1,158 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
On the semi-classical analysis of the groundstate energy of the Dirichlet Pauli operator III: Magnetic fields that change sign
We consider the semi-classical Dirichlet Pauli operator in bounded connected
domains in the plane. Rather optimal results have been obtained in previous
papers by Ekholm-Kova\v{r}\'ik-Portmann and Helffer-Sundqvist for the
asymptotics of the ground state energy in the semi-classical limit when the
magnetic field has constant sign. In this paper, we focus on the case when the
magnetic field changes sign. We show, in particular, that the ground state
energy of this Pauli operator will be exponentially small as the semi-classical
parameter tends to zero and give lower bounds and upper bounds for this decay
rate. Concrete examples of magnetic fields changing sign on the unit disc are
discussed. Various natural conjectures are disproved and this leaves the
research of an optimal result in the general case still open.Comment: 20 pages, 8 figure
2D wind clumping in hot, massive stars from hydrodynamical line-driven instability simulations using a pseudo-planar approach
Context: Clumping in the radiation-driven winds of hot, massive stars arises
naturally due to the strong, intrinsic instability of line-driving (the `LDI').
But LDI wind models have so far mostly been limited to 1D, mainly because of
severe computational challenges regarding calculation of the multi-dimensional
radiation force. Aims: To simulate and examine the dynamics and
multi-dimensional nature of wind structure resulting from the LDI. Methods: We
introduce a `pseudo-planar', `box-in-a-wind' method that allows us to
efficiently compute the line-force in the radial and lateral directions, and
then use this approach to carry out 2D radiation-hydrodynamical simulations of
the time-dependent wind. Results: Our 2D simulations show that the LDI first
manifests itself by mimicking the typical shell-structure seen in 1D models,
but how these shells then quickly break up into complex 2D density and velocity
structures, characterized by small-scale density `clumps' embedded in larger
regions of fast and rarefied gas. Key results of the simulations are that
density-variations in the well-developed wind statistically are quite isotropic
and that characteristic length-scales are small; a typical clump size is ~0.01R
at 2R, thus resulting also in rather low typical clump-masses ~10^17 g.
Overall, our results agree well with the theoretical expectation that the
characteristic scale for LDI-generated wind-structure is of order the Sobolev
length. We further confirm some earlier results that lateral `filling-in' of
radially compressed gas leads to somewhat lower clumping factors in 2D
simulations than in comparable 1D models. We conclude by discussing an
extension of our method toward rotating LDI wind models that exhibit an
intriguing combination of large- and small-scale structure extending down to
the wind base.Comment: 9 pages, 7 figures + 1 Appendix with 1 figure. Recommended for
publication in A&
Monitoring the Safe Disposal of Radioactive Waste: a Combined Technical and Socio-Political Activity
Mass loss from inhomogeneous hot star winds III. An effective-opacity formalism for line radiative transfer in accelerating, clumped two-component media, and first results on theory and diagnostics
[Abridged] We develop and benchmark a fast and easy-to-use effective-opacity
formalism for line and continuum radiative transfer in an accelerating
two-component clumpy medium. The formalism bridges the limits of optically thin
and thick clumps, and is here used to i) design a simple vorosity-modified
Sobolev with exact integration (vmSEI) method for analyzing UV wind resonance
lines in hot, massive stars, and ii) derive simple correction factors to the
line force driving the outflows of such stars. We show that (for a given
ionization factor) UV resonance doublets may be used to analytically predict
the upward corrections in empirically inferred mass-loss rates associated with
porosity in velocity space (a.k.a. velocity-porosity, or vorosity), but that
severe solution degeneracies exist. For an inter-clump density set to 1 % of
the mean density, we for O and B supergiants derive upward empirical mass-loss
corrections of typically factors of either ~5 or ~50, depending on which of the
two applicable solutions is chosen. Overall, our results indicate this solution
dichotomy severely limits the use of UV resonance lines as direct mass-loss
indicators of clumped hot stellar winds. We next apply the effective-opacity
formalism to the standard CAK theory of line-driven winds. By analytic and
numerical hydrodynamics calculations, we show that in cases where vorosity is
important at the critical point setting the mass-loss rate, the reduced
line-force leads to a lower theoretical mass loss, by a factor scaling with the
normalized velocity filling factor fvel. On the other hand, if vorosity is
important only above this critical point, the predicted mass loss is not
affected, but the wind terminal speed is reduced. This shows that porosity in
velocity space can have a significant impact not only on the diagnostics, but
also on the dynamics and theory of radiatively driven winds.Comment: 13 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
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