159,218 research outputs found
Pipelined digital SAR azimuth correlator using hybrid FFT-transversal filter
A synthetic aperture radar system (SAR) having a range correlator is provided with a hybrid azimuth correlator which utilizes a block-pipe-lined fast Fourier transform (FFT). The correlator has a predetermined FFT transform size with delay elements for delaying SAR range correlated data so as to embed in the Fourier transform operation a corner-turning function as the range correlated SAR data is converted from the time domain to a frequency domain. The azimuth correlator is comprised of a transversal filter to receive the SAR data in the frequency domain, a generator for range migration compensation and azimuth reference functions, and an azimuth reference multiplier for correlation of the SAR data. Following the transversal filter is a block-pipelined inverse FFT used to restore azimuth correlated data in the frequency domain to the time domain for imaging
Effect of Dzyaloshinskii Moriya interaction on magnetic vortex
The effect of the Dzyaloshinskii Moriya interaction on the vortex in magnetic
microdisk was investigated by micro magnetic simulation based on the Landau
Lifshitz Gilbert equation. Our results show that the DM interaction modifies
the size of the vortex core, and also induces an out of plane magnetization
component at the edge and inside the disk. The DM interaction can destabilizes
one vortex handedness, generate a bias field to the vortex core and couple the
vortex polarity and chirality. This DM-interaction-induced coupling can
therefore provide a new way to control vortex polarity and chirality
Direct and secondary nuclear excitation with x-ray free-electron lasers
The direct and secondary nuclear excitation produced by an x-ray free
electron laser when interacting with a solid-state nuclear target is
investigated theoretically. When driven at the resonance energy, the x-ray free
electron laser can produce direct photoexcitation. However, the dominant
process in that interaction is the photoelectric effect producing a cold and
very dense plasma in which also secondary processes such as nuclear excitation
by electron capture may occur. We develop a realistic theoretical model to
quantify the temporal dynamics of the plasma and the magnitude of the secondary
excitation therein. Numerical results show that depending on the nuclear
transition energy and the temperature and charge states reached in the plasma,
secondary nuclear excitation by electron capture may dominate the direct
photoexcitation by several orders of magnitude, as it is the case for the 4.8
keV transition from the isomeric state of Mo, or it can be negligible,
as it is the case for the 14.4 keV M\"ossbauer transition in
. These findings are most relevant for future nuclear quantum
optics experiments at x-ray free electron laser facilities.Comment: 17 pages, 7 figures; minor corrections made; accepted by Physics of
Plasma
Symmetry-preserving Loop Regularization and Renormalization of QFTs
A new symmetry-preserving loop regularization method proposed in \cite{ylw}
is further investigated. It is found that its prescription can be understood by
introducing a regulating distribution function to the proper-time formalism of
irreducible loop integrals. The method simulates in many interesting features
to the momentum cutoff, Pauli-Villars and dimensional regularization. The loop
regularization method is also simple and general for the practical calculations
to higher loop graphs and can be applied to both underlying and effective
quantum field theories including gauge, chiral, supersymmetric and
gravitational ones as the new method does not modify either the lagrangian
formalism or the space-time dimension of original theory. The appearance of
characteristic energy scale and sliding energy scale offers a
systematic way for studying the renormalization-group evolution of gauge
theories in the spirit of Wilson-Kadanoff and for exploring important effects
of higher dimensional interaction terms in the infrared regime.Comment: 13 pages, Revtex, extended modified version, more references adde
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