102 research outputs found
The onset of tree-like patterns in negative streamers
We present the first analytical and numerical studies of the initial stage of
the branching process based on an interface dynamics streamer model in the
fully 3-D case. This model follows from fundamental considerations on charge
production by impact ionization and balance laws, and leads to an equation for
the evolution of the interface between ionized and non-ionized regions. We
compare some experimental patterns with the numerically simulated ones, and
give an explicit expression for the growth rate of harmonic modes associated
with the perturbation of a symmetrically expanding discharge. By means of full
numerical simulation, the splitting and formation of characteristic tree-like
patterns of electric discharges is observed and described
Contour dynamics model for electric discharges
A contour dynamics model for electrical discharges is obtained and analyzed.
The model is deduced as the asymptotic limit of the minimal streamer model for
the propagation of electric discharges, in the limit of small electron
diffusion. The dispersion relation for a non planar 2-D discharge is
calculated. The development and propagation of finger-like patterns are studied
and their main features quantified.Comment: 4 pages, 2 fi
Impact ionization fronts in Si diodes: Numerical evidence of superfast propagation due to nonlocalized preionization
We present numerical evidence of a novel propagation mode for superfast
impact ionization fronts in high-voltage Si -- structures. In
nonlinear dynamics terms, this mode corresponds to a pulled front propagating
into an unstable state in the regime of nonlocalized initial conditions. Before
the front starts to travel, field-ehanced emission of electrons from deep-level
impurities preionizes initially depleted base creating spatially nonuniform
free carriers profile. Impact ionization takes place in the whole high-field
region. We find two ionizing fronts that propagate in opposite directions with
velocities up to 10 times higher than the saturated drift velocity.Comment: 3 pages, 4 figure
Electric discharge contour dynamics model: the effects of curvature and finite conductivity
In this paper we present the complete derivation of the effective contour
model for electrical discharges which appears as the asymptotic limit of the
minimal streamer model for the propagation of electric discharges, when the
electron diffusion is small. It consists of two integro-differential equations
defined at the boundary of the plasma region: one for the motion and a second
equation for the net charge density at the interface. We have computed explicit
solutions with cylindrical symmetry and found the dispersion relation for small
symmetry-breaking perturbations in the case of finite resistivity. We implement
a numerical procedure to solve our model in general situations. As a result we
compute the dispersion relation for the cylindrical case and compare it with
the analytical predictions. Comparisons with experimental data for a 2-D
positive streamers discharge are provided and predictions confirmed.Comment: 23 pages, 3 figure
Optomagnetic composite medium with conducting nanoelements
A new type of metal-dielectric composites has been proposed that is
characterised by a resonance-like behaviour of the effective permeability in
the infrared and visible spectral ranges. This material can be referred to as
optomagnetic medium. The analytical formalism developed is based on solving the
scattering problem for considered inclusions with impedance boundary condition,
which yields the current and charge distributions within the inclusions. The
presence of the effective magnetic permeability and its resonant properties
lead to novel optical effects and open new possible applications.Comment: 48 pages, 13 figures. accepted to Phys. Rev. B; to appear vol. 66,
200
A generalization of Snoek's law to ferromagnetic films and composites
The present paper establishes characteristics of the relative magnetic
permeability spectrum (f) of magnetic materials at microwave frequencies.
The integral of the imaginary part of (f) multiplied with the frequency f
gives remarkable properties. A generalisation of Snoek's law consists in this
quantity being bounded by the square of the saturation magnetization multiplied
with a constant. While previous results have been obtained in the case of
non-conductive materials, this work is a generalization to ferromagnetic
materials and ferromagnetic-based composites with significant skin effect. The
influence of truncating the summation to finite upper frequencies is
investigated, and estimates associated to the finite summation are provided. It
is established that, in practice, the integral does not depend on the damping
model under consideration. Numerical experiments are performed in the exactly
solvable case of ferromagnetic thin films with uniform magnetization, and these
numerical experiments are found to confirm our theoretical results. Microwave
permeability measurements on soft amorphous films are reported. The relation
between the integral and the saturation magnetization is verified
experimentally, and some practical applications of the theoretical results are
introduced. The integral can be used to determine the average magnetization
orientation in materials with complex configurations of the magnetization, and
furthermore to demonstrate the accuracy of microwave measurement systems. For
certain applications, such as electromagnetic compatibility or radar absorbing
materials, the relations established herein provide useful indications for the
design of efficient materials, and simple figures of merit to compare the
properties measured on various materials
Effect of transition layers on the electromagnetic properties of composites containing conducting fibres
The approach to calculating the effective dielectric and magnetic response in
bounded composite materials is developed. The method is essentially based on
the renormalisation of the dielectric matrix parameters to account for the
surface polarisation and the displacement currents at the interfaces. This
makes it possible the use of the effective medium theory developed for
unbounded materials, where the spatially-dependent local dielectric constant
and magnetic permeability are introduced. A detailed mathematical analysis is
given for a dielectric layer having conducting fibres with in-plane positions.
The surface effects are most essential at microwave frequencies in
correspondence to the resonance excitation of fibres. In thin layers (having a
thickness of the transition layer), the effective dielectric constant has a
dispersion region at much higher frequencies compared to those for unbounded
materials, exhibiting a strong dependence on the layer thickness. For the
geometry considered, the effective magnetic permeability differs slightly from
unity and corresponds to the renormalised matrix parameter. The magnetic effect
is due entirely to the existence of the surface displacement currents.Comment: PDF, 33 pages, 10 figure
- …