Numerical Simulation of Ultracold Plasmas: How Rapid Intrinsic Heating Limits the Development of Correlation

Kuzmin, S. G.; O’Neil, T. M.

doi:10.1103/physrevlett.88.065003

Cited by 105 publications

(145 citation statements)

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“…This makes them an excellent platform for studying a wide range of plasma phenomena, such as equilibration of strongly coupled plasmas, [3][4][5][6][7][8][9][10][11][12][13][14] ambipolar diffusion with 15 and without 16,17 a magnetic field, electron plasma oscillations, 16,18 Tonks-Dattner resonances 19 and edge modes, 20 ion acoustic waves, 21 an electron drift instability, 22 threebody recombination at ultracold temperatures, 13,[23][24][25][26][27][28][29] and the crossover to an ultracold plasma from a dense gas of Rydberg atoms. [30][31][32][33] Recent experiments creating ultracold plasmas in a seeded supersonic molecular beam 34 introduce molecular processes to the plasma evolution and show promise for yielding more strongly coupled systems.…”

Section: Introductionmentioning

confidence: 99%

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

et al. 2013

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We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma. V C 2013 AIP Publishing LLC.

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Section: Introductionmentioning

confidence: 99%

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

et al. 2013

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“…While the initial ion relaxation reveals some interesting strongcoupling effects as discussed above, disorder-induced heating [7,12] drives the ion component to the border of the strongly coupled fluid regime Γ i ≈ 2 and therefore limits the amount of correlations achievable in UNPs. However, so far this could be verified only for the early stage of the plasma evolution [7,12,19]. The present H-MD approach allows us to study also the long-time behavior of the ion coupling.…”

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confidence: 99%

“…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).…”

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confidence: 99%

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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“…They are created by photoionizing laser cooled atoms and therefore have very precisely controlled initial temperatures and densities [1,2,3,4]. Photoionization causes an impulsive hardening of the interparticle potential [5], and the pathway to (non)equilibrium can be studied in detail [6,7].The electron system equilibrates on the shortest time scales and largely determines how the plasma expands [8,9,10,11,12]. Indirect measurements of the electron temperature agree well with simulations [9,13,14].…”

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confidence: 65%

Recombination Fluorescence in Ultracold Neutral Plasmas

Bergeson

Robicheaux

2008

Phys. Rev. Lett.

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We present the first measurements and simulations of recombination fluorescence in ultracold neutral plasmas. In contrast with previous work, experiment and simulation are in significant disagreement. Comparison with a recombination model suggests that the disagreement could be due to the high energy portion of the electron energy distribution or to large energy changes in electron/Rydberg scattering. Recombination fluorescence opens a new diagnostic window in ultracold plasmas because it probes the deeply-bound Rydberg levels, which depend critically on electron energetics.PACS numbers: 34.80. Lx, 52.27.Gr, 52.27.Cm Ultracold neutral plasmas can provide new insights into relaxation phenomena in strongly coupled Coulomb systems. They are created by photoionizing laser cooled atoms and therefore have very precisely controlled initial temperatures and densities [1,2,3,4]. Photoionization causes an impulsive hardening of the interparticle potential [5], and the pathway to (non)equilibrium can be studied in detail [6,7].The electron system equilibrates on the shortest time scales and largely determines how the plasma expands [8,9,10,11,12]. Indirect measurements of the electron temperature agree well with simulations [9,13,14]. Disorder-induced heating, three body recombination (TBR), and electron/Rydberg scattering are important heating mechanisms in the plasma. The latter two are thought to occur on times that are long compared to the electron plasma frequency. Thus TBR can probe the electron system at early times in the plasma evolution after disorder-induced heating has occurred [15].In higher temperature plasmas, the TBR picture is an electron and ion colliding in the presence of another electron. The TRB rate is the two-body collision frequency (nσv th ) multiplied by the probability that a third body is one collision distance away (nb 3 ). The collision distance and cross section depend on the electron temperature (b = e 2 /4πǫ 0 k B T and σ = πb 2 ), giving the well-known TBR rate α 3 = Rn 2 T −9/2 , where R is the TBR rate coefficient [16].In a strongly-coupled system, this binary collision picture breaks down.When the nearest-neighbor potential energy exceeds the kinetic energy (Γ ≡ e 2 n 1/3 /4πǫ 0 k B T > 1), the plasma is strongly coupled. The average distance between electrons becomes comparable to the collision distance b ∼ n −1/3 . The electrons are in a constant collision and TBR becomes, in a sense, many-body recombination. A numerical simulation of early-time recombination suggest that even moderate initial coupling in the electron system changes the recombination rate by perhaps a factor of two [17]. Other theoretical treatments suggest larger changes [18,19]. However, there is no experimental evidence that TBR departs from the standard formulas [20].In this paper we present a new study of three-body recombination in ultracold neutral plasmas. We measure the time-dependent fluorescence emitted by the plasma following electron/ion recombination for a range of initial plasma densities and electr...

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Numerical Simulation of Ultracold Plasmas: How Rapid Intrinsic Heating Limits the Development of Correlation

Cited by 105 publications

References 8 publications

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

Relaxation to Nonequilibrium in Expanding Ultracold Neutral Plasmas

Recombination Fluorescence in Ultracold Neutral Plasmas

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