141 research outputs found
Self-injection-locked magnetron as an active ring resonator side coupled to a waveguide with a delayed feedback loop
The theoretical analysis and numerical simulations of the magnetron operation
with a feedback loop were performed assuming that the delay of the
electromagnetic wave propagating in the loop is constant whereas the phase of
the complex feedback reflection coefficient is varied. Results of simulations
showed that by a proper adjustment of values of the time delay and phase of
reflection coefficient that determines phase matching between the waves in the
resonator and feedback loop, one can increase the magnetron's output power
significantly without any other additional measures.Comment: 12 pages, 4 figure
X-band microwave generation caused by plasma-sheath instability
It is well known that oscillations at the electron plasma frequency may
appear due to instability of the plasma sheath near a positively biased
electrode immersed in plasma. This instability is caused by transit-time
effects when electrons, collected by this electrode, pass through the sheath.
Such oscillations appear as low-power short spikes due to additional ionization
of a neutral gas in the electrode vicinity. Herein we present first results
obtained when the additional ionization was eliminated. We succeeded to prolong
the oscillations during the whole time a positive bias was applied to the
electrode. These oscillations could be obtained at much higher frequency than
previously reported (tens of GHz compared to few hundreds of MHz) and power of
tens of mW. These results in combination with presented theoretical estimations
may be useful, e.g., for plasma diagnostics.Comment: 12 pages, 7 figure
Total absorption of an electromagnetic wave by an overdense plasma
We show both theoretically and experimentally that an electromagnetic wave
can be totally absorbed by an overdense plasma when a subwavelength diffraction
grating is placed in front of the plasma surface. The absorption is due to
dissipation of surface plasma waves (plasmons-polaritons) that have been
resonantly excited by the evanescent component of the diffracted
electromagnetic wave. The developed theoretical model allows one to determine
the conditions for the total absorption.Comment: To be published in PR
Broad-band polarization-independent total absorption of electromagnetic waves by an overdense plasma
We have shown both experimentally and theoretically that
polarization-independent broad-band absorption of electromagnetic waves by an
overdense plasma, caused by surface plasmon-polaritons (SPP) excitation, can be
achieved due to combination of two factors: a non-zero angle of incidence and a
two-dimensional circular diffraction grating placed at a properly chosen
distance in front of the plasma boundary. Direct detection of SPP has been
achieved for the first time using a miniature antenna imbedded in the plasma.Comment: considerably broadened versio
Ferroelectric Devices Emit Charged Particles and Radiation
Devices called solid-state ferroelectric- based sources (SSFBSs) are under development as sources of electrons, ions, ultraviolet light, and x-rays for diverse applications in characterization and processing of materials. Whereas heretofore it has been necessary to use a different device to generate each of the aforementioned species of charged particles or radiation, a single SSFBS can be configured and operated to selectively generate any of the species as needed using a single source. Relative to comparable prior sources based, variously, on field emission, thermionic emission, and gaseous discharge plasmas, SSFBSs demand less power, and are compact and lightweight. An SSFBS exploits the unique physical characteristics of a ferroelectric material in the presence of a high-frequency pulsed electric field. The basic building block of an SSFBS is a ferroelectric cathode -- a ferroelectric wafer with a solid electrode covering its rear face and a grid electrode on its front face (see figure). The application of a voltage pulse -- typically having amplitude of several kilovolts and duration of several nanoseconds -- causes dense surface plasma to form near the grid wires on the front surface
Calculation of valence electron momentum densities using the projector augmented-wave method
We present valence electron Compton profiles calculated within the
density-functional theory using the all-electron full-potential projector
augmented-wave method (PAW). Our results for covalent (Si), metallic (Li, Al)
and hydrogen-bonded ((H_2O)_2) systems agree well with experiments and
computational results obtained with other band-structure and basis set schemes.
The PAW basis set describes the high-momentum Fourier components of the valence
wave functions accurately when compared with other basis set schemes and
previous all-electron calculations.Comment: Submitted to Journal of Physics and Chemistry of Solids on September
17 2004. Revised version submitted on December 13 200
X-ray Fluorescent Fe Kalpha Lines from Stellar Photospheres
X-ray spectra from stellar coronae are reprocessed by the underlying
photosphere through scattering and photoionization events. While reprocessed
X-ray spectra reaching a distant observer are at a flux level of only a few
percent of that of the corona itself, characteristic lines formed by inner
shell photoionization of some abundant elements can be significantly stronger.
The emergent photospheric spectra are sensitive to the distance and location of
the fluorescing radiation and can provide diagnostics of coronal geometry and
abundance. Here we present Monte Carlo simulations of the photospheric Kalpha
doublet arising from quasi-neutral Fe irradiated by a coronal X-ray source.
Fluorescent line strengths have been computed as a function of the height of
the radiation source, the temperature of the ionising X-ray spectrum, and the
viewing angle. We also illustrate how the fluorescence efficiencies scale with
the photospheric metallicity and the Fe abundance. Based on the results we make
three comments: (1) fluorescent Fe lines seen from pre-main sequence stars
mostly suggest flared disk geometries and/or super-solar disk Fe abundances;
(2) the extreme ~1400 mA line observed from a flare on V1486 Ori can be
explained entirely by X-ray fluorescence if the flare itself were partially
eclipsed by the limb of the star; and (3) the fluorescent Fe line detected by
Swift during a large flare on II Peg is consistent with X-ray excitation and
does not require a collisional ionisation contribution. There is no convincing
evidence supporting the energetically challenging explanation of electron
impact excitation for observed stellar Fe Kalpha lines.Comment: 30 pages; accepted for publication in the Astrophysical Journa
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