11,394 research outputs found
Two-region model for positive and negative plasma sheaths and its application to Hall thruster metallic anodes.
An asymptotic presheath/sheath model for positive and negative sheaths in front of a conducting electrode, with a continuous parametric transition at the no-sheath case, is presented. Key aspects of the model are as follows: full hydrodynamics of both species in the presheath; a kinetic formulation with a truncated distribution function for the repelled species within the sheath; and the fulfillment of the marginal Bohm condition at the sheath edge, in order to match the two formulations of the repelled species. The sheath regime depends on the ratios of particle fluxes and sound speeds between the two species. The presheath model includes the effect of a magnetic field parallel to the wall on electrons. An asymptotic, parametric study of the anode presheath is carried out in terms of the local ion-to-electron flux ratio and Hall parameter. The drift-diffusive model of magnetized electrons fails in a parametric region that includes parts of the negative sheath regime. In the case of the Hall parameter vanishing near the electrode and a weakly collisional plasma, a quasisonic, quasineutral plateau forms next to the sheath edge
Applications and identification of surface correlations
We compare theoretical, experimental, and computational approaches to random
rough surfaces. The aim is to produce rough surfaces with desirable
correlations and to analyze the correlation functions extracted from the
surface profiles. Physical applications include ultracold neutrons in a rough
waveguide, lateral electronic transport, and scattering of longwave particles
and waves. Results provide guidance on how to deal with experimental and
computational data on rough surfaces. A supplemental goal is to optimize the
neutron waveguide for GRANIT experiments. The measured correlators are
identified by fitting functions or by direct spectral analysis. The results are
used to compare the calculated observables with theoretical values. Because of
fluctuations, the fitting procedures lead to inaccurate physical results even
if the quality of the fit is very good unless one guesses the right shape of
the fitting function. Reliable extraction of the correlation function from the
measured surface profile seems virtually impossible without independent
information on the structure of the correlation function. Direct spectral
analysis of raw data rarely works better than the use of a "wrong" fitting
function. Analysis of surfaces with a large correlation radius is hindered by
the presence of domains and interdomain correlations
Itinerario por los hervideros o cultura de los baños: recurso didáctico para el estudio y conservación del paisaje volcánico del Campo de Calatrava (Ciudad Real, España)
Los “hervideros” del Campo de Calatrava son una de las manifestaciones más
características y singulares de la Región Volcánica del Campo de Calatrava. El
itinerario escogido refleja una clara interdisciplinariedad aunando, combinando e integrando principalmente tres criterios: el puramente geológico-geomorfológico por la procedencia y las características físicas de éstos; el histórico-sociológico, al convertirse
en una actividad cotidiana y extendida a ámbitos nacionales e internacionales, durante casi un siglo (desde mediados del siglo XIX hasta la década de los años 50 en el siglo
XX) y el ambiental, dentro de la línea de conservación y protección de un elemento
destacable del paisaje volcánico calatravo. De ellos nos han quedado restos de sus construcciones, actualmente en claro deterioro
Reflectance measurements of cotton leaf senescence altered by mepiquat chloride
Spectrophotometric reflectance measurements were made on plant-attached leaves to evaluate growth chamber-grown cotton leaf (Gossypium hirsutum L.) senescence (chlorophyll degradation as criterion) that was delayed by mepiquat chloride (1,1-dimethylpiperidinium chloride) rates of 0, 10, 40, 70, and 100 g a.i./ha. Mepiquat chloride (MC increased both chlorophyll and leaf water contents as compared with that of untreated leaves. Reflectance was inversely and linearly correlated (r = -0.873**) with eater content at the 1.65 micrometer wavelength and was inversely correlated (r = -0.812**) with chlorophyll concentration at the 0.55 micrometer wavelength but best fit a quadratic equation. Either wavelength measurement might be useful to remotely detect cotton leaf senescence or fields of MC-treated cotton plants
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