9,011 research outputs found
Coupled cavity traveling wave tube with velocity tapering
A coupled cavity traveling wave tube with a velocity taper, which affords beam wave resynchronization and thereby enhances is described. The wave velocity reduction is achieved by reducing the resonant frequencies of the individual resonant cavities as a function of the distance from the electron gun, through changes in internal cavity dimensions. The required changes in cavity dimensions can be accomplished by gradually increasing the cavity radius decreasing the gap length from cavity to cavity. The velocity reduction is carried out without an increase in circuit resistive losses and the upper and lower cut off frequencies are reduced in approximately the same manner
Traveling wave tube circuit
A traveling wave tube (TWT) has a slow wave structure (SWS) which is severed into two or more sections. A signal path, connects the end of an SWS section to the beginning of the following SWS section. The signal path comprises an impedance matching coupler (IMC), followed by an isolator, a variable phase shifter, and a second IMC. The aggregate band pass characteristic of the components in the signal path is chosen to reject, or strongly attenuate, all frequencies outside the desired operating frequency range of the TWT and yet pass, with minimal attenuation in the forward direction, all frequencies within the desired operating frequency range. The isolator is chosen to reject, or strongly attenuate, waves, of all frequencies, which propagate in the backward direction. The aggregate phase shift characteristic of the components in the signal path is chosen to apply signal power to the beginning of the following SWS section with the phase angle yielding maximum efficiency
The Regulation and Development of Bioremediation
The authors describe how federal statutes regulating hazardous wastes create both incentives and disincentives for exploiting the large potential of bioremediation. Ultimately, they argue for regulation attending more to comparative risks and costs
The 20 and 30 GHz MMIC technology for future space communication antenna system
The development of fully monolithic gallium arsenide receive and transmit modules is described. These modules are slated for phased array antenna applications in future 30/20 gigahertz communications satellite systems. Performance goals and various approaches to achieve them are discussed. The latest design and performance results of components, submodules and modules are presented
Designing III-V Multijunction Solar Cells on Silicon
Single junction Si solar cells dominate photovoltaics but are close to their
efficiency limits. This paper presents ideal limiting efficiencies for tandem
and triple junction multijunction solar cells subject only to the constraint of
the Si bandgap and therefore recommending optimum cell structures departing
from the single junction ideal. The use of III-V materials is considered, using
a novel growth method capable of yielding low defect density III-V layers on
Si. In order to evaluate the real potential of these proposed multijunction
designs, a quantitative model is presented, the strength of which is the joint
modelling of external quantum efficiency and current-voltage characteristics
using the same parameters. The method yields a single parameter fit in terms of
the Shockley-Read-Hall lifetime. This model is validated by fitting
experimental data of external quantum efficiency, dark current, and conversion
efficiency of world record tandem and triple junction cells under terrestrial
solar spectra without concentration. We apply this quantitative model to the
design of tandem and triple junction solar cells, yielding cell designs capable
of reaching efficiencies without concentration of 32% for the best tandem cell
and 36% for the best triple junction cell. This demonstrates that efficiencies
within a few percent of world records are realistically achievable without the
use of concentrating optics, with growth methods being developed for
multijunction cells combining III-V and Si materials.Comment: Preprint of the paper submitted to the journal Progress in
Photovoltaics, selected by the Executive Committee of the 28th EU PVSEC 2013
for submission to Progress in Photovoltaics. 10 pages, 7 figure
Aerospace applications of high temperature superconductivity
Space application of high temperature superconducting (HTS) materials may occur before most terrestrial applications because of the passive cooling possibilities in space and because of the economic feasibility of introducing an expensive new technology which has a significant system benefit in space. NASA Lewis Research Center has an ongoing program to develop space technology capitalizing on the potential benefit of HTS materials. The applications being pursued include space communications, power and propulsion systems, and magnetic bearings. In addition, NASA Lewis is pursuing materials research to improve the performance of HTS materials for space applications
Partial reflections of radio waves from the lower ionosphere
The addition of phase difference measurements to partial reflection experiments is discussed, and some advantages of measuring electron density this way are pointed out. The additional information obtained reduces the requirement for an accurate predetermination of collision frequency. Calculations are also made to estimate the errors expected in partial-reflection experiments due to the assumption of Fresnel reflection and to the neglect of coupling between modes. In both cases, the errors are found to be of the same order as known errors in the measurements due to current instrumental limitations
MMIC technology for advanced space communications systems
The current NASA program for 20 and 30 GHz monolithic microwave integrated circuit (MMIC) technology is reviewed. The advantages of MMIC are discussed. Millimeter wavelength MMIC applications and technology for communications systems are discussed. Passive and active MMIC compatible components for millimeter wavelength applications are investigated. The cost of a millimeter wavelength MMIC's is projected
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