8,893 research outputs found
High power operation of an X-band gyrotwistron
We report the first experimental verification of a gyrotwistron amplifier. The device utilized a single 9.858 GHz, TE011 cavity, a heavily attenuated drift tube, and a long tapered output waveguide section. With a 440 kV, 200-245 A, 1 μs electron beam and a sharply tapered axial magnetic field, peak powers above 21 MW were achieved with a gain near 24 dB. Performance was limited by competition from a fundamental TE11 mode. A multimode code was developed to analyze this system, and simulations were in good agreement with the experiment
Lasso Estimation of an Interval-Valued Multiple Regression Model
A multiple interval-valued linear regression model considering all the
cross-relationships between the mids and spreads of the intervals has been
introduced recently. A least-squares estimation of the regression parameters
has been carried out by transforming a quadratic optimization problem with
inequality constraints into a linear complementary problem and using Lemke's
algorithm to solve it. Due to the irrelevance of certain cross-relationships,
an alternative estimation process, the LASSO (Least Absolut Shrinkage and
Selection Operator), is developed. A comparative study showing the differences
between the proposed estimators is provided
Modeling Initial Stage of Ablation Material Pyrolysis: Graphitic Precursor Formation and Interfacial Effects
Reactive molecular dynamics simulations are used to study initial stage of pyrolysis of ablation materials and their composites with carbon nanotubes and carbon fibers. The products formed during pyrolysis are characterized and water is found as the primary product in all cases. The water formation mechanisms are analyzed and the value of the activation energy for water formation is estimated. A detailed study on graphitic precursor formation reveals the presence of two temperature zones. In the lower temperature zone (less than 2000 K) polymerization occurs resulting in formation of large, stable graphitic precursors, and in the high temperature zone (greater than 2000 K) polymer scission results in formation of short polymer chains/molecules. Simulations performed in the high temperature zone on the phenolic resin composites (with carbon nanotubes and carbon fibers) shows that the presence of interfaces had no substantial effect on the chain scission rate or the activation energy value for water formation
Modeling initial stage of phenolic pyrolysis: Graphitic precursor formation and interfacial effects
Reactive molecular dynamics simulations are used to study the initial stage of pyrolysis of phenolic polymers with carbon nanotube and carbon fiber. The products formed are characterized and water is found to be the primary product in all cases. The water formation mechanisms are analyzed and the value of the activation energy for water formation is estimated. A detailed study of graphitic precursor formation reveals the presence of two temperature zones. In the lower temperature zone (\u3c2000 K) polymerization occurs resulting in the formation of large, stable graphitic precursors, while in the high temperature zone (\u3e2000 K) polymer scission results in formation of short polymer chains/molecules. Simulations performed in the high temperature zone of the phenolic resin (with carbon nanotubes and carbon fibers) show that the presence of interfaces does not have a substantial effect on the chain scission rate or the activation energy value for water formation
Long-Slit Observations of Extended C II 1335 Emission Around V854 Centauri and RY Sagittarii
We have obtained long-slit far-ultraviolet (1150--1730 A) spectra of the R
Coronae Borealis (RCB) stars V854 Cen and RY Sgr, near maximum light and
pulsational phase zero, with the Space Telescope Imaging Spectrograph (STIS) on
Hubble Space Telescope (HST). The far-UV spectrum of each star shows a
photospheric continuum rising steeply toward longer wavelengths, and a
prominent emission feature at C II 1335. RY Sgr displays a second, but fainter,
emission attributed to Cl I 1351 (which is radiatively fluoresced by C II
1335), but Cl I is weak or absent in V854 Cen. Most surprisingly, the C II
emission of V854 Cen is significantly extended along the slit by +/- 2.5
arcsec, about 6 x 10^3 AU at the distance of the star. The C II feature of RY
Sgr exhibits no such gross extension. Nevertheless, subtle broadenings of the C
II emissions beyond the point response profile suggests inner clouds of radius
\~0.1 arcsec (250 AU) around both stars. V854 Cen is only the third RCB star
after R CrB and UW Cen known to have a resolved shell.Comment: 15 pages, 2 figures, (Figure 1 is a jpeg file), ApJ, in pres
Quantum Oscillations in CuBiSe in High Magnetic Fields
CuBiSe has drawn much attention as the leading candidate to be
the first topological superconductor and the realization of coveted Majorana
particles in a condensed matter system. However, there has been increasing
controversy about the nature of its superconducting phase. This study sheds
light on present ambiguity in the normal state electronic state, by providing a
complete look at the quantum oscillations in magnetization in
CuBiSe at intense high fields up to 31T. Our study focuses on the
angular dependence of the quantum oscillation pattern in a low carrier
concentration. As magnetic field tilts from along the crystalline c-axis to
ab-plane, the change of the oscillation period follows the prediction of the
ellipsoidal Fermi surface. As the doping level changes, the 3D Fermi surface is
found to transform into quasi-cylindrical at high carrier density. Such a
transition is potentially a Lifshitz transition of the electronic state in
CuBiSe.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
The fall and rise of V854 Centauri: long-term ultraviolet spectroscopy of a highly-active R Coronae Borealis star
We examine long-term low-dispersion IUE, SWP and LWP spectroscopy of the R
Coronae Borealis (RCB) star V854 Cen, obtained across the deep 1991, 1992-1993
and 1994 declines. We also report the optical light curve for the star in the
interval 1987-1998, including multi-color photometry obtained during 1989-1998.
Analysis of the UV emission line spectra indicates most lines decay during
the deep declines on characteristic timescales comparable to that reported for
optical features. Fe, Mg and neutral C lines decay on timescales of typically
50-100 d. Other lines, notably ionized C lines, decay on longer timescales (>
200 d) or appear to be unaffected by the declines. The general nature of the UV
emission lines and other UV features during the declines is consistent with the
E1/E2/BL line-region model developed from the behavior of optical spectral
features during declines. However, the detailed line-behavior indicates large
intrinsic variability between decline events inconsistent with the simple
E1/E2/BL model. Limited temporal coverage prevents detailed examination of the
geometry of the emission line region or the obscuring dust. We also report the
first detection of the transition-region line C IV 1550 in the spectrum of an
RCB star.Comment: AJ in press (June), 7 figures, 4 table
Research notes: Characterization of several abnormal nodulation reactions in soybeans
Several abnormal nodulation reactions in soybeans are known. These range from a complete lack of nodules, caused by the non-nodulating gene (Williams and Lynch, 1954) to plants with normal-appearing nodules (Vest et al., 1973), but low nitrogen fixation as exemplified by the \u27Peking\u27-strain T23 combination. The purpose of the study reported here was threefold
Research notes: Inheritance of abnormal nodulation between Rhizobium japonicum strain 62 and the soybean variety Amsoy 71
To date, four genes are known that result in abnormal nodulation in soybeans. The gene rj1 (Williams and Lynch, 1954; Caldwell, 1966) prevents nodulation with almost all Rhizobium japonicum strains. The genes Rj2 (Caldwell, 1966 ) in combination with strains b7 and bl4 of the 3-24-44 serogroup and bl22 of the 122 serogroup, Rj3 (Vest, 1970) in combination with strain 33, and Rj4 (Vest and Caldwell, 1972) in combination with strain 61 all result in chlorotic plants with abnormal nodulation
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