53,703 research outputs found
Least squares DOA estimation with an informed phase unwrapping and full bandwidth robustness
The weighted least-squares (WLS) direction-of-arrival estimator that minimizes an error based on interchannel phase differences is both computationally simple and flexible. However, the approach has several limitations, including an inability to cope with spatial aliasing and a sensitivity to phase wrapping. The recently proposed phase wrapping robust (PWR)-WLS estimator addresses the latter of these issues, but requires solving a nonconvex optimization problem. In this contribution, we focus on both of the described shortcomings. First, a conceptually simpler alternative to PWR is presented that performs comparably given a good initial estimate. This newly proposed method relies on an unwrapping of the phase differences vector. Secondly, it is demonstrated that all microphone pairs can be utilized at all frequencies with both estimators. When incorporating information from other frequency bins, this permits a localization above the spatial aliasing frequency of the array. Experimental results show that a considerable performance improvement is possible, particularly for arrays with a large microphone spacing
Synchro-Betatron Stop-Bands due to a Single Crab Cavity
We analyze the stop-band due to crab cavities for horizontal tunes that are
either close to integers or close to half integers. The latter case is relevant
for today's electron/positron colliders. We compare this stop-band to that
created by dispersion in an accelerating cavity and show that a single typical
crab cavity creates larger stop-bands than a typical dispersion at an
accelerating cavity.
We furthermore analyze whether it is beneficial to place the crab cavity at a
position where the dispersion and its slope vanish. We find that this choice is
worth while if the horizontal tune is close to a half integer, but not if it is
close to an integer. Furthermore we find that stop-bands can be avoided when
the horizontal tune is located at a favorable side of the integer or the half
integer.
While we are here concerned with the installation of a single crab cavity in
a storage ring, we show that the stop-bands can be weakened, although not
eliminated, significantly when two crab cavities per ring are chosen suitably.Comment: 7 pages, 9 figure
Firehose and Mirror Instabilities in a Collisionless Shearing Plasma
Hybrid-kinetic numerical simulations of firehose and mirror instabilities in
a collisionless plasma are performed in which pressure anisotropy is driven as
the magnetic field is changed by a persistent linear shear . For a
decreasing field, it is found that mostly oblique firehose fluctuations grow at
ion Larmor scales and saturate with energies ; the pressure
anisotropy is pinned at the stability threshold by particle scattering off
microscale fluctuations. In contrast, nonlinear mirror fluctuations are large
compared to the ion Larmor scale and grow secularly in time; marginality is
maintained by an increasing population of resonant particles trapped in
magnetic mirrors. After one shear time, saturated order-unity magnetic mirrors
are formed and particles scatter off their sharp edges. Both instabilities
drive sub-ion-Larmor--scale fluctuations, which appear to be
kinetic-Alfv\'{e}n-wave turbulence. Our results impact theories of momentum and
heat transport in astrophysical and space plasmas, in which the stretching of a
magnetic field by shear is a generic process.Comment: 5 pages, 8 figures, accepted for publication in Physical Review
Letter
Simple quantum feedback of a solid-state qubit
We propose an experiment on quantum feedback control of a solid-state qubit,
which is almost within the reach of the present-day technology. Similar to the
earlier proposal, the feedback loop is used to maintain the coherent (Rabi)
oscillations in a qubit for an arbitrary long time; however, this is done in a
significantly simpler way, which requires much smaller bandwidth of the control
circuitry. The main idea is to use the quadrature components of the noisy
detector current to monitor approximately the phase of qubit oscillations.
The price for simplicity is a less-than-ideal operation: the fidelity is
limited by about 95%. The feedback loop operation can be experimentally
verified by appearance of a positive in-phase component of the detector current
relative to an external oscillating signal used for synchronization.Comment: 5 page
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Molecular dynamics study of oxygen diffusion in Pr<sub>2</sub>NiO<sub>4+δ</sub>
Oxygen transport in tetragonal Pr2NiO4+δ has been investigated using molecular dynamics simulations in conjunction with a set of Born model potentials. Oxygen diffusion in Pr2NiO4+δ is highly anisotropic, occurring almost entirely via an interstitialcy mechanism in the a-b plane. The calculated oxygen diffusivity has a weak dependence upon the concentration of oxygen interstitials, in agreement with experimental observations. In the temperature range 800-1500 K, the activation energy for migration varied between 0.49 and 0.64 eV depending upon the degree of hyperstoichiometry. The present results are compared to previous work on oxygen self-diffusion in related K2NiF4 structure materials
Magneto-immutable turbulence in weakly collisional plasmas
We propose that pressure anisotropy causes weakly collisional turbulent
plasmas to self-organize so as to resist changes in magnetic-field strength. We
term this effect "magneto-immutability" by analogy with incompressibility
(resistance to changes in pressure). The effect is important when the pressure
anisotropy becomes comparable to the magnetic pressure, suggesting that in
collisionless, weakly magnetized (high-) plasmas its dynamical relevance
is similar to that of incompressibility. Simulations of magnetized turbulence
using the weakly collisional Braginskii model show that magneto-immutable
turbulence is surprisingly similar, in most statistical measures, to critically
balanced MHD turbulence. However, in order to minimize magnetic-field
variation, the flow direction becomes more constrained than in MHD, and the
turbulence is more strongly dominated by magnetic energy (a nonzero "residual
energy"). These effects represent key differences between pressure-anisotropic
and fluid turbulence, and should be observable in the turbulent
solar wind.Comment: Accepted for publication in J. Plasma Phy
Sensing Noncollinear Magnetism at the Atomic Scale Combining Magnetic Exchange and Spin-Polarized Imaging
Storing and accessing information in atomic-scale magnets requires magnetic
imaging techniques with single-atom resolution. Here, we show simultaneous
detection of the spin-polarization and exchange force, with or without the flow
of current, with a new method, which combines scanning tunneling microscopy and
non-contact atomic force microscopy. To demonstrate the application of this new
method, we characterize the prototypical nano-skyrmion lattice formed on a
monolayer of Fe/Ir(111). We resolve the square magnetic lattice by employing
magnetic exchange force microscopy, demonstrating its applicability to
non-collinear magnetic structures, for the first time. Utilizing
distance-dependent force and current spectroscopy, we quantify the exchange
forces in comparison to the spin-polarization. For strongly spin-polarized
tips, we distinguish different signs of the exchange force which we suggest
arises from a change in exchange mechanisms between the probe and a skyrmion.
This new approach may enable both non-perturbative readout combined with
writing by current-driven reversal of atomic-scale magnets
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