 cm–3 were published in 1999. The coupling parameter under such initial conditions can reach 23, with the electrons remaining nondegenerate. In an experiment, an ultracold plasma at the initial time is a gas of cold free electrons with the same initial kinetic energy and even colder ions. During the
recombination of an ultracold plasma, the collisional processes are decisive over the time of its observation t = 10–4 s, because the population of the upper atomic levels with an energy lower than or of the order of Te is determined exclusively by the threebody (electron– electron–ion) collision process, while all of the remaining processes in the plasma
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