652 research outputs found
Efficient Raw Signal Generation Based on Equivalent Scatterer and Subaperture Processing for SAR with Arbitrary Motion
An efficient SAR raw signal generation method based on equivalent scatterer and subaperture processing is proposed in this paper. It considers the radarās motion track, which can obtain the precise raw signal for the real SAR. First, the imaging geometry with arbitrary motion is established, and then the scene is divided into several equidistant rings. Based on the equivalent scatterer model, the approximate expression of the SAR system transfer function is derived, thus each pulseās raw signal can be generated by the convolution of the transmitted signal and system transfer function, performed by the fast Fourier transform (FFT). To further improve the simulation efficiency, the subaperture and polar subscene processing is used. The system transfer function of pluses for the same subaperture is calculated simultaneously by the weighted sum of all subscenesā equivalent backscattering coefficient in the same equidistant ring, performed by the nonuniform FFT (NUFFT). The method only involves the FFT, NUFFT and complex multiplication operations, which means the easier implementation and higher efficiency. Simulation results are given to prove the validity of this method
Crystal Structure, Infrared Spectra, and Microwave Dielectric Properties of Temperature-Stable Zircon-Type (Y,Bi)VO<inf>4</inf> Solid-Solution Ceramics
A series of (Bi 1-x Y x )VO 4 (0.4 ā¤ x ā¤ 1.0) ceramics were synthesized using the traditional solid-state reaction method. In the composition range of 0.4 ā¤ x ā¤ 1.0, a zircon-type solid solution was formed between 900 and 1550 Ā°C. Combined with our previous work (scheelite monoclinic and zircon-type phases coexist in the range of x < 0.40), a pseudobinary phase diagram of BiVO 4 -YVO 4 is presented. As x decreased from 1.0 to 0.40, the microwave permittivity (Ļµ r ) of (Bi 1-x Y x )VO 4 ceramics increased linearly from 11.03 to 30.9, coincident with an increase in the temperature coefficient of resonant frequency (TCF) from -61.3 to +103 ppm/Ā°C. Excellent microwave dielectric properties were obtained for (Bi 0.3 Y 0.7 )VO 4 sintered at 1025 Ā°C and (Bi 0.2 Y 0.8 )VO 4 sintered at 1075 Ā°C with Ļµ r ā¼ 19.35, microwave quality factor (Qf) ā¼ 25 760 GHz, and TCF ā¼ +17.8 ppm/Ā°C and Ļµ r ā¼ 16.3, Qf ā¼ 31 100 GHz, and TCF ā¼ -11.9 ppm/Ā°C, respectively. Raman spectra, Shannon's additive rule, a classical oscillator model, and far-infrared spectra were employed to study the structure-property relations in detail. All evidence supported the premise that Bi-based vibrations dominate the dielectric permittivity in the microwave region
High Quality Factor, Ultralow Sintering Temperature Li6B4O9 Microwave Dielectric Ceramics with Ultralow Density for Antenna Substrates
Dense Li6B4O9microwave dielectric ceramics were synthesized at low temperature via solid-state reaction using Li2CO3and LiBO2. Optimum permittivity ā¼ 5.95, quality factor ā¼ 41 800 GHz and temperature coefficient of resonant frequency ā¼ - 72 ppm/Ā°C were obtained in ceramics sintered at 640 Ā°C with a ultrasmall bulk density ā¼2.003 g/cm3(ā¼95% relative density, the smallest among all the reported microwave dielectric ceramics). Li6B4O9ceramics were shown to be chemically compatible with silver electrodes but reacted with aluminum forming Li3AlB2O6and Li2AlBO4secondary phases. A prototype patch antenna was fabricated by tape casting and screen printing. The antenna resonated at 4.255 GHz with a bandwidth ā¼279 MHz at -10 dB transmission loss (S11) in agreement with simulated results. The Li6B4O9microwave dielectric ceramic possesses similar microwave dielectric properties to the commercial materials but much lower density and could be a good candidate for both antenna substrate and low-temperature cofired ceramics technology
Structureāproperty relationships of low sintering temperature scheelite-structured (1 ā x)BiVO 4 āxLaNbO 4 microwave dielectric ceramics
A series of (1 ā x)BiVO4āxLaNbO4 (0.0 ā¤ x ā¤ 1.0) ceramics were prepared via a solid state reaction method. A scheelite-structured solid solution was formed for x ā¤ 0.5 but for x > 0.5, tetragonal scheelite, monoclinic LaNbO4-type and La1/3NbO3 phases co-existed. As x increased from 0 to 0.1, the room temperature crystal structure gradually changed from monoclinic to tetragonal scheelite, associated with a decrease in the ferroelastic phase transition temperature from 255 Ā°C (BiVO4) to room temperature or even below. High sintering temperatures were also found to accelerate this phase transition for compositions with x ā¤ 0.08. Temperature independent high quality factor Qf >10 000 GHz in a wide temperature range 25ā140 Ā°C and high microwave permittivity Īµr ā¼76.3 Ā± 0.5 was obtained for the x = 0.06 ceramic sintered at 800 Ā°C. However, small changes in composition resulted in a change in the sign and magnitude of the temperature coefficient of resonant frequency (TCF) due to the proximity of the ferroelastic transition to room temperature. If TCF can be controlled and tuned through zero, then (1 ā x)BiVO4āxLaNbO4 (0.0 ā¤ x ā¤ 1.0) is a strong candidate for microwave device applications
A polarized beam splitter using an anisotropic medium slab
The propagation of electromagnetic waves in the anisotropic medium with a
single-sheeted hyperboloid dispersion relation is investigated. It is found
that in such an anisotropic medium E- and H-polarized waves have the same
dispersion relation, while E- and H-polarized waves exhibit opposite amphoteric
refraction characteristics. E- (or H-) polarized waves are positively refracted
whereas H- (or E-) polarized waves are negatively refracted at the interface
associated with the anisotropic medium. By suitably using the properties of
anomalous refraction in the anisotropic medium it is possible to realize a very
simple and very efficient beam splitter to route the light. It is shown that
the splitting angle and the splitting distance between E- and H- polarized beam
is the function of anisotropic parameters, incident angle and slab thickness.Comment: 14 pages, 6 figure
Cosmic ray tests of the D0 preshower detector
The D0 preshower detector consists of scintillator strips with embedded
wavelength-shifting fibers, and a readout using Visible Light Photon Counters.
The response to minimum ionizing particles has been tested with cosmic ray
muons. We report results on the gain calibration and light-yield distributions.
The spatial resolution is investigated taking into account the light sharing
between strips, the effects of multiple scattering and various systematic
uncertainties. The detection efficiency and noise contamination are also
investigated.Comment: 27 pages, 24 figures, submitted to NIM
The effect of grain-domain-size on levitation force of melt growth processing YBCO bulk superconductors
Collimation of high current fast electrons in dense plasmas with a tightly focused precursor intense laser pulse
High-current fast electrons at the mega-ampere level provide a unique way to generate high-energy density states of matter, which are related to many applications. However, the large divergence angle of fast electrons typically over 50 degrees is a significant disadvantage. The guiding effect of the self-generated azimuthal magnetic fields on fast electron current is found to be very limited due to the cone-shaped spatial structure of the fields. In this work, we present a new understanding of the collimation conditions of fast electrons under such a magnetic field structure. It is shown that the transverse peak position of the magnetic field layer plays a more crucial role in collimating the fast electrons than its magnitude. Based upon this, a new two-pulse collimating scheme is proposed, where a guiding precursor pulse is adopted to form proper azimuthal magnetic fields and the main pulse is for fast electron generation as usual. The present scheme can be implemented relatively easily with the precursor lasers at the 10 TW level with a duration of 200 femtoseconds, with which the divergence angle of fast electrons driven by the main pulse can be confined within a few degrees. Practical applications of our scheme can be found in high-energy density science
Crystal structure, impedance and broadband dielectric spectra of ordered scheelite-structured Bi(Sc1/3Mo2/3)O4 ceramic
Bi(Sc 1/3 Mo 2/3 )O 4 ceramics were prepared via solid state reaction method. It crystallized with an ordered scheelite-related structure (a = 16.9821(9) Ć
, b = 11.6097(3) Ć
, c = 5.3099(3) Ć
and Ī² = 104.649(2)Ā°) with a space group C12/C1, in which Bi 3+ , Sc 3+ and Mo 6+ are -8, -6 and -4 coordinated, respectively. Bi(Sc 1/3 Mo 2/3 )O 4 ceramics were densifiedat 915 Ā°C, giving a permittivity (Īµ r ) 24.4, quality factor (Qf, Q = 1/dielectric loss, f = resonant frequency) ~48, 100 GHz and temperature coefficient of resonant frequency (TCF) -68 ppm/Ā°C. Impedance spectroscopy revealed that there was only a bulk response for conductivity with activation energy (E a ) ~0.97 eV, suggesting the compound is electrically and chemically homogeneous. Wide band dielectric spectra were employed to study the dielectric response of Bi(Sc 1/3 Mo 2/3 )O 4 from 20 Hz to 30 THz. Īµ r was stable from 20 Hz to the GHz region, in which only ionic and electron displacive polarization contributed to the Īµ r
Directed fast electron beams in ultraintense picosecond laser irradiated solid targets
We report on fast electron transport and emission patterns from solid targets irradiated by s-polarized, relativistically intense, picosecond laser pulses. A beam of multi-MeV electrons is found to be transported along the target surface in the laser polarization direction. The spatial-intensity and energy distributions of this beam are compared with the beam produced along the laser propagation axis. It is shown that even for peak laser intensities an order of magnitude higher than the relativistic threshold; laser polarization still plays an important role in electron energy transport. Results from 3D particle-in-cell simulations confirm the findings. The characterization of directional beam emission is important for applications requiring efficient energy transfer, including secondary photon and ion source development
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