69,461 research outputs found
A cusp electron gun for millimeter wave gyrodevices
The experimental results of a thermionic cusp electron gun, to drive millimeter and submillimeter wave harmonic gyrodevices, are reported in this paper. Using a "smooth" magnetic field reversal formed by two coils this gun generated an annular-shaped, axis-encircling electron beam with 1.5 A current, and an adjustable velocity ratio alpha of up to 1.56 at a beam voltage of 40 kV. The beam cross-sectional shape and transported beam current were measured by a witness plate technique and Faraday cup, respectively. These measured results were found to be in excellent agreement with the simulated results using the three-dimensional code MAGIC
Sunward-propagating Alfv\'enic fluctuations observed in the heliosphere
The mixture/interaction of anti-sunward-propagating Alfv\'enic fluctuations
(AFs) and sunward-propagating Alfv\'enic fluctuations (SAFs) is believed to
result in the decrease of the Alfv\'enicity of solar wind fluctuations with
increasing heliocentric distance. However, SAFs are rarely observed at 1 au and
solar wind AFs are found to be generally outward. Using the measurements from
Voyager 2 and Wind, we perform a statistical survey of SAFs in the heliosphere
inside 6 au. We first report two SAF events observed by Voyager 2. One is in
the anti-sunward magnetic sector with a strong positive correlation between the
fluctuations of magnetic field and solar wind velocity. The other one is in the
sunward magnetic sector with a strong negative magnetic field-velocity
correlation. Statistically, the percentage of SAFs increases gradually with
heliocentric distance, from about 2.7% at 1.0 au to about 8.7% at 5.5 au. These
results provide new clues for understanding the generation mechanism of SAFs
A new image decomposition and reconstruction approach -- adaptive fourier decomposition
© Springer International Publishing Switzerland 2015. Fourier has been a powerful mathematical tool for representing a signal into an expression consist of sin and cos. Recently a new developed signal decomposition theory is proposed by Pro. Tao Qian named Adaptive Fourier Decomposition, which has the advantage in time frequency over Fourier decomposition and without the need for a fixed window size problem such as short-time frequency transform. Studies show that AFD can fast decompose signals into positive-frequency functions with good analytical properties. In this paper we apply AFD into image decomposition and reconstruction area first time in the literature, which shows a promising result and gives the fundamental prospect for image compression
Impaction of spray droplets on leaves: influence of formulation and leaf character on shatter, bounce and adhesion
This paper combines experimental data with simple mathematical models to
investigate the influence of spray formulation type and leaf character
(wettability) on shatter, bounce and adhesion of droplets impacting with
cotton, rice and wheat leaves. Impaction criteria that allow for different
angles of the leaf surface and the droplet impact trajectory are presented;
their predictions are based on whether combinations of droplet size and
velocity lie above or below bounce and shatter boundaries. In the experimental
component, real leaves are used, with all their inherent natural variability.
Further, commercial agricultural spray nozzles are employed, resulting in a
range of droplet characteristics. Given this natural variability, there is
broad agreement between the data and predictions. As predicted, the shatter of
droplets was found to increase as droplet size and velocity increased, and the
surface became harder to wet. Bouncing of droplets occurred most frequently on
hard to wet surfaces with high surface tension mixtures. On the other hand, a
number of small droplets with low impact velocity were observed to bounce when
predicted to lie well within the adhering regime. We believe this discrepancy
between the predictions and experimental data could be due to air layer effects
that were not taken into account in the current bounce equations. Other
discrepancies between experiment and theory are thought to be due to the
current assumption of a dry impact surface, whereas, in practice, the leaf
surfaces became increasingly covered with fluid throughout the spray test runs.Comment: 19 pages, 6 figures, accepted for publication by Experiments in
Fluid
Theory and simulations of a gyrotron backward wave oscillator using a helical interaction waveguide
A gyrotron backward wave oscillator (gyro-BWO) with a helically corrugated interaction waveguide demonstrated its potential as a powerful microwave source with high efficiency and a wide frequency tuning range. This letter presents the theory describing the dispersion properties of such a waveguide and the linear beam-wave interaction. Numerical simulation results using the PIC code MAGIC were found to be in excellent agreement with the output measured from a gyro-BWO experiment
A new atmospheric aerosol phase equilibrium model (UHAERO): organic systems
In atmospheric aerosols, water and volatile inorganic and organic species are distributed between the gas and aerosol phases in accordance with thermodynamic equilibrium. Within an atmospheric particle, liquid and solid phases can exist at equilibrium. Models exist for computation of phase equilibria for inorganic/water mixtures typical of atmospheric aerosols; when organic species are present, the phase equilibrium problem is complicated by organic/water interactions as well as the potentially large number of organic species. We present here an extension of the UHAERO inorganic thermodynamic model (Amundson et al., 2006c) to organic/water systems. Phase diagrams for a number of model organic/water systems characteristic of both primary and secondary organic aerosols are computed. Also calculated are inorganic/organic/water phase diagrams that show the effect of organics on inorganic deliquescence behavior. The effect of the choice of activity coefficient model for organics on the computed phase equilibria is explored
Filamentary superconductivity across the phase diagram of Ba(Fe,Co)As
We show magnetotransport results on Ba(FeCo)As () single crystals. We identify the low temperature resistance step
at 23 K in the parent compound with the onset of filamentary superconductivity
(FLSC), which is suppressed by an applied magnetic field in a similar manner to
the suppression of bulk superconductivity (SC) in doped samples. FLSC is found
to persist across the phase diagram until the long range antiferromagnetic
order is completely suppressed. A significant suppression of FLSC occurs for
, the doping concentration where bulk SC emerges. Based on these
results and the recent report of an electronic anisotropy maximum for 0.02
0.04 [Science 329, 824 (2010)], we speculate that, besides spin
fluctuations, orbital fluctuations may also play an important role in the
emergence of SC in iron-based superconductors.Comment: 5 pages, 3 figure
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