684 research outputs found
Ionization fronts in negative corona discharges
In this paper we use a hydrodynamic minimal streamer model to study negative
corona discharge. By reformulating the model in terms of a quantity called
shielding factor, we deduce laws for the evolution in time of both the radius
and the intensity of ionization fronts. We also compute the evolution of the
front thickness under the conditions for which it diffuses due to the geometry
of the problem and show its self-similar character.Comment: 4 pages, 4 figure
Power laws and self-similar behavior in negative ionization fronts
We study anode-directed ionization fronts in curved geometries. When the
magnetic effects can be neglected, an electric shielding factor determines the
behavior of the electric field and the charged particle densities. From a
minimal streamer model, a Burgers type equation which governs the dynamics of
the electric shielding factor is obtained. A Lagrangian formulation is then
derived to analyze the ionization fronts. Power laws for the velocity and the
amplitude of streamer fronts are observed numerically and calculated
analytically by using the shielding factor formulation. The phenomenon of
geometrical diffusion is explained and clarified, and a universal self-similar
asymptotic behavior is derived.Comment: 25 pages, 9 figure
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
Positive and negative streamers in ambient air: modeling evolution and velocities
We simulate short positive and negative streamers in air at standard
temperature and pressure. They evolve in homogeneous electric fields or emerge
from needle electrodes with voltages of 10 to 20 kV. The streamer velocity at
given streamer length depends only weakly on the initial ionization seed,
except in the case of negative streamers in homogeneous fields. We characterize
the streamers by length, head radius, head charge and field enhancement. We
show that the velocity of positive streamers is mainly determined by their
radius and in quantitative agreement with recent experimental results both for
radius and velocity. The velocity of negative streamers is dominated by
electron drift in the enhanced field; in the low local fields of the present
simulations, it is little influenced by photo-ionization. Though negative
streamer fronts always move at least with the electron drift velocity in the
local field, this drift motion broadens the streamer head, decreases the field
enhancement and ultimately leads to slower propagation or even extinction of
the negative streamer.Comment: 18 pages, 10 figure
On the low-temperature performances of THGEM and THGEM/G-APD multipliers in gaseous and two-phase Xe
The performances of THGEM multipliers in two-phase Xe avalanche mode are
presented for the first time. Additional results on THGEM operation in gaseous
Xe at cryogenic temperatures are provided. Stable operation of a double-THGEM
multiplier was demonstrated in two-phase Xe with gains reaching 600. These are
compared to existing data, summarized here for two-phase Ar, Kr and Xe
avalanche detectors incorporating GEM and THGEM multipliers. The optical
readout of THGEMs with Geiger-mode Avalanche Photodiodes (G-APDs) has been
investigated in gaseous Xe at cryogenic temperature; avalanche scintillations
were recorded in the Near Infrared (NIR) at wavelengths of up to 950 nm. At
avalanche charge gain of 350, the double-THGEM/G-APD multiplier yielded 0.07
photoelectrons per initial ionization electron, corresponding to an avalanche
scintillation yield of 0.7 NIR photons per avalanche electron over 4pi. The
results are compared with those of two-phase Ar avalanche detectors. The
advantages, limitations and possible applications are discussed.Comment: 22 pages, 14 figures. Revised Figs. 10,11 and Table 1. To be
published in JINS
Observation of large arrays of plasma filaments in air breakdown by 1.5-MW 110-GHz gyrotron pulses
We report the observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-mu s pulses. Each element is a plasma filament elongated in the electric field direction and regularly spaced about one-quarter wavelength apart in the plane perpendicular to the electric field. The development of the array is explained as a result of diffraction of the beam around the filaments, leading to the sequential generation of high intensity spots, at which new filaments are created, about a quarter wavelength upstream from each existing filament. Electromagnetic wave simulations corroborate this explanation and show very good correlation to the observed pattern of filaments.open424
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
Electrification of granular systems of identical insulators
Insulating particles can become highly electrified during powder handling,
volcanic eruptions, and the wind-blown transport of dust, sand, and snow.
Measurements in these granular systems have found that smaller particles
generally charge negatively, while larger particles charge positively. These
observations are puzzling, since particles in these systems are generally
chemically identical, and thus have no contact potential difference. We show
here that simple geometry leads to a net transfer of electrons from larger to
smaller particles, in agreement with these observations. We integrate this
charging mechanism into the first quantitative charging scheme for a granular
system of identical insulators, and show that its predictions are in agreement
with measurements. Our theory thus seems to provide an explanation for the
hitherto puzzling phenomenon of the size-dependent charging of granular systems
of identical insulators.Comment: 8 pages, 5 figures, published in Physical Review
GEM operation in helium and neon at low temperatures
We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne
and Ne+H2 at temperatures in the range of 2.6-293 K. In He, at temperatures
between 62 and 293 K, the triple-GEM structures often operate at rather high
gains, exceeding 1000. There is an indication that this high gain is achieved
by Penning effect in the gas impurities released by outgassing. At lower
temperatures the gain-voltage characteristics are significantly modified
probably due to the freeze-out of impurities. In particular, the double-GEM and
single-GEM structures can operate down to 2.6 K at gains reaching only several
tens at a gas density of about 0.5 g/l; at higher densities the maximum gain
drops further. In Ne, the maximum gain also drops at cryogenic temperatures.
The gain drop in Ne at low temperatures can be reestablished in Penning
mixtures of Ne+H2: very high gains, exceeding 10000, have been obtained in
these mixtures at 50-60 K, at a density of 9.2 g/l corresponding to that of
saturated Ne vapor near 27 K. The results obtained are relevant in the fields
of two-phase He and Ne detectors for solar neutrino detection and electron
avalanching at low temperatures.Comment: 13 pages, 14 figures. Accepted for publishing in Nucl. Instr. and
Meth.
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