1,721,015 research outputs found
On the variational approach to axisymmetric magnetohydrodynamic equilibria (vol 15, 092108, 2008)
No full textDouble mid-latitude dynamical reconnection at the magnetopause: An efficient mechanism allowing solar wind to enter the Earth’s magnetosphere
No full textThree-dimensional simulations of the Kelvin-Helmholtz (KH) instability in a magnetic configuration reproducing typical conditions at the flank Earth’s magnetosphere during northward periods show the system’s ability to generate favorable conditions for magnetic reconnection to occur at mid-latitude. Once these conditions are established, magnetic reconnection proceeds spontaneously in both hemispheres generating field lines that close on Earth but are connected to the solar wind at low latitude, allowing direct entrance of solar wind plasma into the magnetosphere. These results are consistent with recent observations of KH vortices showing the signature of reconnection events occurring well outside the equatorial plane (Bavassano M.B. et al., Ann. Geophys., 28 (2010) 893)
MHD equilibrium variational principles with symmetry
No full textThe chain rule for functionals is used to reduce the noncanonical Poisson bracket for magnetohydrodynamics (MHD) to one for axisymmetric and translationally symmetric MHD and hydrodynamics. The procedure for obtaining Casimir invariants from noncanonical Poisson brackets is reviewed and then used to obtain the Casimir invariants for the considered symmetrical theories. It is shown why extrema of the energy plus Casimir invariants correspond to equilibria, thereby giving an explanation for the ad hoc variational principles that have existed in plasma physics. Variational principles for general equilibria are obtained in this way
On the variational approach to axisymmetric magnetohydrodynamic equilibria
No full textThe variational formulation of the axisymmetric magnetohydrodynamic equilibrium equations with plasma flows is addressed and a more comprehensive method is presented that allows, in particular, for open boundary conditions and discontinuous (shock) solutions. A numerical procedure based on the variational formulation is described and a validation test for an open conical geometry, including also hydrodynamic shocks, is investigated. (c) 2008 American Institute of Physics
Magnetized plasma flows and magnetoplasmadynamic thrusters
No full textAn axisymmetric magnetohydrodynamics (MHD) model of the acceleration channel of an applied-field magnetoplasmadynamic thruster is presented. A set of general relationships between the flow features and the thruster performance is obtained. The boundary conditions and the flow regime, which depends on the Alfven Mach number, are shown to provide the ideal limits of steady state thruster operation. In the present analysis, a Hamiltonian formulation of the MHD plasma flow model is adopted. This formulation makes it possible to cast the model equations in a variational form, which is then solved by using a finite element numerical algorithm. (C) 2010 American Institute of Physics. [doi : 10.1063/1.3447876
Plasma characterization in Hall thrusters by Langmuir probes
No full textOne of the main factors that limits the Hall thruster lifetime is the degradation of surfaces exposed to plasma caused by the flux of energetic ions. Since the magnetic field shapes the plasma properties inside the thruster channel, non-standard magnetic configurations have been proposed in recent years to substantially increase the thruster lifetime. The present article reports an experimental investigation of different magnetic topologies of a Hall thruster operating on xenon and krypton. Plasma properties were measured using probes installed on the channel walls and a fast diving probe. Data are analysed by means of a Bayesian methodology. Results show that magnetic shielding correctly reduces the interaction between the plasma and the channel walls, which can result in lower erosion rates. A comparison between xenon and krypton operation showed that magnetic shielding is less effective for krypton, as the grazing line presents higher temperatures. Fast probe results also show how krypton discharges present wider acceleration regions and higher temperatures inside the channel
Hamiltonian magnetohydrodynamics: Lagrangian, Eulerian, and dynamically accessible stability: Theory
Full text linkStability conditions of magnetized plasma flows are obtained by exploiting the Hamiltonian structure of the magneto-hydrodynamics (MHD) equations and, in particular, by using three kinds of energy principles. First, the Lagrangian variable energy principle is described and sufficient stability conditions are presented. Next, plasma flows are described in terms of Eulerian variables and the noncanonical Hamiltonian formulation of MHD is exploited. For symmetric equilibria, the
energy-Casimir principle is expanded to second order and sufficient conditions for stability to symmetric perturbation are obtained. Then, dynamically accessible variations, i.e., variations that explicitly preserve invariants of the system, are introduced and the respective energy principle is
considered. General criteria for stability are obtained, along with comparisons between the three different approaches
Hamiltonian magnetohydrodynamics: symmetric formulation, Casimir invariants and equilibrium variational principles
No full textThe noncanonical Hamiltonian formulation of magnetohydrodynamics (MHD) is used to construct variational principles for symmetric equilibrium configurations of magnetized plasma including flow. In particular, helical symmetry is considered
and results on axial and translational symmetries are retrieved as special cases of the helical configurations. The symmetry
condition, which allows the description in terms of a magnetic flux function, is exploited to deduce a symmetric form of the noncanonical Poisson bracket of MHD. Casimir invariants are then obtained directly from the Poisson bracket. Equilibria are obtained from an energy-Casimir principle and reduced forms of this variational principle are obtained by the elimination of algebraic constraints
Measurement of the Breathing Mode Oscillations in Hall Thruster Plasmas with a Fast-Diving Triple Langmuir Probe
No full textHamiltonian magnetohydrodynamics: Lagrangian, Eulerian, and dynamically accessible stability - Examples with translation symmetry
No full textBecause different constraints are imposed, stability conditions for dissipationless fluids and magnetofluids may take different forms when derived within the Lagrangian, Eulerian (energy- Casimir), or dynamically accessible frameworks. This is in particular the case when flows are present. These differences are explored explicitly by working out in detail two magnetohydrodynamic examples: convection against gravity in a stratified fluid and translationally invariant perturbations of a rotating magnetized plasma pinch. In this second example, we show in explicit form how to perform the time-dependent relabeling introduced in Andreussi et al. [Phys. Plasmas 20, 092104 (2013)] that makes it possible to reformulate Eulerian equilibria with flows as Lagrangian equilibria in the relabeled variables. The procedures detailed in the present article provide a paradigm that can be applied to more general plasma configurations and in addition extended to more general plasma descriptions where dissipation is absent
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