489 research outputs found
A Self-Similar Solution for the Propagation of a Relativistic Shock in an Exponential Atmosphere
We derive a fully relativistic, self-similar solution to describe the
propagation of a shock along an exponentially decreasing atmosphere, in the
limit of very large Lorentz factor. We solve the problem in planar symmetry and
compute the acceleration of the shock in terms of the density gradient crossed
during its evolution. We apply our solution to the acceleration of shocks
within the atmosphere of a HyperNova, and show that velocities consistent with
the requirements of GRB models can be achieved with exponential atmospheres
spanning a wide density range.Comment: ApJL in pres
Moving boundary approximation for curved streamer ionization fronts: Solvability analysis
The minimal density model for negative streamer ionization fronts is
investigated. An earlier moving boundary approximation for this model consisted
of a "kinetic undercooling" type boundary condition in a Laplacian growth
problem of Hele-Shaw type. Here we derive a curvature correction to the moving
boundary approximation that resembles surface tension. The calculation is based
on solvability analysis with unconventional features, namely, there are three
relevant zero modes of the adjoint operator, one of them diverging;
furthermore, the inner/outer matching ahead of the front has to be performed on
a line rather than on an extended region; and the whole calculation can be
performed analytically. The analysis reveals a relation between the fields
ahead and behind a slowly evolving curved front, the curvature and the
generated conductivity. This relation forces us to give up the ideal
conductivity approximation, and we suggest to replace it by a constant
conductivity approximation. This implies that the electric potential in the
streamer interior is no longer constant but solves a Laplace equation; this
leads to a Muskat-type problem.Comment: 22 pages, 6 figure
Giant magnetoresistance in semiconductor / granular film heterostructures with cobalt nanoparticles
We have studied the electron transport in SiO(Co)/GaAs and
SiO(Co)/Si heterostructures, where the SiO(Co) structure is the
granular SiO film with Co nanoparticles. In SiO(Co)/GaAs
heterostructures giant magnetoresistance effect is observed. The effect has
positive values, is expressed, when electrons are injected from the granular
film into the GaAs semiconductor, and has the temperature-peak type character.
The temperature location of the effect depends on the Co concentration and can
be shifted by the applied electrical field. For the SiO(Co)/GaAs
heterostructure with 71 at.% Co the magnetoresistance reaches 1000 ( %)
at room temperature. On the contrary, for SiO(Co)/Si heterostructures
magnetoresistance values are very small (4%) and for SiO(Co) films the
magnetoresistance has an opposite value. High values of the magnetoresistance
effect in SiO(Co)/GaAs heterostructures have been explained by
magnetic-field-controlled process of impact ionization in the vicinity of the
spin-dependent potential barrier formed in the semiconductor near the
interface. Kinetic energy of electrons, which pass through the barrier and
trigger the avalanche process, is reduced by the applied magnetic field. This
electron energy suppression postpones the onset of the impact ionization to
higher electric fields and results in the giant magnetoresistance. The
spin-dependent potential barrier is due to the exchange interaction between
electrons in the accumulation electron layer in the semiconductor and
-electrons of Co.Comment: 25 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
The moving boundary problem in the presence of a dipole magnetic field
An exact analytic solution is obtained for a uniformly expanding, neutral,
infinitely conducting plasma sphere in an external dipole magnetic field. The
electrodynamical aspects related to the radiation and transformation of energy
were considered as well. The results obtained can be used in analyzing the
recent experimental and simulation data.Comment: 17 pages, 1 figure, Submitted to J. Phys. A, Math. and Genera
Investigation of initiation of gigantic jets connecting thunderclouds to the ionosphere
The initiation of giant electrical discharges called as "gigantic jets"
connecting thunderclouds to the ionosphere is investigated by numerical
simulation method in this paper. Using similarity relations, the triggering
conditions of streamer formation in laboratory situations are extended to form
a criterion of initiation of gigantic jets. The energy source causing a
gigantic jet is considered due to the quasi-electrostatic field generated by
thunderclouds. The electron dynamics from ionization threshold to streamer
initiation are simulated by the Monte Carlo technique. It is found that
gigantic jets are initiated at a height of ~18-24 km. This is in agreement with
the observations. The method presented in this paper could be also applied to
the analysis of the initiation of other discharges such as blue jets and red
sprites.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture
Positive streamers need a source of free electrons ahead of them to
propagate. A streamer can supply these electrons by itself through
photo-ionization, or the electrons can be present due to external background
ionization. Here we investigate the effects of background ionization on
streamer propagation and morphology by changing the gas composition and the
repetition rate of the voltage pulses, and by adding a small amount of
radioactive Krypton 85.
We find that the general morphology of a positive streamer discharge in high
purity nitrogen depends on background ionization: at lower background
ionization levels the streamers branch more and have a more feather-like
appearance. This is observed both when varying the repetition rate and when
adding Krypton 85, though side branches are longer with the radioactive
admixture. But velocities and minimal diameters of streamers are virtually
independent of the background ionization level. In air, the inception cloud
breaks up into streamers at a smaller radius when the repetition rate and
therefore the background ionization level is higher. When measuring the effects
of the pulse repetition rate and of the radioactive admixture on the discharge
morphology, we found that our estimates of background ionization levels are
consistent with these observations; this gives confidence in the estimates.
Streamer channels generally do not follow the paths of previous discharge
channels for repetition rates of up to 10 Hz. We estimate the effect of
recombination and diffusion of ions and free electrons from the previous
discharge and conclude that the old trail has largely disappeared at the moment
of the next voltage pulse; therefore the next streamers indeed cannot follow
the old trail.Comment: 30 pages, 13 figure
Pressurized rf cavities in ionizing beams
A muon collider or Higgs factory requires significant reduction of the six dimensional emittance of the beam prior to acceleration. One method to accomplish this involves building a cooling channel using high pressure gas filled radio frequency cavities. The performance of such a cavity when subjected to an intense particle beam must be investigated before this technology can be validated. To this end, a high pressure gas filled radio frequency (rf) test cell was built and placed in a 400 MeV beam line from the Fermilab linac to study the plasma evolution and its effect on the cavity. Hydrogen, deuterium, helium and nitrogen gases were studied. Additionally, sulfur hexafluoride and dry air were used as dopants to aid in the removal of plasma electrons. Measurements were made using a variety of beam intensities, gas pressures, dopant concentrations, and cavity rf electric fields, both with and without a 3 T external solenoidal magnetic field. Energy dissipation per electron-ion pair, electron-ion recombination rates, ion-ion recombination rates, and electron attachment times to SF6 and O-2 were measured.ope
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