23,808 research outputs found
Optimization of micromachined relex klystrons for operation at terahertz frequencies
New micromachining techniques now provide us
with the technology to fabricate reflex klystron oscillators with dimensions suitable for operation in the terahertz region of the electromagnetic spectrum. For the success of these devices, accurate designs are required since the optimization of certain parameters is critical to obtaining useful amounts of ac power. Classical models for device design have long been in existence,
but these are no longer valid at terahertz frequencies. For this reason, we have developed a simulation tool, specifically aimed at the design of terahertz frequency reflex klystrons. The tool, based on the Monte Carlo algorithm, includes loss mechanisms and takes into account the main peculiarities expected for device
operation at terahertz frequencies. In this study, the tool is used to study the influence of the electron beam aperture angle and cavity dimensions (particularly the grid spacing) on ac power generation. The results demonstrate that aperture angles of less than 10 are necessary for the optimization of output power. It is
also found that the power output is highly sensitive to the distance between the grids
Holographic non-computers
We introduce the notion of holographic non-computer as a system which
exhibits parametrically large delays in the growth of complexity, as calculated
within the Complexity-Action proposal. Some known examples of this behavior
include extremal black holes and near-extremal hyperbolic black holes. Generic
black holes in higher-dimensional gravity also show non-computing features.
Within the expansion of General Relativity, we show that large-
scalings which capture the qualitative features of complexity, such as a linear
growth regime and a plateau at exponentially long times, also exhibit an
initial computational delay proportional to . While consistent for large AdS
black holes, the required `non-computing' scalings are incompatible with
thermodynamic stability for Schwarzschild black holes, unless they are tightly
caged.Comment: 23 pages, 7 figures. V3: References added. Figures updated. New
discussion of small black holes in the canonical ensembl
Exact and approximate symmetries for light propagation equations with higher order nonlinearity
For the first time exact analytical solutions to the eikonal equations in
(1+1) dimensions with a refractive index being a saturated function of
intensity are constructed. It is demonstrated that the solutions exhibit
collapse; an explicit analytical expression for the self-focusing position,
where the intensity tends to infinity, is found. Based on an approximated Lie
symmetry group, solutions to the eikonal equations with arbitrary nonlinear
refractive index are constructed. Comparison between exact and approximate
solutions is presented. Approximate solutions to the nonlinear Schrodinger
equation in (1+2) dimensions with arbitrary refractive index and initial
intensity distribution are obtained. A particular case of refractive index
consisting of Kerr refraction and multiphoton ionization is considered. It is
demonstrated that the beam collapse can take place not only at the beam axis
but also in an off-axis ring region around it. An analytical condition
distinguishing these two cases is obtained and explicit formula for the
self-focusing position is presented.Comment: 25 pages, 5 figure
Edge and waveguide THz surface plasmon modes in graphene micro-ribbons
Surface plasmon modes supported by graphene ribbon waveguides are studied and
classified. The properties of both modes with the field concentration within
the ribbon area (waveguiding modes) and on the edges (edge modes) are
discussed. The waveguide and edge modes are shown to be separated from each
other by a gap in wavenumbers. The even-parity hybridized edge mode results to
be the fundamental electromagnetic mode of the ribbon, possessing also the
lowest losses. All the plasmonic modes in the ribbons have an optimum
frequency, at which the absorption losses are minimum, due to competition
between the plasmon confinement and the frequency dependence of absorption in
graphene.Comment: 4 pages, 4 figure
Change in the North Atlantic circulation associated with the mid-Pleistocene transition
The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20-MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37 degrees 34.285' N, 10 degrees 7.562' W; 2585m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55 degrees N, basin-wide paleoceano-graphic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south-north, and east-west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.info:eu-repo/semantics/publishedVersio
Dynamic cyclic performance of phenol-formaldehyde resin derived carbons for pre-combustion CO2 capture : An experimental study
Acknowledgments This work was carried out with financial support from the Spanish MINECO (Project ENE2011-23467), co-financed by the European Regional Development Fund (ERDF).Peer reviewedPublisher PD
Wavelength de-multiplexing properties of a single aperture flanked by periodic arrays of indentations
In this paper we explore the transmission properties of single subwavelength
apertures perforated in thin metallic films flanked by asymmetric
configurations of periodic arrays of indentations. It is shown how the
corrugation in the input side can be used to transmit selectively only two
different wavelengths. Also, by tuning the geometrical parameters defining the
corrugation of the output side, these two chosen wavelengths can emerge from
the structure as two very narrow beams propagating at well-defined directions.
This new ability of structured metals can be used as a base to build
micron-sized wavelength de-multiplexers.Comment: Accepted for publication in Photonics and Nanostructure
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