Molecular Dynamics Simulations of Classical Stopping Power

Grabowski, Paul E.; Surh, Michael P.; Richards, David F.; Graziani, Frank; Murillo, Michael S.

doi:10.1103/physrevlett.111.215002

Cited by 88 publications

(79 citation statements)

References 38 publications

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“…In contrast, we found a wellresolved disagreement between the measured rate and predictions based on only weak coupling considerations, making these results the first demonstration of strong coupling influence on electron-ion collision rates in a system free of significant interaction with neutrals. While previous strong coupling theoretical extensions naively indicate 50 % corrections for our conditions [1,6,17,18], we see a factor of 4 increase instead -a much larger effect that is described in detail below.To create our UCP, we first made an 85 Rb magneto- …”

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confidence: 39%

Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

2017

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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...

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confidence: 39%

Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

2017

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“…Further, we mention a classical molecular dynamics approach to charged-particle stopping in warm dense matter by Grabowski et al [6] where the classical wake potential was computed. Finally, an even simpler approach that is based on a quantum hydrodynamic model (QHD) has been applied that predicts an attractive interaction between ions even in the absence of streaming, u e = 0 [29].…”

Section: Introductionmentioning

confidence: 99%

Ion potential in warm dense matter: Wake effects due to streaming degenerate electrons

et al. 2015

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The effective dynamically screened potential of a classical ion in a stationary flowing quantum plasma at finite temperature is investigated. This is a key quantity for thermodynamics and transport of dense plasmas in the warm dense matter regime. This potential has been studied before within hydrodynamic approaches or based on the zero temperature Lindhard dielectric function. Here we extend the kinetic analysis by including the effects of finite temperature and of collisions based on the Mermin dielectric function. The resulting ion potential exhibits an oscillatory structure with attractive minima (wakes) and, thus, strongly deviates from the static Yukawa potential of equilibrium plasmas. This potential is analyzed in detail for high-density plasmas with values of the Brueckner parameter in the range 0.1 ≤ rs ≤ 1, for a broad range of plasma temperature and electron streaming velocity. It is shown that wake effects become weaker with increasing temperature of the electrons. Finally, we obtain the minimal electron streaming velocity for which attraction between ions occurs. This velocity turns out to be less than the electron Fermi velocity. Our results allow, for the first time, for reliable predictions of the strength of wake effects in nonequilibrium quantum plasmas with fast streaming electrons showing that these effects are crucial for transport under warm dense matter conditions, in particular for laser-matter interaction, electron-ion temperature equilibration and for stopping power.

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“…In complex plasmas [1], dynamical screening and wake effects have been investigated in a large number of studies, including experimental [2,3,4,5] as well as theoretical [6,7,8,9,10] work. ‡ Computational approaches include first-principle molecular dynamics simulations [13,14,15], fluid codes [16,17] and particle-in-cell (PIC) simulations, e.g. [18,19,20,21].…”

Section: Introductionmentioning

confidence: 99%

Screened Coulomb potential in a flowing magnetized plasma

Joost

Ludwig

Kählert

et al. 2014

Plasma Phys. Control. Fusion

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Abstract. The electrostatic potential of a moving dust grain in a complex plasma with magnetized ions is computed using linear response theory, thereby extending our previous work for unmagnetized plasmas [P. Ludwig et al., New J. Phys. 14, 053016 (2012)]. In addition to the magnetic field, our approach accounts for a finite ion temperature as well as ion-neutral collisions. Our recently introduced code Kielstream is used for an efficient calculation of the dust potential. Increasing the magnetization of the ions, we find that the shape of the potential crucially depends on the Mach number M . In the regime of subsonic ion flow (M < 1), a strong magnetization gives rise to a potential distribution that is qualitatively different from the unmagnetized limit, while for M > 1 the magnetic field effectively suppresses the plasma wakefield.

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Molecular Dynamics Simulations of Classical Stopping Power

Cited by 88 publications

References 38 publications

Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

Ion potential in warm dense matter: Wake effects due to streaming degenerate electrons

Screened Coulomb potential in a flowing magnetized plasma

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