Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session YI2: Invited Basic: Basic Plasma Theory, Dynamo, Turbulence |
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Chair: Boris Breizman Room: Floridian Ballroom AB |
Friday, October 25, 2019 9:30AM - 10:00AM |
YI2.00001: Mean Force Kinetic Theory Invited Speaker: Scott Baalrud Traditional plasma kinetic theories can be derived from the BBGKY hierarchy by expanding either in terms of the range or strength of interactions, as in the Boltzmann or Lenard-Balescu theories, respectively. Either infrared or ultraviolet divergences arise as a consequence. These are resolved by invoking concepts of weak Coulomb coupling, such as Debye shielding or the distance of closest approach in a binary collision. Because these theories rely on ad hoc arguments, it is difficult to attempt generalizations to other important situations such as moderate or strong Coulomb coupling or strong magnetization. This work presents a new closure of the BBGKY hierarchy based on a single expansion parameter. It does not depend explicitly on the strength or range of interactions, which avoids the usual divergences. Rather, it is based on enforcing that the exact equilibrium limit is maintained at all orders of the hierarchy. The resulting kinetic equation shows that particles effectively interact via the potential of mean force and that the range of this force determines the size of the collision volume. The resulting collision operator is the same as the effective potential theory proposed in [1] based on a physical argument. In addition to providing a systematic derivation of this collision operator, the new theory [2] includes an additional term that is shown to be associated with the excess (non-ideal) component of the pressure and internal energy in the hydrodynamic limit. Tests of the transport and equation of state properties predicted by this theory are provided from a combination of molecular dynamics simulations and experiments in both ultracold plasmas and warm dense matter. These show that the theory provides an accurate extension of the plasma kinetic theory well into the strongly coupled regime. [1] Baalrud and Daligault, Phys. Rev. Lett. 110, 235001 (2013). [2] Baalrud and Daligault; arXiv:1904.09208 (2019). [Preview Abstract] |
Friday, October 25, 2019 10:00AM - 10:30AM |
YI2.00002: An efficient treatment of the full Coulomb collision operator with applications Invited Speaker: Rogerio Jorge A formulation of full Coulomb (or Landau) collision operator is provided that allows for an efficient numerical implementation, both in unmagnetized and magnetized plasmas [1]. The method is based on projecting the Boltzmann equations and the collision operator onto a Hermite-Laguerre velocity-space polynomial basis, obtaining a moment-hierarchy. This approach is implemented in a numerical simulation code and studies of systems of increasing complexity are being carried out, shedding light on the importance of retaining the full Coulomb collision operator, with respect to widely used simplified operators. First, the dynamics of electron-plasma waves is described at arbitrary collisionality for the first time by considering the full Coulomb collision operator [2]. In particular, a purely damped entropy mode, characteristic of a plasma where pitch-angle scattering effects are dominant with respect to collisionless effects, is shown to emerge numerically, and its dispersion relation is analytically derived. This mode is absent when simplified collision operators are used, and like-particle collisions strongly influence its damping rate. Second, the linear properties of drift-waves are investigated, which allows for the comparison of the Coulomb collision operator to collision operators used in state-of-the-art turbulence simulation codes [3]. Established collisional and collisionless limits are retrieved and an analysis on both the growth rate and eigenmode spectrum shows the need for retaining the Coulomb collision operator, specially at the intermediate levels of collisionality relevant for present and future magnetic confinement fusion devices. ([1] R. Jorge et al, Journal of Plasma Physics 83, 6 (2017); [2] R. Jorge et. al, Journal of Plasma Physics 85, 2 (2019); [3] R. Jorge et al, Physical Review Letters 121, 16 (2018)) [Preview Abstract] |
Friday, October 25, 2019 10:30AM - 11:00AM |
YI2.00003: Nonadiabatic Ab-initio Simulations: Ab Initio Stopping Power of High Energy Ions Invited Speaker: Alexander White We perform ab initio simulations of electronic, ionic, and nonadiabatic dynamics in the warm dense matter (WDM) and hot dense plasma (HDP) regimes. Using density-functional theory (DFT) in two guises: orbital-based Kohn-Sham (KS) and orbital-free (OF) Thomas-Fermi-Dirac approximations, permits a wide coverage of extreme conditions. This approach provides a consistent set static, dynamic, and optical properties such as equation of state, mass transport, opacity, conductivity and ion stopping power. We have developed explicitly time-dependent (TD) versions of both the KS and OF approaches in order to treat electron conductivities and stopping power. The TD schemes offer a path into the upper ranges of the WDM regime, into HDP. Additionally, the response to high frequency (temporal) perturbations can be simulated, as can non-linear or non-equilibrium interactions, $e.g.$ between the plasma and an intense laser pulse. Our recent simulations of the, inherently non-linear and non-adiabatic, electronic stopping of high energy ions will be presented. The efficiency of OF TD-DFT allows for large simulation sizes at high temperatures. This is required for the direct calculation of stopping power for MeV projectiles, relevant to ICF plasma heating. Comparison of OF and KS stopping show excellent agreement for high velocities. We have derived a current-dependent kinetic energy functional that improves agreement at low velocities. \begin{enumerate} \item Y.H. Ding, \underline {A.J.White}, O. Certik, S.X. Hu, and L.A. Collins, ``Ab initio studies of stopping power in warm dense matter using time-dependent density functional theory,'' \textit{Phys. Rev. Lett.} \textbf{121}, 145001 \item \underline {A.J.White}, O. Certik, Y.H. Ding, S.X. Hu, and L.A. Collins, ``Time-dependent orbital-free density functional theory for electronic stopping power: Comparison to the Mermin-Kohn-Sham theory at high temperatures,'' \textit{Phys. Rev. B.} \textbf{98}, 144302 \end{enumerate} [Preview Abstract] |
Friday, October 25, 2019 11:00AM - 11:30AM |
YI2.00004: Fluctuation dynamo in collisionless and weakly collisional, magnetized plasmas Invited Speaker: Denis St-Onge The amplification of cosmic magnetic fields by chaotic fluid motions is hampered by the adiabatic production of magnetic-field-aligned pressure anisotropy. This anisotropy drives a viscous stress parallel to the field that inhibits the plasma's ability to stretch magnetic-field lines. However, in high-$\beta$ plasmas, kinetic ion-Larmor scale instabilities---namely, firehose and mirror---sever the adiabatic link between the thermal and magnetic pressures, reducing this viscous stress and thereby allowing the dynamo to operate. We identify two distinct regimes of the fluctuation dynamo in a magnetized plasma: one in which these instabilities efficiently regulate the pressure anisotropy so that it does not venture much beyond the firehose and mirror instability thresholds, and one in which this regulation is imperfect. Using kinetic and Braginskii-MHD simulations and analytic theory, we elucidate the role of these kinetic instabilities and determine how the fields and flows self-organize to allow the dynamo to operate in the face of parallel viscous stresses. In the case of efficient pressure-anisotropy regulation, the plasma dynamo closely resembles its more traditional ${\rm Pm}\sim 1 $ MHD counterpart. When the regulation is imperfect, the dynamo exhibits characteristics remarkably similar to those found in the saturated state of the MHD dynamo. An analytical model for the latter regime is developed that exploits this similarity. The model predicts that the plasma dynamo ceases to operate if the ratio of field-aligned to field-perpendicular viscosities is too large, a behavior confirmed by numerical simulation. Leveraging these results, we construct a novel set of microphysical closures for fluid simulations that bridges these two regimes---one that exhibits explosive magnetic-field growth caused by a field-strength-dependent viscosity set by the firehose and mirror instabilities. [Preview Abstract] |
Friday, October 25, 2019 11:30AM - 12:00PM |
YI2.00005: Subcritical turbulence spreading and avalanche birth Invited Speaker: Robin Heinonen Turbulence in confined plasma is known to self-propagate via nonlinear scattering. This phenomenon of ``turbulence spreading'' is of interest because it decouples the relationship between the local driving gradient and the local fluctuation intensity, in particular allowing linearly stable regions to be contaminated with fluctuations. This process has been traditionally modeled using a Fisher-KPP equation, a supercritical reaction-diffusion equation. However, such an approach suffers from a number of drawbacks. For one, it begs the question of why the turbulence hasn't already saturated due to linear instability. Moreover, the Fisher-KPP fails to predict any but the weakest of penetration into stable regions, which is dubiously consistent with clear observations of fluctuations in such regions. As a final reason to reconsider the older model, we note that a growing body of numerical and analytical work suggests the possibility of nonlinear instability and subcritical turbulence, neither of which are described by Fisher-KPP. In this work, we resolve the above issues by introducing a new \textit{subcritical} model for turbulence spreading, featuring nonlinear instability drive. In addition to predicting stronger penetration of turbulence into stable regions via ballistically propagating fronts, this model predicts the possibility of bursty, intermittent propagation of turbulence similar to avalanches. We show that such an avalanche can be triggered when a threshold is exceeded, say due to noise, and estimate the threshold with a simple physical argument. These predictions provide avenues to test the model in experiment or simulation.\\ \\In collaboration with P.H. Diamond. [Preview Abstract] |
Friday, October 25, 2019 12:00PM - 12:30PM |
YI2.00006: Saturation of Shear-flow Turbulence in Magnetized Plasmas Invited Speaker: Adrian Fraser Shear-flow-instability saturation is examined for turbulent hydrodynamic, gyrokinetic, and MHD systems, showing that critical nonlinear behavior can be modeled and incorporated into improved transport models. It is shown that large-scale, linearly stable (damped) eigenmodes are nonlinearly driven to large amplitude, playing an important role in saturation [Fraser et al. PoP (2017)]. In that situation, accounting for stable modes in Reynolds stress models improves the ability of these models to recover nonlinear parameter scalings. \\ Unlike previous work on stable modes in gyroradius-scale or quasi-homogeneous systems, this analysis considers stable modes in a macroscopic, fully inhomogeneous instability. These modes are inviscid, with their linear decay corresponding to reversible energy transfer to the base flow, reflected in their significant modifications to the Reynolds stress. Gyrokinetic simulations of a driven, shear-unstable flow show that, for most cases considered, stable and unstable mode amplitudes are nearly equal in the turbulent state [Fraser et al. PoP (2018)]. It is further shown that stable modes are a crucial ingredient in reduced models of Reynolds stress when they are present. These findings are compared to MHD simulations of a shear layer with a flow-aligned magnetic field. There, the role of the magnetic field in determining the amplitudes of stable and unstable modes, and their role in determining the partition of magnetic and kinetic energy at different spatial scales, is of interest. It is observed that as the field is advected by the flow, it triggers secondary instabilities with corresponding stable modes whose energy is predominantly magnetic, and whose inclusion is necessary for reduced models. [Preview Abstract] |
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