216 research outputs found
Design considerations for the beam-waveguide retrofit of a ground antenna station
Retrofitting an antenna that was originally designed without a beam waveguide introduces special difficulties because it is desirable to minimize alteration of the original mechanical truss work and to image the actual feed without distortion at the focal point of the dual-shaped reflector. To obtain an acceptable image, certain Geometrical Optics (GO) design criteria are followed as closely as possible. The problems associated with applying these design criteria to a 34-meter dual-shaped DSN (Deep Space Network) antenna are discussed. The use of various diffraction analysis techniques in the design process is also discussed. GTD and FFT algorithms are particularly necessary at the higher frequencies, while Physical Optics and Spherical Wave Expansions proved necessary at the lower frequencies
Nonlinear effects in superconducting thin film microwave resonators
We discuss how reactive and dissipative non-linearities affect the intrinsic
response of superconducting thin-film resonators. We explain how most, if not
all, of the complex phenomena commonly seen can be described by a model in
which the underlying resonance is a single-pole Lorentzian, but whose centre
frequency and quality factor change as external parameters, such as readout
power and frequency, are varied. What is seen during a vector-network-analyser
measurement is series of samples taken from an ideal Lorentzian that is
shifting and spreading as the readout frequency is changed. According to this
model, it is perfectly proper to refer to, and measure, the resonant frequency
and quality factor of the underlying resonance, even though the swept-frequency
curves appear highly distorted and hysteretic. In those cases where the
resonance curve is highly distorted, the specific shape of the trajectory in
the Argand plane gives valuable insights into the second-order physical
processes present. We discuss the formulation and consequences of this approach
in the case of non-linear kinetic inductance, two-level-system loss,
quasiparticle generation, and a generic model based on a power-law form. The
generic model captures the key features of specific dissipative
non-linearities, but additionally leads to insights into how general
dissipative processes create characteristic forms in the Argand plane. We
provide detailed formulations in each case, and indicate how they lead to the
wide variety of phenomena commonly seen in experimental data. We also explain
how the properties of the underlying resonance can be extracted from this data.
Overall, our paper provides a self-contained compendium of behaviour that will
help practitioners interpret and determine important parameters from distorted
swept-frequency measurements
Electrothermal feedback in kinetic inductance detectors
In kinetic inductance detectors (KIDs) and other similar applications of superconducting microresonators, both the large and small-signal behaviour of the device may be affected by electrothermal feedback. Microwave power applied to read out the device is absorbed by and heats the superconductor quasiparticles, changing the superconductor conductivity and hence the readout power absorbed in a positive or negative feedback loop. In this work, we explore numerically the implications of an extensible theoretical model of a generic superconducting microresonator device for a typical KID, incorporating recent work on the power flow between superconductor quasiparticles and phonons. This model calculates the large-signal (changes in operating point) and small-signal behaviour of a device, allowing us to determine the effect of electrothermal feedback on device responsivity and noise characteristics under various operating conditions. We also investigate how thermally isolating the device from the bath, for example by designing the device on a membrane only connected to the bulk substrate by thin legs, affects device performance. We find that at a typical device operating point, positive electrothermal feedback reduces the effective thermal conductance from the superconductor quasiparticles to the bath, and so increases responsivity to signal (pair-breaking) power, increases noise from temperature fluctuations, and decreases the noise equivalent power (NEP). Similarly, increasing the thermal isolation of the device while keeping the quasiparticle temperature constant decreases the NEP, but also decreases the device response bandwidth
Experimental Verification of Electromagnetic Simulations of a HIFI Mixer Sub-Assembly
Phase II of the study "Far-Infrared Optics Design &
Verification", commissioned by the European Space Agency (ESA), we
investigate the ability of several commercial software packages (GRASP,
CODEV, GLAD and ASAP) to predict the performance of a representative
example of a submillimeter-wave optical system. In this paper, we use the
software packages to predict the behaviour of a Mixer Sub-Assembly (MSA) of
HIFI, and we compare the simulations with near-field measurements at 480 GHz.
In order to be able to distinguish between the predictions of the packages, we
move the corrugated horn of the MSA through its nominal focus position. A
unique feature of the experimental arrangement is that the measured position of
every field point is known absolutely to within fractions of a wavelength. In this
paper we present the results of this through-focus experiment, which give a good
first-order indication of the agreement between measured and simulated
behaviour of a typical submillimeter-wave optical system
Diffraction Considerations for Planar Detectors in the Few-Mode Limit
Filled arrays of bolometers are currently being employed for use in astronomy
from the far-infrared through millimeter parts of the electromagnetic spectrum.
Because of the large range of wavelengths for which such detectors are
applicable, the number of modes supported by a pixel will vary according to the
specific application of a given available technology. We study the dependence
of image fidelity and induced polarization on the size of the pixel by
employing a formalism in which diffraction due to the pixel boundary is treated
by propagating the second-order statistical correlations of the radiation field
through a model optical system. We construct polarized beam pattern images of
square pixels for various ratios of p/\lambda where p is the pixel size and
\lambda is the wavelength of the radiation under consideration. For the limit
in which few modes are supported by the pixel (p/\lambda<1), we find that the
diffraction due to the pixel edges is non-negligible and hence must be
considered along with the telescope diffraction pattern in modeling the
ultimate spatial resolution of an imaging system. For the case in which the
pixel is over-moded (p/\lambda>1), the geometric limit is approached as
expected. This technique gives a quantitative approach to optimize the imaging
properties of arrays of planar detectors in the few-mode limit.Comment: 20 pages, 8 figure
Intoxicants and the invention of 'consumption'
In 1600 the word ‘consumption’ was a term of medical pathology describing the ‘wasting, petrification of things’. By 1700 it was also a term of economic discourse: ‘In commodities, the value rises as its quantity is less and vent greater, which depends upon it being preferred in its consumption’. The article traces the emergence of this key category of economic analysis to debates over the economy in the 1620s and subsequent disputes over the excise tax, showing how ‘consumption’ was an early term in the developing lexicon of political economy. In so doing the article demonstrates the important role of ‘intoxicants’ – i.e. addictive and intoxicating commodities like alcohols and tobaccos – in shaping these early meanings and uses of ‘consumption’. It outlines the discursive importance of intoxicants, both as the foci for discussions of ‘superfluous’ and ‘necessary’ consumption and the target of legislation on consumption. And it argues that while these discussions had an ideological dimension, or dimensions, they were also responses to material increases in the volume and diversity of intoxicants in early seventeenth-century England. By way of conclusion the article suggests the significance of the Low Countries as a point of reference for English writers, as well as a more capacious and semantically sensitive approach to changes in early-modern consumption practices
HARP/ACSIS: A submillimetre spectral imaging system on the James Clerk Maxwell Telescope
This paper describes a new Heterodyne Array Receiver Programme (HARP) and
Auto-Correlation Spectral Imaging System (ACSIS) that have recently been
installed and commissioned on the James Clerk Maxwell Telescope (JCMT). The
16-element focal-plane array receiver, operating in the submillimetre from 325
to 375 GHz, offers high (three-dimensional) mapping speeds, along with
significant improvements over single-detector counterparts in calibration and
image quality. Receiver temperatures are 120 K across the whole band and
system temperatures of 300K are reached routinely under good weather
conditions. The system includes a single-sideband filter so these are SSB
figures. Used in conjunction with ACSIS, the system can produce large-scale
maps rapidly, in one or more frequency settings, at high spatial and spectral
resolution. Fully-sampled maps of size 1 square degree can be observed in under
1 hour.
The scientific need for array receivers arises from the requirement for
programmes to study samples of objects of statistically significant size, in
large-scale unbiased surveys of galactic and extra-galactic regions. Along with
morphological information, the new spectral imaging system can be used to study
the physical and chemical properties of regions of interest. Its
three-dimensional imaging capabilities are critical for research into
turbulence and dynamics. In addition, HARP/ACSIS will provide highly
complementary science programmes to wide-field continuum studies, and produce
the essential preparatory work for submillimetre interferometers such as the
SMA and ALMA.Comment: MNRAS Accepted 2009 July 2. 18 pages, 25 figures and 6 table
The non-equilibrium response of a superconductor to pair-breaking radiation measured over a broad frequency band
We have measured the absorption of terahertz radiation in a BCS
superconductor over a broad range of frequencies from 200 GHz to 1.1 THz, using
a broadband antenna-lens system and a tantalum microwave resonator. From low
frequencies, the response of the resonator rises rapidly to a maximum at the
gap edge of the superconductor. From there on the response drops to half the
maximum response at twice the pair-breaking energy. At higher frequencies, the
response rises again due to trapping of pair-breaking phonons in the
superconductor. In practice this is the first measurement of the frequency
dependence of the quasiparticle creation efficiency due to pair-breaking in a
superconductor. The efficiency, calculated from the different non-equilibrium
quasiparticle distribution functions at each frequency, is in agreement with
the measurements.We would like to thank Jan Barkhof for help with the FTS calibration. This work was in part supported by ERC starting Grant Nos. ERC-2009-StG and 240602 TFPA. T. M. Klapwijk acknowledges financial support from the Ministry of Science and Education of Russia under Contract No. 14.B25.31.0007 and from the European Research Council Advanced Grant No. 339306 (METIQUM). P. J. de Visser acknowledges support from a Niels Stensen Fellowship.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.492309
Detector Modeling and CMB Polarimetry Technology Development at GSFC
Pixel size limits the resolution in the focal plane. This should be accounted for in optical design. Alternatively, this reduces the effective number of independent detectors. Polarization and scattering are intrinsically related, and both are more severe at low pnambda. Future work: Quantification of the pixel cross-coupling- calculate a theoretical covariance matrix to predict performance of future detector arrays
- …