High pressure operation of the triple-GEM detector in pure Ne, Ar and Xe

We study the performance of the triple-GEM (Gas Electron Multiplier) detector in pure noble gases Ne, Ar and Xe, at different pressures varying from 1 to 10 atm. In Ar and Xe, the maximum attainable gain of the detector abruptly drops down for pressures exceeding 3 atm. In contrast, the maximum gain in Ne was found to increase with pressure, reaching a value of 100,000 at 7 atm. The results obtained are of particular interest for developing noble gas-based cryogenic particle detectors for solar neutrino and dark matter search.Comment: 7 pages, 4 figures. Submitted to Nucl. Instr. and Meth. A as a letter to the Edito

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

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

Anomalous Capacitive Sheath with Deep Radio Frequency Electric Field Penetration

A novel nonlinear effect of anomalously deep penetration of an external radio frequency electric field into a plasma is discribed. A self-consistent kinetic treatment reveals a transition region between the sheath and the plasma. Because of the electron velocity modulation in the sheath, bunches in the energetic electron density are formed in the transition region adjusted to the sheath. The width of the region is of order $V_{T}/\omega$, where V_{T} is the electron thermal velocity, and $\omega$ is frequency of the electric field. The presence of the electric field in the transition region results in a cooling of the energetic electrons and an additional heating of the cold electrons in comparison with the case when the transition region is neglected.Comment: 14,4 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

Calogênese in vitro de Elaeis oleifera.

O caiaué ou dendê americano(Elaeis oleifera) constitui-se na única fonte atualmente disponível de tolerância ao Amarelecimento Fatal (AF), considerado na grande ameaça à dendeicultura latino-americana dado o seu alto alto grau de letalidade. Esta espécie também se destaca pelo menor crescimento em altura, qualidade do óleo e relativa facilidade de hibridação com o dendê africano(E.guineensis). Apesar das bases fisiológicas da multiplicação vegetativa serem as mesmas para a maioria das espécies e, bastante exploradas para o E.guineensis, são necessários estudos específicos que possibilitem a clonagem massal do caiaué, e esta variabilidade genética possa ser explorada nos programas de melhoramento genético do dendê.Com vistas à propagação via embriogênese somática, o presente trabalho teve como objetivo avaliar o efeito de diferentes reguladores na indução de calos a partir de embriões zigóticos de E.oleifera. O ensaio foi conduzido no Laboratório de Cultura de Tecidos de Plantas da Embrapa Amazônia Ocidental, onde embriões zigóticos maduros de caiaué após assepsia foram inoculados em meio de cultura composto pelos sais e vitaminas de Murashige e Skoog (1962) acrescido de inositol (100 mgL-1), caseína hidrolisada(400 mgL-1), glutamina (500 mg L-1), ácido aspártico (100 mg L-1), arginina (100 mg L-1), sacarose (3%), ágar (0,7%), 2,4-D nas concentrações 9μM (tratamento 1) e 500 μM com carvão ativado a 0,3%(tratamento 2), e picloram nas concentrações 8,3μM (tratamento 3) e 596 μM com carvão ativado a 0,3%(tratamento 4). As culturas foram mantidas em ambiente escuro com temperatura de 26±1°Ce avaliados a formação de calos, germinação, oxidação e vigor (com ou sem atividade morfogênica). Ao final de 60 dias, os tratamentos 1 e 2 resultaram em 50 e 55% de explantes com formação de calos, respectivamente. No tratamento 3 somente 11,3% dos embriões foram responsivos à calogênese e no tratamento 4 esta ocorrência foi nula. Ao contrário do que se esperava, a porcentagem de embriões germinados foi bastante elevada nos tratamentos 3 e 4, com 30,3 e 35 %, respectivamente. A oxidação ocorreu em 3,3% dos explantes do T3 e em 10% do T4. Danos mecânicos ocorridos durante a retirada e inoculação do embrião in vitro pode em parte explicar a ausência de resposta morfogênica dos explantes. Estes resultados demonstraram que o 2,4-D foi indutor de calos embriogênicos, enquanto o picloram favoreceu a germinação de embriões zigóticos maduros de caiau

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

Giant magnetoresistance in semiconductor / granular film heterostructures with cobalt nanoparticles

We have studied the electron transport in SiO${}_2$(Co)/GaAs and SiO${}_2$(Co)/Si heterostructures, where the SiO${}_2$(Co) structure is the granular SiO${}_2$ film with Co nanoparticles. In SiO${}_2$(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${}_2$(Co)/GaAs heterostructure with 71 at.% Co the magnetoresistance reaches 1000 ($10^5$ %) at room temperature. On the contrary, for SiO${}_2$(Co)/Si heterostructures magnetoresistance values are very small (4%) and for SiO${}_2$(Co) films the magnetoresistance has an opposite value. High values of the magnetoresistance effect in SiO${}_2$(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 $d$-electrons of Co.Comment: 25 pages, 16 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

Instability of ion kinetic waves in a weakly ionized plasma

The fundamental higher-order Landau plasma modes are known to be generally heavily damped. We show that these modes for the ion component in a weakly ionized plasma can be substantially modified by ion-neutral collisions and a dc electric field driving ion flow so that some of them can become unstable. This instability is expected to naturally occur in presheaths of gas discharges at sufficiently small pressures and thus affect sheaths and discharge structures.Comment: Published in Phys. Rev. E, see http://link.aps.org/doi/10.1103/PhysRevE.85.02641