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Search Results: 1 - 10 of 331535 matches for " C. B. Forest "
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Fast dynamos in spherical boundary-driven flows
I. V. Khalzov,C. M. Cooper,C. B. Forest
Physics , 2013, DOI: 10.1103/PhysRevLett.111.125001
Abstract: We numerically demonstrate the feasibility of kinematic fast dynamos for a class of time-periodic axisymmetric flows of conducting fluid confined inside a sphere. The novelty of our work is in considering the realistic flows, which are self-consistently determined from the Navier-Stokes equation with specified boundary driving. Such flows can be achieved in a new plasma experiment, whose spherical boundary is capable of differential driving of plasma flows in the azimuthal direction. We show that magnetic fields are self-excited over a range of flow parameters such as amplitude and frequency of flow oscillations, fluid Reynolds (Re) and magnetic Reynolds (Rm) numbers. In the limit of large Rm, the growth rates of the excited magnetic fields are of the order of the advective time scales and practically independent of Rm, which is an indication of the fast dynamo.
A Spherical Plasma Dynamo Experiment
E. J. Spence,K. Reuter,C. B. Forest
Physics , 2009, DOI: 10.1088/0004-637X/700/1/470
Abstract: We propose a plasma experiment to be used to investigate fundamental properties of astrophysical dynamos. The highly conducting, fast-flowing plasma will allow experimenters to explore systems with magnetic Reynolds numbers an order of magnitude larger than those accessible with liquid-metal experiments. The plasma is confined using a ring-cusp strategy and subject to a toroidal differentially rotating outer boundary condition. As proof of principle, we present magnetohydrodynamic simulations of the proposed experiment. When a von K\'arm\'an-type boundary condition is specified, and the magnetic Reynolds number is large enough, dynamo action is observed. At different values of the magnetic Prandtl and Reynolds numbers the simulations demonstrate either laminar or turbulent dynamo action.
Wave-driven dynamo action in spherical MHD systems
K. Reuter,F. Jenko,A. Tilgner,C. B. Forest
Physics , 2009, DOI: 10.1103/PhysRevE.80.056304
Abstract: Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically forced two-vortex flow inside a sphere are reported. The simulations are performed in the intermediate regime between the laminar flow and developed turbulence where a hydrodynamic instability is found to generate internal waves with a characteristic m=2 zonal wave number. It is shown that this time-periodic flow acts as a dynamo although snapshots of the flow as well as the mean flow are not dynamos. The magnetic fields' growth rate exhibits resonance effects depending on the wave frequency. Furthermore, a cyclic self-killing and self-recovering dynamo based on the relative alignment of the velocity and magnetic fields is presented. The phenomena are explained in terms of a mixing of non-orthogonal eigenstates of the time dependent linear operator of the magnetic induction equation. The potential relevance of this mechanism to dynamo experiments is discussed.
Optimized boundary driven flows for dynamos in a sphere
I. V. Khalzov,B. P. Brown,C. M. Cooper,D. B. Weisberg,C. B. Forest
Physics , 2012, DOI: 10.1063/1.4764048
Abstract: We perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number Rm_cr required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Karman type satisfying the steady-state Navier-Stokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment (MPDX), whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds number Re. It is found that the overall lowest Rm_cr~200 is achieved at Re~240 for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range Rm_cr
Modeling the Parker instability in a rotating plasma screw pinch
I. V. Khalzov,B. P. Brown,N. Katz,C. B. Forest
Physics , 2012, DOI: 10.1063/1.3684240
Abstract: We analytically and numerically study the analogue of the Parker (magnetic buoyancy) instability in a uniformly rotating plasma screw pinch confined in a cylinder. Uniform plasma rotation is imposed to create a centrifugal acceleration, which mimics the gravity required for the classical Parker instability. The goal of this study is to determine how the Parker instability could be unambiguously identified in a weakly magnetized, rapidly rotating screw pinch, in which the rotation provides an effective gravity and a radially varying azimuthal field is controlled to give conditions for which the plasma is magnetically buoyant to inward motion. We show that an axial magnetic field is also required to circumvent conventional current driven magnetohydrodynamic (MHD) instabilities such as the sausage and kink modes that would obscure the Parker instability. These conditions can be realized in the Madison Plasma Couette Experiment (MPCX). Simulations are performed using the extended MHD code NIMROD for an isothermal compressible plasma model. Both linear and nonlinear regimes of the instability are studied, and the results obtained for the linear regime are compared with analytical results from a slab geometry. Based on this comparison, it is found that in a cylindrical pinch the magnetic buoyancy mechanism dominates at relatively large Mach numbers (M>5), while at low Mach numbers (M<1) the instability is due to the curvature of magnetic field lines. At intermediate values of Mach number (1
Numerical simulation of laminar plasma dynamos in a cylindrical von Kármán flow
I. V. Khalzov,B. P. Brown,F. Ebrahimi,D. D. Schnack,C. B. Forest
Physics , 2011, DOI: 10.1063/1.3559472
Abstract: The results of a numerical study of the magnetic dynamo effect in cylindrical von K\'arm\'an plasma flow are presented with parameters relevant to the Madison Plasma Couette Experiment. This experiment is designed to investigate a broad class of phenomena in flowing plasmas. In a plasma, the magnetic Prandtl number Pm can be of order unity (i.e., the fluid Reynolds number Re is comparable to the magnetic Reynolds number Rm). This is in contrast to liquid metal experiments, where Pm is small (so, Re>>Rm) and the flows are always turbulent. We explore dynamo action through simulations using the extended magnetohydrodynamic NIMROD code for an isothermal and compressible plasma model.We also study two-fluid effects in simulations by including the Hall term in Ohm's law. We find that the counter-rotating von K\'arm\'an flow results in sustained dynamo action and the self-generation of magnetic field when the magnetic Reynolds number exceeds a critical value. For the plasma parameters of the experiment, this field saturates at an amplitude corresponding to a new stable equilibrium (a laminar dynamo). We show that compressibility in the plasma results in an increase of the critical magnetic Reynolds number, while inclusion of the Hall term in Ohm's law changes the amplitude of the saturated dynamo field but not the critical value for the onset of dynamo action.
Measurements of the magnetic field induced by a turbulent flow of liquid metal
M. D. Nornberg,E. J. Spence,R. D. Kendrick,C. M. Jacobson,C. B. Forest
Physics , 2005, DOI: 10.1063/1.2173614
Abstract: Initial results from the Madison Dynamo Experiment provide details of the inductive response of a turbulent flow of liquid sodium to an applied magnetic field. The magnetic field structure is reconstructed from both internal and external measurements. A mean toroidal magnetic field is induced by the flow when an axial field is applied, thereby demonstrating the omega effect. Poloidal magnetic flux is expelled from the fluid by the poloidal flow. Small-scale magnetic field structures are generated by turbulence in the flow. The resulting magnetic power spectrum exhibits a power-law scaling consistent with the equipartition of the magnetic field with a turbulent velocity field. The magnetic power spectrum has an apparent knee at the resistive dissipation scale. Large-scale eddies in the flow cause significant changes to the instantaneous flow profile resulting in intermittent bursts of non-axisymmetric magnetic fields, demonstrating that the transition to a dynamo is not smooth for a turbulent flow.
Intermittent magnetic field excitation by a turbulent flow of liquid sodium
M. D. Nornberg,E. J. Spence,R. D. Kendrick,C. M. Jacobson,C. B. Forest
Physics , 2006, DOI: 10.1103/PhysRevLett.97.044503
Abstract: The magnetic field measured in the Madison Dynamo Experiment shows intermittent periods of growth when an axial magnetic field is applied. The geometry of the intermittent field is consistent with the fastest growing magnetic eigenmode predicted by kinematic dynamo theory using a laminar model of the mean flow. Though the eigenmodes of the mean flow are decaying, it is postulated that turbulent fluctuations of the velocity field change the flow geometry such that the eigenmode growth rate is temporarily positive. Therefore, it is expected that a characteristic of the onset of a turbulent dynamo is magnetic intermittency.
Observation of a Turbulence-Induced Large Scale Magnetic Field
E. J. Spence,M. D. Nornberg,C. M. Jacobson,R. D. Kendrick,C. B. Forest
Physics , 2005, DOI: 10.1103/PhysRevLett.96.055002
Abstract: An axisymmetric magnetic field is applied to a spherical, turbulent flow of liquid sodium. An induced magnetic dipole moment is measured which cannot be generated by the interaction of the axisymmetric mean flow with the applied field, indicating the presence of a turbulent electromotive force. It is shown that the induced dipole moment should vanish for any axisymmetric laminar flow. Also observed is the production of toroidal magnetic field from applied poloidal magnetic field (the omega-effect). Its potential role in the production of the induced dipole is discussed.
Femtometer Toroidal Structures in Nuclei
J. L. Forest,V. R. Pandharipande,Steven C. Pieper,R. B. Wiringa,R. Schiavilla,A. Arriaga
Physics , 1996, DOI: 10.1103/PhysRevC.54.646
Abstract: The two-nucleon density distributions in states with isospin $T=0$, spin $S$=1 and projection $M_S$=0 and $\pm$1 are studied in $^2$H, $^{3,4}$He, $^{6,7}$Li and $^{16}$O. The equidensity surfaces for $M_S$=0 distributions are found to be toroidal in shape, while those of $M_S$=$\pm$1 have dumbbell shapes at large density. The dumbbell shapes are generated by rotating tori. The toroidal shapes indicate that the tensor correlations have near maximal strength at $r<2$ fm in all these nuclei. They provide new insights and simple explanations of the structure and electromagnetic form factors of the deuteron, the quasi-deuteron model, and the $dp$, $dd$ and $\alpha d$ $L$=2 ($D$-wave) components in $^3$He, $^4$He and $^6$Li. The toroidal distribution has a maximum-density diameter of $\sim$1 fm and a half-maximum density thickness of $\sim$0.9 fm. Many realistic models of nuclear forces predict these values, which are supported by the observed electromagnetic form factors of the deuteron, and also predicted by classical Skyrme effective Lagrangians, related to QCD in the limit of infinite colors. Due to the rather small size of this structure, it could have a revealing relation to certain aspects of QCD.
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