5,142 research outputs found
Magnetorotational Instability in a Couette Flow of Plasma
All experiments, which have been proposed so far to model the
magnetorotational instability (MRI) in the laboratory, involve a Couette flow
of liquid metals in a rotating annulus. All liquid metals have small magnetic
Prandtl numbers, Pm, of about 10^{-6} (the ratio of kinematic viscosity to
magnetic diffusivity). With plasmas both large and small Pm are achievable by
varying the temperature and the density of plasma. Compressibility and fast
rotation of the plasma result in radial stratification of the equilibrium
plasma density. Evolution of perturbations in radially stratified viscous and
resistive plasma permeated by an axial uniform magnetic field is considered.
The differential rotation of the plasma is induced by the ExB drift in applied
radial electric field. Global unstable eigenmodes are calculated by our newly
developed matrix code. The plasma is shown to be MRI unstable for parameters
easily achievable in experimental setup.Comment: 6 pages, 2 figures; to be published in the Proceedings of the 3d
Workshop on Non-Neutral Plasmas, July 2003, Santa Fe, US
Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk
The stability of nonaxisymmetric perturbations in differentially rotating
astrophysical accretion disks is analyzed by fully incorporating the properties
of shear flows. We verify the presence of discrete unstable eigenmodes with
complex and pure imaginary eigenvalues, without any artificial disk edge
boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the
mathematical theory of a non-self-adjoint system, we investigate the nonlocal
behavior of eigenmodes in the vicinity of Alfven singularities at
omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and
omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of
discrete eigenmodes is discussed and the magnetic field and wavenumber
dependence of the growth rate are obtained. Exponentially growing modes are
present even in a region where the local dispersion relation theory claims to
have stable eigenvalues. The velocity field created by an eigenmode is
obtained, which explains the anomalous angular momentum transport in the
nonlinear stage of this stability.Comment: 11pages, 11figures, to be published in ApJ. For associated eps files,
see http://dino.ph.utexas.edu/~knoguchi
Magnetorotational Instability in Liquid Metal Couette Flow
Despite the importance of the magnetorotational instability (MRI) as a
fundamental mechanism for angular momentum transport in magnetized accretion
disks, it has yet to be demonstrated in the laboratory. A liquid sodium
alpha-omega dynamo experiment at the New Mexico Institute of Mining and
Technology provides an ideal environment to study the MRI in a rotating metal
annulus (Couette flow). A local stability analysis is performed as a function
of shear, magnetic field strength, magnetic Reynolds number, and turbulent
Prandtl number. The later takes into account the minimum turbulence induced by
the formation of an Ekman layer against the rigidly rotating end walls of a
cylindrical vessel. Stability conditions are presented and unstable conditions
for the sodium experiment are compared with another proposed MRI experiment
with liquid gallium. Due to the relatively large magnetic Reynolds number
achievable in the sodium experiment, it should be possible to observe the
excitation of the MRI for a wide range of wavenumbers and further to observe
the transition to the turbulent state.Comment: 12 pages, 22 figures, 1 table. To appear in the Astrophysical Journa
EXPERIMENTAL-STUDY ABOUT THE INFLUENCE OF LIVER DYSFUNCTION UPON CYCLOSPORINE PHARMACOKINETICS
One-Dimensional Confinement and Enhanced Jahn-Teller Instability in LaVO
Ordering and quantum fluctuations of orbital degrees of freedom are studied
theoretically for LaVO in spin-C-type antiferromagnetic state. The
effective Hamiltonian for the orbital pseudospin shows strong one-dimensional
anisotropy due to the negative interference among various exchange processes.
This significantly enhances the instability toward lattice distortions for the
realistic estimate of the Jahn-Teller coupling by first-principle LDA+
calculations, instead of favoring the orbital singlet formation. This explains
well the experimental results on the anisotropic optical spectra as well as the
proximity of the two transition temperatures for spin and orbital orderings.Comment: 4 pages including 4 figure
- …