5,970 research outputs found
Multi-channel GaAs-based planar gunn diodes
We present a multi-channel GaAs-based planar Gunn
diode. By introducing extra channels, the output RF power has
been significantly improved compared to single-channel GaAsbased
planar Gunn diodes. For a 1.14 μm length and 60 μm wide
device, the highest power achieved was approximately -4 dBm
operating in fundamental mode at 109 GHz, and -26.6 dBm at its
second-harmonic at 218 GHz
Radiative Hydrodynamic Simulations of HD209458b: Temporal Variability
We present a new approach for simulating the atmospheric dynamics of the
close-in giant planet HD209458b that allows for the decoupling of radiative and
thermal energies, direct stellar heating of the interior, and the solution of
the full 3D Navier Stokes equations. Simulations reveal two distinct
temperature inversions (increasing temperature with decreasing pressure) at the
sub-stellar point due to the combined effects of opacity and dynamical flow
structure and exhibit instabilities leading to changing velocities and
temperatures on the nightside for a range of viscosities. Imposed on the
quasi-static background, temperature variations of up to 15% are seen near the
terminators and the location of the coldest spot is seen to vary by more than
20 degrees, occasionally appearing west of the anti-solar point. Our new
approach introduces four major improvements to our previous methods including
simultaneously solving both the thermal energy and radiative equations in both
the optical and infrared, incorporating updated opacities, including a more
accurate treatment of stellar energy deposition that incorporates the opacity
relevant for higher energy stellar photons, and the addition of explicit
turbulent viscosity.Comment: Accepted for publication in Ap
A wideband CPW ring power combiner with low insertion loss and high port isolation
In this paper we present a coplanar waveguide (CPW)-based ring power combiner that exhibits less than 0.8 dB insertion loss, better than 15 dB port match and higher than 22 dB isolation loss over the frequency range from 50 GHz to 100 GHz. Compared with the conventional 2-way Wilkinson combiner, the proposed ring power combiner replaces the resistor between the two input ports with two quasi quarter-wave CPWs, a 180º CPW phase inverter, and two resistors that lead to frequency-insensitive port isolation and wideband port match. The power combiner is realized using an electron beam-based GaAs MMIC process along with simple electron beam airbridge technology. These results agree well with 3D full-wave simulations
Biocompatibility of a lab-on-a-pill sensor in artificial gastrointestinal environments
n this paper, we present a radiotelemetry sensor, designed as a lab-in-a-pill, which incorporates a two-channel microfabricated sensor platform for real-time measurements of temperature and pH. These two parameters have potential application for use in remote biological sensing (for example they may be used as markers that reflect the physiological environment or as indicators for disease, within the gastrointestinal tract). We have investigated the effects of biofouling on these sensors, by exploring their response time and sensitivity in a model in vitro gastrointestinal system. The artificial gastric and intestinal solutions used represent a model both for fasting, as well as for the ingestion of food and subsequent digestion to gastrointestinal chyme. The results showed a decrease in pH sensitivity after exposure of the sensors for 3 h. The response time also increased from an initial measurement time of 10 s in pure GI juice, to ca. 25 s following the ingestion of food and 80 s in simulated chyme. These in vitro results indicate that changes in viscosity in our model gastrointestinal system had a pronounced effect on the unmodified sensor
Filamentation Instability of Interacting Current Sheets in Striped Relativistic Winds: The Origin of Low Sigma?
I outline a mechanism, akin to Weibel instabilities of interpenetrating
beams, in which the neighboring current sheets in a striped wind from an
oblique rotator interact through a two stream-like mechanism (a Weibel
instability in flatland), to create an anomalous resistivity that heats the
sheets and causes the magnetic field to diffusively annihilate in the wind
upstream of the termination shock. The heating has consequences for observable
unpulsed emission from pulsars.Comment: 7 pages, 9 figures. To be published in the proceedings of ``40 Years
of Pulsars'
Theory of Pulsar Wind Nebulae
Our understanding of Pulsar Wind Nebulae (PWNe), has greatly improved in the
last years thanks to unprecedented high resolution images taken from the
HUBBLE, CHANDRA and XMM satellites. The discovery of complex but similar inner
features, with the presence of unexpected axisymmetric rings and jets, has
prompted a new investigation into the dynamics of the interaction of the pulsar
winds with the surrounding SNR, which, thanks to the improvement in the
computational resources, has let to a better understanding of the properties of
these objects. On the other hand the discovery of non-thermal emission from bow
shock PWNe, and of systems with a complex interaction between pulsar and SNR,
has led to the development of more reliable evolutionary models. I will review
the standard theory of PWNe, their evolution, and the current status in the
modeling of their emission properties, in particular I will show that our
evolutionary models are able to describe the observations, and that the X-ray
emission can now be reproduced with sufficient accuracy, to the point that we
can use these nebulae to investigate fundamental issues as the properties of
relativistic outflows and particle acceleration.Comment: 9 page, 5 figures, Proceeding of the conference "40 Years of
Pulsars", 12-17 August 2007, Montreal, Canada. (figures are not properly
displayed in .ps or .pdf version please download archive for them
Twenty Years of Searching for (and Finding) Globular Cluster Pulsars
Globular clusters produce orders of magnitude more millisecond pulsars per
unit mass than the Galactic disk. Since the first cluster pulsar was uncovered
twenty years ago, at least 138 have been identified - most of which are binary
millisecond pulsars. Because of their origins involving stellar encounters,
many of these systems are exotic objects that would never be observed in the
Galactic disk. Examples include pulsar-main sequence binaries, extremely rapid
rotators (including the current record holder), and millisecond pulsars in
highly eccentric orbits. These systems are allowing new probes of the
interstellar medium, the equation of state of material at supra-nuclear
density, the mass distribution of neutron stars, and the dynamics of globular
clusters.Comment: 9 pages, 6 figures. Submitted review for the "40 Years of Pulsars"
conference in Montreal, Aug 2007. To be published by the AI
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