509 research outputs found
Fluctuations in the electron system of a superconductor exposed to a photon flux
We report on fluctuations in the electron system, Cooper pairs and
quasiparticles, of a superconducting aluminium film. The superconductor is
exposed to pair-breaking photons (1.54 THz), which are coupled through an
antenna. The change in the complex conductivity of the superconductor upon a
change in the quasiparticle number is read out by a microwave resonator. A
large range in radiation power can be chosen by carefully filtering the
radiation from a blackbody source. We identify two regimes. At high radiation
power, fluctuations in the electron system caused by the random arrival rate of
the photons are resolved, giving a straightforward measure of the optical
efficiency (48%). At low radiation power fluctuations are dominated by excess
quasiparticles, the number of which is measured through their recombination
lifetime
Confocal Ellipsoidal Reflector System for a Mechanically Scanned Active Terahertz Imager
We present the design of a reflector system that can rapidly scan and refocus a terahertz beam for high-resolution standoff imaging applications. The proposed optical system utilizes a confocal Gregorian geometry with a small mechanical rotating mirror and an axial displacement of the feed. For operation at submillimeter wavelengths and standoff ranges of many meters, the imaging targets are electrically very close to the antenna aperture. Therefore the main reflector surface must be an ellipse, instead of a parabola, in order to achieve the best imaging performance. Here we demonstrate how a simple design equivalence can be used to generalize the design of a Gregorian reflector system based on a paraboloidal main reflector to one with an ellipsoidal main reflector. The system parameters are determined by minimizing the optical path length error, and the results are validated with numerical simulations from the commercial antenna software package GRASP. The system is able to scan the beam over 0.5 m in cross-range at a 25 m standoff range with less than 1% increase of the half-power beam-width
Time-Delay Multiplexing of Two Beams in a Terahertz Imaging Radar
We demonstrate a time-delay multiplexing technique
that doubles the frame rate of a 660–690-GHz imaging radar
with minimal additional instrument complexity. This is done by
simultaneously projecting two offset, orthogonally polarized radar
beams generated and detected by a common source and receiver.
Beam splitting and polarization rotation is accomplished with a
custom designed waveguide hybrid coupler and twist. A relative
time lag of approximately 2 ns between the beams’ waveforms is
introduced using a quasi-optical delay line, followed by spatial
recombination using a selectively reflective wire grid. This delay is
much longer than the approximately 20-ps time-of-flight resolution
of the 30-GHz bandwidth radar, permitting the two beams’
reflected signals from a compact target to be easily distinguished
in digital post-processing of the single receiver channel
Operation and Maintenance in Solar Plants: Eight study cases
The use of solar photovoltaic generation is growing up nowadays. New developments have been achieved lowering the prices of the modules and their installation. Nevertheless, the operation and maintenance of these plants still needs improvements in order to enhance their efficiency and economic balance.
This paper presents a detailed analysis of eight photovoltaic (PV) plants revealing improvements on the monitoring system and the maintenance procedures.
The common failures in PV plants have been established and their losses have been estimated. A correct diagnostic method using good information and monitoring systems could have reduced the losses
Surface wave control for large arrays of microwave kinetic inductance detectors
Large ultra-sensitive detector arrays are needed for present and future
observatories for far infra-red, submillimeter wave (THz), and millimeter wave
astronomy. With increasing array size, it is increasingly important to control
stray radiation inside the detector chips themselves, the surface wave. We
demonstrate this effect with focal plane arrays of 880 lens-antenna coupled
Microwave Kinetic Inductance Detectors (MKIDs). Presented here are near field
measurements of the MKID optical response versus the position on the array of a
reimaged optical source. We demonstrate that the optical response of a detector
in these arrays saturates off-pixel at the dB level compared to the
peak pixel response. The result is that the power detected from a point source
at the pixel position is almost identical to the stray response integrated over
the chip area. With such a contribution, it would be impossible to measure
extended sources, while the point source sensitivity is degraded due to an
increase of the stray loading. However, we show that by incorporating an
on-chip stray light absorber, the surface wave contribution is reduced by a
factor 10. With the on-chip stray light absorber the point source response
is close to simulations down to the dB level, the simulation based on
an ideal Gaussian illumination of the optics. In addition, as a crosscheck we
show that the extended source response of a single pixel in the array with the
absorbing grid is in agreement with the integral of the point source
measurements.Comment: accepted for publication in IEEE Transactions on Terahertz Science
and Technolog
Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar
We show experimentally that a high-resolution imaging radar operating at 576–605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4–25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources
Machine Learning models for the estimation of the production of large utility-scale photovoltaic plants
Photovoltaic (PV) energy development has increased in the last years mainly based on large utility-scale plants. These plants are characterised by a huge number of panels connected to high-power inverters occupying a large land area. An accurate estimation of the power production of the PV plants is needed for failure detection, identifying production deviations, and the integration of the plants into the power grid. Various studies have used Machine Learning estimation techniques developed on very small PV plants. This paper deals with large utility-scale plants and uses all the available information to represent the non-uniform radiation over the whole studied solar field. Variables measured in up to four meteorological stations and distributed across the plant are used. Three PV plants with 1, 2 and 4 meteorological stations have been used to develop Machine Learning models. The hyperparameters were systematically optimised, demonstrating the improvements by comparing with a simple model based on Multiple Linear Regression. The best results were obtained with the Random Forest technique for the three PV plants, providing a RMS error value ranging from 1.9% to 5.4%. The final models were compared with those found in the literature for tiny PV plants showing in general much better performance
Eliminating stray radiation inside large area imaging arrays
With increasing array size, it is increasingly important to control stray
radiation inside the detector chips themselves. We demonstrate this effect with
focal plane arrays of absorber coupled Lumped Element microwave Kinetic
Inductance Detectors (LEKIDs) and lens-antenna coupled distributed quarter
wavelength Microwave Kinetic Inductance Detectors (MKIDs). In these arrays the
response from a point source at the pixel position is at a similar level to the
stray response integrated over the entire chip area. For the antenna coupled
arrays, we show that this effect can be suppressed by incorporating an on-chip
stray light absorber. A similar method should be possible with the LEKID array,
especially when they are lens coupled.Comment: arXiv admin note: substantial text overlap with arXiv:1707.0214
Valor de la Resonancia Nuclear Magnética en Ortopedia Oncológica
Presentamos nuestra experiencia preliminar sobre la utilidad diagnóstica de la
Resonancia Nuclear Magnética (RM) en la evaluación pre- y postoperatoria de pacientes con tumores
musculoesqueléticos. Se han revisado 21 pacientes con tumores caracterizados histológicamente.
Del total, 10 casos eran sarcomas de partes blandas, 7 correspondían a tumores óseos
y 4 eran metástasis óseas o recidivas locales. Las imágenes se obtuvieron mediante un sistema
de RM de campo medio provisto de un imán superconductor operando a 0.5 Teslas. En todos
los casos se obtuvieron imágenes T1 y T2, en los planos axial y coronal. Como norma se objetivó
un excelente contraste entre la señal de la lesión y la de las estructuras normales adyacentes.
Sólo en un caso, un osteosarcoma del extremo proximal del peroné, las imágenes de extensión
a partes blandas vecinas resultaron ser negativas en la exploración quirúrgica. El análisis de los
cambios de intensidad de la señal no permitieron distinguir la especificidad tisular del tumor,
ni diferenciar lesiones benignas y malignas. En nuestra experiencia, la RM nos ha permitido
un mejor diagnóstico anatómico de la extensión tumoral, facilitándonos la planificación quirúrgica
que requieren las modernas técnicas reconstructivas en ortopedia oncológica.The preliminary experience using Magnetic Resonance imaging for pre- and
post-operative assessment of orthopaedic oncologic patients is hereby reported. Twenty-one
patients with histologically characterized bone and soft tissue tumors have been reviewed. Seventeen
patients had primary musculoskeletal neoplasia: 10 had soft tissue sarcomas and 7 bone
tumors. The remained 4 patients consisted of bone metastasis or local racidive. Magnetic
resonance images were acquired using a superconductive magnet operating at 0.5 Tesla. T1-
and T2-weighted transaxial and coronal images were obtained in all cases. An excellent contrast
between the signal of the lesion and the normal adjacent structures was usually obtained.
Only in one osteosarcoma of the proximal fibula, an extraosseous extension was presumed but
not found during surgical resection. Changes in image intensity did not permit to identify tumor
tissue specificity neither distinguish between benign and malignat lesions. In our experience,
Magnetic Resonance shows a great advantage in order to determine tumor anatomical extension,
providing a useful information for the surgical planning required by current reconstructive
techniques in orthopaedic oncology
Dermatitis crónica por sensibilización al níquel en un paciente con fractura del astrágalo tratada mediante osteosíntesis
Se presenta un varón de 25 años, con fractura del astrágalo izquierdo tratada
mediante osteosíntesis con 2 tornillos de acero inoxidable con un contenido en níquel del 12-14%.
A los 4 meses de la intervención, el paciente desarrolló una dermatitis pruriginosa en cara
lateral externa del pie, en la zona cutánea supra-adyacente al lugar de la osteosíntesis. La lesión
no cedió con el tratamiento tópico convencional a base de corticoides. Las pruebas epicutáneas
realizadas con la batería estándar del Grupo Español de Investigación en Dermatitis
de Contacto revelaron un parche positivo vesiculoso (+ +) al sulfato de níquel al 2,5% y al
cloruro de cobalto al 1%, ambos aplicados en vaselina, en la lectura a las 48 y 96 horas. En
vista de que las lesiones permanecían estables al mes de tratamiento con un potente corticoide
en cura oclusiva y antihistamínicos sistémicos, se procedió a retirar los tornillos de osteosíntesis.
A los pocos días de la intervención comenzó a desaparecer el prurito y, en el curso de
2 semanas, se produjo la curación completa de la dermatitisSe presenta un varón de 25 años, con fractura del astrágalo izquierdo tratada
mediante osteosíntesis con 2 tornillos de acero inoxidable con un contenido en níquel del 12-14%.
A los 4 meses de la intervención, el paciente desarrolló una dermatitis pruriginosa en cara
lateral externa del pie, en la zona cutánea supra-adyacente al lugar de la osteosíntesis. La lesión
no cedió con el tratamiento tópico convencional a base de corticoides. Las pruebas epicutáneas
realizadas con la batería estándar del Grupo Español de Investigación en Dermatitis
de Contacto revelaron un parche positivo vesiculoso (+ +) al sulfato de níquel al 2,5% y al
cloruro de cobalto al 1%, ambos aplicados en vaselina, en la lectura a las 48 y 96 horas. En
vista de que las lesiones permanecían estables al mes de tratamiento con un potente corticoide
en cura oclusiva y antihistamínicos sistémicos, se procedió a retirar los tornillos de osteosíntesis.
A los pocos días de la intervención comenzó a desaparecer el prurito y, en el curso de
2 semanas, se produjo la curación completa de la dermatitis
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