Abstract. We describe a hybrid molecular dynamics approach for the description of ultracold neutral plasmas, based on an adiabatic treatment of the electron gas and a full molecular dynamics simulation of the ions, which allows us to follow the long-time evolution of the plasma including the effect of the strongly coupled ion motion. The plasma shows a rather complex relaxation behavior, connected with temporal as well as spatial oscillations of the ion temperature. Furthermore, additional laser cooling of the ions during the plasma evolution drastically modifies the expansion dynamics, so that crystallization of the ion component can occur in this nonequilibrium system, leading to lattice-like structures or even long-range order resulting in concentric shells.
IntroductionExperiments in cooling and trapping of neutral gases have paved the way toward a new parameter regime of ionized gases, namely the regime of ultracold neutral plasmas (UNPs). Experimentally, UNPs are produced by photoionizing a cloud of laser-cooled atoms collected in a magneto-optical trap [1], with temperatures down to 10 µK. By tuning the frequency of the ionizing laser, initial electron kinetic energies of E e /k B = 1K − 1000K have been achieved. The time evolution of several quantities characterizing the state of the plasma, such as the plasma density [1,2,3], the rate of expansion of the plasma cloud into the surrounding vacuum [2], the energy-resolved population of bound Rydberg states formed through recombination [4], or electronic [5,6] as well as ionic [3] temperature have been measured using various plasma diagnostic methods.Despite the low typical densities of ≈ 10 9 cm −3 , the very low initial temperatures suggest that these plasmas have been produced well within the strongly coupled regime, with Coulomb coupling parameters up to Γ e = 10 for the electrons and even Γ i = 30000 for the ions. Thus, UNPs seem to offer a unique opportunity for a laboratory study of neutral plasmas where, depending on the initial electronic kinetic energy, either one component (namely the ions) or both components (ions and electrons) may be strongly coupled. Moreover, the plasma is created in a completely uncorrelated state, i.e. far away from thermodynamical equilibrium. The relaxation of a strongly correlated system towards equilibrium is an interesting topic in nonequilibrium thermodynamics and has been studied for decades. The history of this problem must be traced back to the important contributions of Klimontovich [7,8,9], who pointed out that kinetic energy conserving collision integrals such as the Boltzmann, Landau and Lenard-Balescu collision integrals are not appropriate for such a situation, and derived non-Markovian kinetic