914 research outputs found
Negative-resistance models for parametrically flux-pumped superconducting quantum interference devices
A Superconducting QUantum Interference Device (SQUID) modulated by a fast
oscillating magnetic flux can be used as a parametric amplifier, providing gain
with very little added noise. Here, we develop linearized models to describe
the parametrically flux-pumped SQUID in terms of an impedance. An unpumped
SQUID acts as an inductance, the Josephson inductance, whereas a flux-pumped
SQUID develops an additional, parallel element which we have coined the
``pumpistor.'' Parametric gain can be understood as a result of a negative
resistance of the pumpistor. In the degenerate case, the gain is sensitive to
the relative phase between the pump and signal. In the nondegenerate case, gain
is independent of this phase.
We develop our models first for degenerate parametric pumping in the
three-wave and four-wave cases, where the pump frequency is either twice or
equal to the signal frequency, respectively. We then derive expressions for the
nondegenerate case where the pump frequency is not a multiple of the signal
frequency, where it becomes necessary to consider idler tones which develop.
For the nondegenerate three-wave case, we present an intuitive picture for a
parametric amplifier containing a flux-pumped SQUID where current at the signal
frequency depends upon the load impedance at an idler frequency. This
understanding provides insight and readily testable predictions of circuits
containing flux-pumped SQUIDs.Comment: 27 pages, 6 figures, 1 tabl
The pumpistor: a linearized model of a flux-pumped SQUID for use as a negative-resistance parametric amplifier
We describe a circuit model for a flux-driven SQUID. This is useful for
developing insight into how these devices perform as active elements in
parametric amplifiers. The key concept is that frequency mixing in a
flux-pumped SQUID allows for the appearance of an effective negative
resistance. In the three-wave, degenerate case treated here, a negative
resistance appears only over a certain range of allowed input signal phase.
This model readily lends itself to testable predictions of more complicated
circuits.Comment: 4 pages, 3 figure
Long term variability of Cygnus X-1: VII. Orbital variability of the focussed wind in Cyg X-1 / HDE 226868 system
Binary systems with an accreting compact object are a unique chance to
investigate the strong, clumpy, line-driven winds of early type supergiants by
using the compact object's X-rays to probe the wind structure. We analyze the
two-component wind of HDE 226868, the O9.7Iab giant companion of the black hole
Cyg X-1 using 4.77 Ms of RXTE observations of the system taken over the course
of 16 years. Absorption changes strongly over the 5.6 d binary orbit, but also
shows a large scatter at a given orbital phase, especially at superior
conjunction. The orbital variability is most prominent when the black hole is
in the hard X-ray state. Our data are poorer for the intermediate and soft
state, but show signs for orbital variability of the absorption column in the
intermediate state. We quantitatively compare the data in the hard state to a
toy model of a focussed Castor-Abbott-Klein-wind: as it does not incorporate
clumping, the model does not describe the observations well. A qualitative
comparison to a simplified simulation of clumpy winds with spherical clumps
shows good agreement in the distribution of the equivalent hydrogen column
density for models with a porosity length on the order of the stellar radius at
inferior conjunction; we conjecture that the deviations between data and model
at superior conjunction could be either due to lack of a focussed wind
component in the model or a more complicated clump structure.Comment: proposed for acceptance in A&A, 11 pages, 11 figures (two in
appendix
emiT: an apparatus to test time reversal invariance in polarized neutron decay
We describe an apparatus used to measure the triple-correlation term (\D
\hat{\sigma}_n\cdot p_e\times p_\nu) in the beta-decay of polarized neutrons.
The \D-coefficient is sensitive to possible violations of time reversal
invariance. The detector has an octagonal symmetry that optimizes
electron-proton coincidence rates and reduces systematic effects. A beam of
longitudinally polarized cold neutrons passes through the detector chamber,
where a small fraction beta-decay. The final-state protons are accelerated and
focused onto arrays of cooled semiconductor diodes, while the coincident
electrons are detected using panels of plastic scintillator. Details regarding
the design and performance of the proton detectors, beta detectors and the
electronics used in the data collection system are presented. The neutron beam
characteristics, the spin-transport magnetic fields, and polarization
measurements are also described.Comment: 15 pages, 13 figure
Comparison of CDMS [100] and [111] oriented germanium detectors
The Cryogenic Dark Matter Search (CDMS) utilizes large mass, 3" diameter
1" thick target masses as particle detectors. The target is
instrumented with both phonon and ionization sensors and comparison of energy
in each channel provides event-by-event classification of electron and nuclear
recoils. Fiducial volume is determined by the ability to obtain good phonon and
ionization signal at a particular location. Due to electronic band structure in
germanium, electron mass is described by an anisotropic tensor with heavy mass
aligned along the symmetry axis defined by the [111] Miller index (L valley),
resulting in large lateral component to the transport. The spatial distribution
of electrons varies significantly for detectors which have their longitudinal
axis orientations described by either the [100] or [111] Miller indices.
Electric fields with large fringing component at high detector radius also
affect the spatial distribution of electrons and holes. Both effects are
studied in a 3 dimensional Monte Carlo and the impact on fiducial volume is
discussed.Comment: Low Temperature Detector 14 conference proceedings to be published in
the Journal of Low Temperature Physic
Validation of Phonon Physics in the CDMS Detector Monte Carlo
The SuperCDMS collaboration is a dark matter search effort aimed at detecting
the scattering of WIMP dark matter from nuclei in cryogenic germanium targets.
The CDMS Detector Monte Carlo (CDMS-DMC) is a simulation tool aimed at
achieving a deeper understanding of the performance of the SuperCDMS detectors
and aiding the dark matter search analysis. We present results from validation
of the phonon physics described in the CDMS-DMC and outline work towards
utilizing it in future WIMP search analyses.Comment: 6 Pages, 5 Figures, Proceedings of Low Temperature Detectors 14
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