Mode selection in an S-band relativistic backward wave oscillator based on a coaxial waveguide

The competition of oscillations in a relativistic backward wave oscillator (RBWO) based on a coaxial waveguide has been studied by theoretical and experimental methods. It is established that the starting current for asymmetric magnetic TE _m1 type waves is lower than that for the TEM wave. Predominant excitation of the former waves is confirmed by numerical simulations using the PIC-code KARAT. In an experimental RBWO prototype, the excitation of asymmetric oscillations was suppressed by longitudinal cuts in the central conductor of the coaxial waveguide. As a result, stable generation of the TEM wave has been achieved at a frequency of 1.23 GHz, a peak power of 260 MW, and a 28% efficiency of electron-beam-power conversion into radiation power for a microwave-pulse duration of 33 ns.     

Effect of the built-in electric field on optical and electrical properties of AlGaAs/InGaAs/GaAs P-HEMT nanoheterostructures

This study is concerned with the photoluminescence spectra and electrical parameters of AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistor (P-HEMT) structures with a quantum well grown at different depths L _b with respect to the surface. The samples are produced so as to make the concentration of electrons in the quantum well unchanged, as the barrier layer thickness L _b is reduced. It is established that the photoluminescence spectra of all of the samples exhibit peaks at the photon energies ħ _ω = 1.28−1.30 and 1.35–1.38 eV. The ratio between the intensities of these peaks increases as L _b is decreased. Calculations of the band structure show that variations in the spectra are due to the fact that the built-in electric field increases as the quantum well is set closer to the surface.     

Effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of metamorphic In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As HEMT nanoheterostructures

The results of studies of the effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of high electron mobility In_0.7Al_0.3As/In_0.75Ga_0.25As/In_0.7Al_0.3As/GaAs nanoheterostructures are reported. Using molecular-beam epitaxy, two identical structures with a stepped compositional profile of the metamorphic In _x Al_{1 ? x} As (Δ _x = 0.05) buffer are grown on substrates of two types: a singular GaAs substrate with the orientation (100) ± 0.5° and a GaAs (100) substrate misoriented by (2 ± 0.5)° in the $\left[ {0\bar 1\bar 1} \right]$ direction. It is found that, in the case of the misoriented substrate, the concentration of the two-dimensional electron gas is ～40% higher. Broadening of the photoluminescence spectra and a shift of the peaks to lower photon energies, as experimentally observed in the case of the misoriented substrate, are attributed to the increased roughness of the heterointerfaces and strengthened fluctuations of the quantum-well width.     

Calculation of Minimum Particle Mixture Fluidization Velocity in a Wet Biomass Torrefaction Reactor

Chemical and Petroleum Engineering - A simple method is proposed for calculating the minimum velocity of fluidization of a bed consisting of particles differing considerably in shape and size. The...     

Fine needle aspiration cytologic findings in malignant small cell tumor of the thoracopulmonary region (Askin tumor)

From 1980 to 1992, five cases of Askin tumor were diagnosed by fine needle aspiration cytology at the Shiraz University Hospitals, Namazi, Faghihi and Beheshti, Iran. All patients were female, their ages ranging between 15 and 22 years. Three of them had a mass on the right side and two on the left side of the chest. Chest roentgenography revealed no bone involvement. Two tumors involved the lungs and pleura. Urinary catecholamine levels were within normal limits. The fine needle aspiration smears revealed many small, round malignant cells with indistinct cytoplasm, a high nuclear/cytoplasmic ratio and prominent, multiple nucleoli. Homer-Wright rosettes were seen frequently. The cells were often arranged in rows simulating rouleaux formation. They were positively stained by periodic acid-Schiff stain. The histologic sections confirmed the cytologic diagnosis of Askin tumor. Four of the patients died within 7-11 months after treatment began.     

Water in Betelgeuse and Antares

Absorption lines of hot water have been identified in the infrared spectra of Betelgeuse (alpha Orionis) and Antares (alpha Scorpii) near 12.3 micrometers (811 to 819 wavenumbers). The water lines originate in the atmospheres of the stars, not in their circumstellar material. The spectra are similar in structure to umbral sunspot spectra. Pure rotation water lines of this type will occur throughout the spectra of cool stars at wavelengths greater than 10 micrometers. From the water spectra, the upper limit for the temperature in the line formation region in both stars is 2800 kelvin. The water column density in both stars is (3 +/- 2) x 10(18) molecules per square centimeter, yielding an abundance relative to atomic hydrogen of n(H2O)/n(H) approximately 10(-)7.     

Mercury in alligators (Alligator mississippiensis) in the southeastern United States

Mercury methylation may be enhanced in wetlands and humic-rich, blackwater systems that crocodiles and alligators typically inhabit. Given their high trophic level and long life-spans, crocodilians could accumulate significant burdens of Hg. Our objectives were to survey Hg concentrations in alligators from several areas in the southeastern United States to test their utility as sentinels of Hg contamination, to examine relationships among Hg concentrations in various tissues and to look for any differences in tissue Hg concentrations among locations. We measured total Hg concentrations in alligators collected in the Florida Everglades (n = 18), the Okefenokee National Wildlife Refuge, Georgia (n = 9), the Savannah River Site (SRS), South Carolina (n = 49) and various locations in central Florida (n = 21), sampling tissues including blood, brain, liver, kidney, muscle, bone, fat, spleen, claws and dermal scutes. Alligators from the Everglades were mostly juvenile, but Hg concentrations in tissues were high (means: liver 41.0, kidney 36.4, muscle 5.6 mg Hg/kg dry wt.). Concentrations in alligators from other locations in Florida were lower (means: liver 14.6, kidney 12.6, muscle 1.8 mg Hg/kg dry wt.), although they tended to be larger adults. Alligators from the Okefenokee were smallest and had the lowest Hg concentrations (means: liver 4.3, kidney 4.8, muscle 0.8 mg Hg/kg dry wt.). SRS alligators had the greatest size range and intermediate Hg levels (means: liver 14.9, muscle 4.8 mg Hg/kg dry wt.). At some locations, alligator length was correlated with Hg concentrations in some internal organs. However, at three of the four locations, muscle Hg was not related to length. Tissue Hg concentrations were correlated at most locations however, claw or dermal scute Hg explained less than 74% of the variation of Hg in muscle or organs, suggesting readily-obtained tissues, such as scutes or claws, have limited value for non-destructive screening of Hg in alligator populations.