272,931 research outputs found

### Magnetoelliptic Instabilities

We consider the stability of a configuration consisting of a vertical
magnetic field in a planar flow on elliptical streamlines in ideal
hydromagnetics. In the absence of a magnetic field the elliptical flow is
universally unstable (the ``elliptical instability''). We find this universal
instability persists in the presence of magnetic fields of arbitrary strength,
although the growthrate decreases somewhat. We also find further instabilities
due to the presence of the magnetic field. One of these, a destabilization of
Alfven waves, requires the magnetic parameter to exceed a certain critical
value. A second, involving a mixing of hydrodynamic and magnetic modes, occurs
for all magnetic-field strengths. These instabilities may be important in
tidally distorted or otherwise elliptical disks. A disk of finite thickness is
stable if the magnetic fieldstrength exceeds a critical value, similar to the
fieldstrength which suppresses the magnetorotational instability.Comment: Accepted for publication in Astrophysical Journa

### Multi-condensate states in BCS superconductors

A BCS (Bardeen-Cooper-Schrieffer) superconductor, which is placed out of
equilibrium, can develop quantum instabilities, which manifest themselves in
oscillations of the superconductor's order parameter (pairing amplitude
$\Delta$). These instabilities are a manifestations of the Cooper instability.
Inelastic collisions are essential in resolving those instabilities.
Incorporating the quantum instabilities and collisions in a unified approach
based on Richardson's exact solution of the pairing Hamiltonian, we find that a
BCS superconductor may end up in a state in which the spectrum has more than
one gap.Comment: Text expanded, figures added, Journal Ref and DOI adde

### Instabilities in the Flux Line Lattice of Anisotropic Superconductors

The stability of the flux line lattice has been investigated within
anisotropic London theory. This is the first full-scale investigation of
instabilities in the `chain' state. It has been found that the lattice is
stable at large fields, but that instabilities occur as the field is reduced.
The field at which these instabilities first arise, $b^*(\epsilon,\theta)$,
depends on the anisotropy $\epsilon$ and the angle $\theta$ at which the
lattice is tilted away from the $c$-axis. These instabilities initially occur
at wavevector $k^*(\epsilon,\theta)$, and the component of $k^*$ along the
average direction of the flux lines, $k_z$, is always finite. As the
instability occurs at finite $k_z$ the dependence of the cutoff on $k_z$ is
important, and we have used a cutoff suggested by Sudb\ospace and Brandt. The
instabilities only occur for values of the anisotropy $\epsilon$ appropriate to
a material like BSCCO, and not for anisotropies more appropriate to YBCO. The
lower critical field $H_{c_1}(\phi)$ is calculated as a function of the angle
$\phi$ at which the applied field is tilted away from the crystal axis. The
presence of kinks in $H_{c_1}(\phi)$ is seen to be related to instabilities in
the equilibrium flux line structure.Comment: Extensively revised paper, with modified analysis of elastic
instabilities. Calculation of the lower critical field is included, and the
presence of kinks in $H_{c_1}$ is seen to be related to the elastic
instabilities. 29 pages including 16 figures, LaTeX with epsf styl

### A robust method for measurement of fluctuation parallel wavenumber in laboratory plasmas

Measuring the parallel wavenumber is fundamental for the experimental characterization of electrostatic instabilities. It becomes particularly important in toroidal geometry, where spatial inhomogeneities and curvature can excite both drift instabilities, whose wavenumber parallel to the magnetic field is finite, and interchange instabilities, which typically have vanishing parallel wavenumber. We demonstrate that multipoint measurements can provide a robust method for the discrimination between the two cases

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