233 research outputs found
Frequency spectra of the sources 3C 295 and 3C 380
Interferometric flux density measurements on stellar light source
Dynamic avalanche breakdown of a p-n junction: deterministic triggering of a plane streamer front
We discuss the dynamic impact ionization breakdown of high voltage p-n
junction which occurs when the electric field is increased above the threshold
of avalanche impact ionization on a time scale smaller than the inverse
thermogeneration rate. The avalanche-to-streamer transition characterized by
generation of dense electron-hole plasma capable to screen the applied external
electric field occurs in such regimes. We argue that the experimentally
observed deterministic triggering of the plane streamer front at the electric
field strength above the threshold of avalanche impact ionization but yet below
the threshold of band-to-band tunneling is generally caused by field-enhanced
ionization of deep-level centers. We suggest that the process-induced sulfur
centers and native defects such as EL2, HB2, HB5 centers initiate the front in
Si and GaAs structures, respectively. In deep-level free structures the plane
streamer front is triggered by Zener band-to-band tunneling.Comment: 4 pages, 2 figure
The Effect of Air Density on Atmospheric Electric Fields Required for Lightning Initiation from a Long Airborne Object
The purpose of the work was to determine minimum atmospheric electric fields required for lightning initiation from an airborne vehicle at various altitudes up to 10 km. The problem was reduced to the determination of a condition for initiation of a viable positive leader from a conductive object in an ambient electric field. It was shown that, depending on air density and shape and dimensions of the object, critical atmospheric fields are governed by the condition for leader viability or that for corona onset. To establish quantitative criteria for reduced air densities, available observations of spark discharges in long laboratory gaps were analyzed, the effect of air density on leader velocity was discussed and evolution in time of the properties of plasma in the leader channel was numerically simulated. The results obtained were used to evaluate the effect of pressure on the quantitative relationships between the potential difference near the leader tip, leader current and its velocity; based on these relationships, criteria for steady development of a leader were determined for various air pressures. Atmospheric electric fields required for lightning initiation from rods and ellipsoidal objects of various dimensions were calculated at different air densities. It was shown that there is no simple way to extend critical ambient fields obtained for some given objects and pressures to other objects and pressures
Constraints to do realistic modeling of the electric field ahead of the tip of a lightning leader
Several computer models exist to explain the observation of terrestrial
gamma-ray flashes (TGFs). Some of these models estimate the electric field
ahead of lightning leaders and its effects on electron acceleration and
multiplication. In this paper, we derive a new set of constraints to do more
realistic modeling. We determine initial conditions based on in situ
measurements of electric field and vertical separation between the main charge
layers of thunderclouds. A maximum electric field strength of 50 kV/cm at sea
level is introduced as the upper constraint for the leader electric field. The
threshold for electron avalanches to develop of 2.86 kV/cm at sea level is
introduced as the lower value. With these constraints, we determine a region
where acceleration and multiplication of electrons occur. The maximum potential
difference in this region is found to be 52 MV, and the corresponding
number of avalanche multiplication lengths is 3.5. We then quantify the
effect of the ambient electric field compared to the leader field at the upper
altitude of the negative tip. Finally, we argue that only leaders with the
highest potential difference between its tips (600 MV) can be candidates
for the production of TGFs. However, with the assumptions we have used, these
cannot explain the observed maximum energies of at least 40 MeV. Open questions
with regard to the temporal development of the streamer zone and its effect on
the shape of the electric field remain
An adaptive grid refinement strategy for the simulation of negative streamers
The evolution of negative streamers during electric breakdown of a
non-attaching gas can be described by a two-fluid model for electrons and
positive ions. It consists of continuity equations for the charged particles
including drift, diffusion and reaction in the local electric field, coupled to
the Poisson equation for the electric potential. The model generates field
enhancement and steep propagating ionization fronts at the tip of growing
ionized filaments. An adaptive grid refinement method for the simulation of
these structures is presented. It uses finite volume spatial discretizations
and explicit time stepping, which allows the decoupling of the grids for the
continuity equations from those for the Poisson equation. Standard refinement
methods in which the refinement criterion is based on local error monitors fail
due to the pulled character of the streamer front that propagates into a
linearly unstable state. We present a refinement method which deals with all
these features. Tests on one-dimensional streamer fronts as well as on
three-dimensional streamers with cylindrical symmetry (hence effectively 2D for
numerical purposes) are carried out successfully. Results on fine grids are
presented, they show that such an adaptive grid method is needed to capture the
streamer characteristics well. This refinement strategy enables us to
adequately compute negative streamers in pure gases in the parameter regime
where a physical instability appears: branching streamers.Comment: 46 pages, 19 figures, to appear in J. Comp. Phy
A moving boundary problem motivated by electric breakdown: I. Spectrum of linear perturbations
An interfacial approximation of the streamer stage in the evolution of sparks
and lightning can be written as a Laplacian growth model regularized by a
`kinetic undercooling' boundary condition. We study the linear stability of
uniformly translating circles that solve the problem in two dimensions. In a
space of smooth perturbations of the circular shape, the stability operator is
found to have a pure point spectrum. Except for the zero eigenvalue for
infinitesimal translations, all eigenvalues are shown to have negative real
part. Therefore perturbations decay exponentially in time. We calculate the
spectrum through a combination of asymptotic and series evaluation. In the
limit of vanishing regularization parameter, all eigenvalues are found to
approach zero in a singular fashion, and this asymptotic behavior is worked out
in detail. A consideration of the eigenfunctions indicates that a strong
intermediate growth may occur for generic initial perturbations. Both the
linear and the nonlinear initial value problem are considered in a second
paper.Comment: 37 pages, 6 figures, revised for Physica
Experimental Evidence of Giant Electron - Gamma Bursts Generated by Extensive Atmospheric Showers in Thunderclouds
The existence of a new phenomena - giant electron-gamma bursts is
established. The bursts are generated in thunderclouds as a result of the
combined action of runaway breakdown and extensive atmosphere showers (RB-EAS).
The experiments were fulfilled at the Tien Shan Mountain Scientific Station
using EAS-Radio installation. This specially constructed installation consists
of a wide spread EAS trigger array and a high time resolution
radiointerferometer.Comment: 30 pages, 16 figure
Spontaneous Branching of Anode-Directed Streamers between Planar Electrodes
Non-ionized media subject to strong fields can become locally ionized by
penetration of finger-shaped streamers. We study negative streamers between
planar electrodes in a simple deterministic continuum approximation. We observe
that for sufficiently large fields, the streamer tip can split. This happens
close to Firsov's limit of `ideal conductivity'. Qualitatively the tip
splitting is due to a Laplacian instability quite like in viscous fingering.
For future quantitative analytical progress, our stability analysis of planar
fronts identifies the screening length as a regularization mechanism.Comment: 4 pages, 6 figures, submitted to PRL on Nov. 16, 2001, revised
version of March 10, 200
Theory of superfast fronts of impact ionization in semiconductor structures
We present an analytical theory for impact ionization fronts in reversely
biased p^{+}-n-n^{+} structures. The front propagates into a depleted n base
with a velocity that exceeds the saturated drift velocity. The front passage
generates a dense electron-hole plasma and in this way switches the structure
from low to high conductivity. For a planar front we determine the
concentration of the generated plasma, the maximum electric field, the front
width and the voltage over the n base as functions of front velocity and doping
of the n base. Theory takes into account that drift velocities and impact
ionization coefficients differ between electrons and holes, and it makes
quantitative predictions for any semiconductor material possible.Comment: 18 pagers, 10 figure
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