Molecular Dynamics Simulations of the Dynamics of Correlations and Relaxation in an OCP

Zwicknagel, G.

doi:10.1002/ctpp.2150390138

Cited by 37 publications

(59 citation statements)

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“…Compared to the homogeneous plasmas considered in [23], the oscillations of the average ionic temperature damp out much quicker in the present case. This can be attributed to the fact that the Gaussian density profile of the UNPs created in current experiments leads to a spatial dependence of the correlation timescale t corr , the build-up of correlations being fastest in the center of the plasma where the density is highest, and becoming slower towards the edge of the plasma cloud.…”

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confidence: 51%

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Relaxation to Nonequilibrium in Expanding Ultracold Neutral Plasmas

2005

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We investigate the strongly correlated ion dynamics and the degree of coupling achievable in the evolution of freely expanding ultracold neutral plasmas. We demonstrate that the ionic Coulomb coupling parameter Γi increases considerably in later stages of the expansion, reaching the strongly coupled regime despite the well-known initial drop of Γi to order unity due to disorder-induced heating. Furthermore, we formulate a suitable measure of correlation and show that Γi calculated from the ionic temperature and density reflects the degree of order in the system if it is sufficiently close to a quasisteady state. At later times, however, the expansion of the plasma cloud becomes faster than the relaxation of correlations, and the system does not reach thermodynamic equilibrium anymore.PACS numbers: 52.27. Gr,32.80.Pj,05.70.Ln Freely expanding ultracold neutral plasmas (UNPs) [1] have attracted wide attention both experimentally [2,3,4,5] and theoretically [6,7,8,9,10]. A main motivation of the early experiments was the creation of a strongly coupled plasma, with the Coulomb coupling parameter (CCP) Γ = e 2 /(ak B T ) ≫ 1 (where T is temperature and a is the Wigner-Seitz radius). From the experimental setup of [1], the CCPs of electrons and ions were estimated to be of the orders of Γ e ≈ 30 and Γ i ≈ 30000, respectively. By changing the frequency of the ionizing laser, the electronic temperature can be varied, offering the prospect of controlling the coupling strength of the electrons and creating UNPs where either one, namely the ionic, or both components could be strongly coupled.However, due to unavoidable heating effects [6,11,12] these hopes have not materialized yet, and only Γ e ≈ 0.2 and Γ i ≈ 2 have been confirmed. Furthermore, the evolution of the expanding plasma turns out to be a rather intricate problem of non-equilibrium plasma physics for which a clear definition of the degree of correlation is not obvious to begin with.The goal of this letter is twofold: Firstly, we will formulate a consistent measure of correlation for expanding ultracold plasmas, and secondly we demonstrate that the strongly correlated regime with Γ i ≈ 10 for the ionic plasma component can be reached by simply waiting until the plasma has (adiabatically) expanded long enough under already realized experimental conditions. This is remarkable in the light of alternatives proposed to increase Γ i [12,13,14,15,16,17] which are experimentally rather involved.Substantiating both of our statements theoretically requires the ability to propagate the plasma numerically over a long time with full account of the ionic correlations. To this end, we have developed a hybrid molecular dynamics (H-MD) method [9] for the description of ultracold neutral plasmas. In our approach, ions and recombined atoms are propagated in the electronic mean-field potential with the full ion-ion interaction taken into account. The much faster and weakly coupled electrons, on the other hand, are treated on a hydrodynamical level. Elastic as well as inelastic ...

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confidence: 51%

“…plasmas show that the ion temperature starts to undergo damped oscillations around its equilibrium value if both of these timescales become equal, which happens for Γ i (0) > ∼ 0.5 [23]. Therefore, the nonmonotonic ion relaxation observed in ultracold plasmas may be seen as a direct manifestation of the violation of Bogoliubov's hypothesis.…”

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confidence: 98%

Relaxation to Nonequilibrium in Expanding Ultracold Neutral Plasmas

2005

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“…Kinetic-energy oscillations at 2! pi have been observed in molecular dynamics simulations of equilibrating strongly coupled systems [9,11,12]. Calculations [6,28] also show oscillations in the velocity autocorrelation function in equilibrium strongly coupled one-component plasmas.…”

Section: Prl 93 265003 (2004) P H Y S I C a L R E V I E W L E T T E mentioning

confidence: 83%

“…For strongly coupled plasmas, however, these time scales both become equal to ! ÿ1 pi [9]. Kinetic-energy oscillations at 2!…”

Section: Prl 93 265003 (2004) P H Y S I C a L R E V I E W L E T T E mentioning

confidence: 99%

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Electron Screening and Kinetic-Energy Oscillations in a Strongly Coupled Plasma

et al. 2004

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We study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma. DOI: 10.1103 There has been significant theoretical study of the equilibration of strongly coupled plasmas [6 -12], especially in the context of plasmas produced with high-intensity lasers. In addition to generating fundamental interest, this problem challenges computational resources and techniques. Experimental results have been lacking, however, because of the fast time scales involved and limited diagnostics.Ultracold neutral plasmas [13], produced by photoionizing clouds of laser-cooled and trapped atoms, are ideal for experimental studies. The equilibration of the plasma is relatively slow ( 100 ns) due to lower plasma density. Ultracold neutral plasmas also offer a high level of control and diagnostics. By varying laser intensities and wavelengths, it is possible to accurately set the initial density and energy of the system. Optical imaging [14] provides an in situ probe of plasma properties with excellent spatial, temporal, and spectral resolution.In this Letter, we explore ion equilibration during the first microsecond after the plasma is created. The density sets the time and the energy scale for equilibration, but electron screening effects are evident. Even when the number of electrons per Debye sphere is small, the equilibration temperature of the ions agrees with a model [15] that uses a Yukawa ion-ion potential.We also observed oscillations of the ion kinetic energy. For many years, this phenomenon has been the subject of intense study through analytic calculations [7] and simulations [6,[8][9][10][11][12]] of one-component strongly coupled plasmas, but it has not previously been observed experimentally. The oscillations and their damping reflect universal dynamics of a Coulomb system with spatial correlations.Details on laser cooling, plasma formation, and imaging are given in [14,16]. The experiment starts with strontium atoms that are cooled and trapped in a magneto-optical trap (MOT). The neutral atom cloud is characterized by a temperature of about 10 mK, 2 10 8 atoms, and a Gaussian density distribution. We vary the atom density by changing the MOT parameters, or by turning the MOT off and releasing the atoms in a ballistic expansion. Up to 30% of the neutral atoms are then ionized with one photon from the cooling laser and one photon from a pulsed dye laser. The ion density distribution equals the atom distribution at the time of photoionization and is given by n i r n 0i exp ÿr 2 =2 2 , with from 0.6 to 1 mm and n 0i from 2 10 9 to 1:4 10 10 cm ÿ3 . The electron density, n e r , closely follows ...

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Ultrafast Dynamics in Extended Systems

Saalmann

Rost

2014

Attosecond Nanophysics

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Molecular Dynamics Simulations of the Dynamics of Correlations and Relaxation in an OCP

Cited by 37 publications

References 9 publications

Relaxation to Nonequilibrium in Expanding Ultracold Neutral Plasmas

Relaxation to Nonequilibrium in Expanding Ultracold Neutral Plasmas

Electron Screening and Kinetic-Energy Oscillations in a Strongly Coupled Plasma

Ultrafast Dynamics in Extended Systems

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