Search Results - (Author, Cooperation:N. Nikolov)

2015-12-18

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Atmosphere/*chemistry ; Extraterrestrial Environment/*chemistry ; Jupiter ; *Planets ; Pressure ; Spectrophotometry, Infrared ; Telescopes ; Temperature ; Water/*analysis

1089-7550

AIP Digital Archive

Physics

Quality and pumping of an electron beam by means of the inhomogeneous electrostatic field at an anode aperture are investigated using numerical simulation. A specially developed relativistic particle-in-cell code, described here, is employed. The longitudinal velocity spread and the pitch ratio of the beam electrons are derived for different aperture radii and cathode-aperture distances. The presented results could be practically useful for operation of high-power cyclotron resonance masers with a sharpened cathode and an anode aperture. The beam pumping in such devices is typically achieved by more complicated systems (pumping magnet, adiabatically changing magnetic field, etc.). The numerical simulations show a significant pumping effect coming from the anode aperture itself.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The operation of a virtual cathode oscillator (vircator) with strong axial magnetic field has been experimentally studied. Depending on the cathode–anode gap and cathode diameter, the operating voltage varies from 200 kV up to 480 kV with 2–7 kA diode current. Microwave emission is produced by the oscillating virtual cathode. The central microwave frequency follows the beam plasma frequency. It varies by 11.5 GHz up to 22 GHz, depending on the current density. The oscillation frequency does not depend on the guide magnetic field magnitude. A maximal output power of 15±5 MW in asymmetric transverse magnetic (TM) modes is achieved by the axially extracted vircator. Variation of the magnetic field intensity in a range of 0–40 kG has an insignificant effect upon the emitted microwave power. An electron beam power to microwave power conversion efficiency of approximately 1% is obtained.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Results from 350 kV, 7 kA virtual cathode oscillator experiments are presented. High-frequency (11–17.6 GHz) high-power (15±7 MW) microwave radiation by a virtual cathode oscillator with a guide magnetic field has been obtained. It has been found that the oscillating virtual cathode and the beam electrons trapped between the real and the virtual cathodes radiate microwaves at two distinct frequencies. The dominant source of radiation is the reflexing electrons. A nonsymmetrical microwave mode TM11 has been observed.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Intense emission of the TE12 mode has been detected in an experiment based on the cyclotron resonance maser mechanism with nonadiabatic electrostatic pumping of an electron beam. The pitch-ratio α of beam electrons has been found experimentally and estimated from a computer simulation. Both results are in good agreement with each other.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A photoemission study of the interface between spin-cast films of a conducting polymer blend consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4′-bis(carbazol-9-yl)biphenyl, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1-1′biphenyl-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′- biphenyl-4,4′-diamine reveals a hole injection barrier between 0.5 and 0.9 eV at the glycerol-modified PEDOT-PSS/HTL interface. The measured energy barriers imply a reasonable charge injection, which is very encouraging for further development of the novel anode structures based on a conducting polymer/small molecule interface to be utilized in electro-optic applications such as organic light-emitting devices. © 2001 American Institute of Physics.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

A Doppler-shifted cyclotron resonance maser experiment using a 500 keV, 30 nsec high-current electron beam emitted by a Blumlein pulse accelerator has produced approximately 20 and 8 MW at frequencies of 13 and 37 GHz in TE01 and TE03 waveguide modes, respectively. The beam has been electrostatically pumped by means of a strongly inhomogeneous electrostatic field created in the diode region. The experimental results are in a good agreement with the numerical calculations of a cold system electrostatic field and with simulation studies of beam propagation. The transverse electron velocity has been estimated on the basis of the performed computations and experimental measurements.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Results from a cyclotron autoresonance maser experiment are reported. Two maxima of the output microwave power (8 and 10 MW) at a wavelength of 5 and 5.5 mm, respectively, have been observed. The nonadiabatic beam pumping is achieved by transit of the electrons through an external magnetic-field local inhomogeneity due to the presence of a copper aperture.

Electronic Resource

0005-2795

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Biology

Electronic Resource

0031-9422

6,8-di-C-glycosylflavones ; Crataegus monogyna ; Rosaceae ; isoschaftoside ; neoisoschaftoside. ; neoschaftoside ; schaftoside ; vicenin-1 ; vicenin-2 ; vicenin-3

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Biology

Chemistry and Pharmacology

Electronic Resource

0375-9601

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

0273-1177

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Electronic Resource

0167-8760

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Medicine

Psychology

Electronic Resource

0167-8760

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Medicine

Psychology

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

Abstract Large-scaleU andB plates obtained with the 2 m Ritchey-Chrétien telescope of the Rozhen Observatory (Bulgaria) were searched for new resolved star groups and for independent delineation of the boundaries of the known ones in M31. We detected 210 groups as real O-associations the mean diameter of which is 80 pc. Many of Hodge's open clusters are also reclassified as O-associations. The majority of van den Bergh's OB-associations were recognized as star complexes and their mean diameter is 650 pc. Almost all O-associations are located inside the star complexes. A dozen of new star complexes (mainly around the dark lanes between OB78 and OB22) and numerous groups presumably not containing O-stars were found out. The nature of these groups has to be object of further investigations. Young star groups closer than 3 kpc to the center of M31 were not identified.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

Abstract The 2 m RCC telescope of the Rozhen National Observatory (Bulgaria) was used to obtain the colour-magnitude and colour-colour diagrams of OB110 and OB112 associations which form the largest star grouping in the southern region of M33. The stars withV andU≲20.4 andB≲21 were measured. TheV magnitude, the coloursB - V, U - B and the respective identification charts are given. This allowed to construct the luminosity function of OB110+OB112. The group has an integrated magnitudeM VT=−11.8 which is by 0 m .5 fainter than OB78 (M31). The mean absorption and the foreground were examined as well.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

Abstract We used the catalogue of the mean light-curves and colours of the Cepheids in the UBV system (Nikolov, 1968), for 240 Cepheids, and constructed the curves which represent plots of their U-B versus B-V colour indices. These curves form three groups: linear curves, open curves, and curves possessing a loop. An opinion that the presence of a loop may be a criterion for Population II objects is not confirmed. It is shown that the slope of such curves (corrected for reddening) with respect to the B-V axis increases with the period. The area delimited by the curves on the U-B/B-V diagram remains almost the same up to a period of about 9 to 10 days, whereafter it increases with the period. The possible causes for this behaviour of the Cepheids on the colour-colour diagram, which is connected with the ultraviolet emission of these stars, are discussed.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

Abstract On the basis of the PLC relation (1) or the PL relation by Van den Bergh (2) and the PC relation by Deanet al. (1978), the distances of 284 galactic cepheids with photoelectric observations have been derived. The space distribution of these cepheids with 111 additional ones without photoelectric observations, is studied. In spite of the strong influence of the absorption matter, which makes a great number of distant cepheids unknown (Figure 4), a conclusion is drawn that the cepheids do not trace spiral arms with only one possible exception: the Carina arm. The cepheidz-coordinate distribution confirms the finding of Fernie (1968) that the cepheid layer is inclined towards the formal galactic plane. On the basis of cepheid space density, a number of vast star complexes (Table I) are identified in which other young objects, together with cepheids fall. The existence of these complexes is explained by star formation in giant molecular clouds. The cepheid mean period increase towards the galactic centre is most probably connected with the existence of a ring between the Sun and the centre of the Galaxy, with the highest density of hydrogen and the highest rate of star formation.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

Abstract A new PL-relation (10) — Figure 2 for the Cepheids in the Galaxy, the Magellanic Clouds and M31 has been constructed. On deriving this relation both the period-radius (3) and period-colour relations (7), (8) and (9) are essentially used. The PC-relation (7) determined after the colours of 88 galactic cepheids (Table I), which are obtained from the colour-spectrum relation (6) — Figure 1, common for Cepheids and non-variable supergiants, are used also for the M31 Cepheids, whereas (8) and (9) are for the Large and Small MC Cepheids, respectively, all three PC-relations having a common slope. The comparison of the relations (8) and (9) with (7) shows that the LMC and SMC Cepheids are bluer than the galactic ones with 0m.04 and 0m.19, respectively, probably because of their metal-poor abundance. The places of thes-Cepheids in Figure 2 show that these Cepheids possess a dissimilar PL-relation with a different slope. The reason for such a difference is that thes-Cepheids are first harmonic pulsators. The distance moduli of the three galaxies under discussion are obtained from the PL-relation (10). The colour-coefficient of period-colour-luminosity-relation is briefly discussed. The general conclusion based on a comparison of the PL-relation in the present paper with those by other authors (Table V) is that our PL-relation differs in the zero-point by less than 0m.2; therefore, the manner of constructing the PL-relation by means of PR and PC-relations is reasonable and useful.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

Abstract An analysis is presented which seeks to reveal the possible causes for discrepancy in the results on the dependence of Cepheid amplitudes on star positions in the instability strip noted by different investigators. A comparison of the data used in two of these investigations—that of Yakimova (1970) and that of Sandage and Tammann (1971)—shows that it is most unlikely to be able to explain completely the conflicting results obtained in these studies. But if we analyse the data of Sandage and Tammann following the approach of Yakimova — namely, by considering the Cepheids in only one stellar system and taking into account the stellar colours in maximum light — we obtain Yakimova's result: the light amplitude is the larger, the nearer to the low temperature edge of the instability strip a Cepheid is. Therefore, it appears that differences between the various approaches contribute greatly to the different results of Sandage and Tammann and Yakimova (and the other investigators).

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