Wenbin Lin

Nonthermal production of weakly interacting massive particles and the subgalactic structure of the universe

There is increasing evidence that conventional cold dark matter (CDM) models lead to conflicts between observations and numerical simulations of dark matter halos on sub-galactic scales. Spergel and Steinhardt showed that if the CDM is strongly self-interacting, then the conflicts disappear. However, the assumption of strong self-interaction would rule out the favored candidates for CDM, namely weakly interacting massive particles (WIMPs), such as the neutralino. In this paper we propose a mechanism of non-thermal production of WIMPs and study its implications on the power spectrum. We find that the non-vanishing velocity of the WIMPs suppresses the power spectrum on small scales compared to what it obtained in the conventional CDM model. Our results show that, in this context, WIMPs as candidates for dark matter can work well both on large scales and on sub-galactic scales.

DESIGN OF AN ULTRA-WIDEBAND POWER DIVIDER VIA THE COARSE-GRAINED PARALLEL MICRO- GENETIC ALGORITHM

An ultra-wideband (UWB) power divider is designed in this paper. The UWB performance of this power divider is obtained by using a tapered microstrip line that consists of exponential and elliptic sections. The coarse grained parallel micro-genetic algorithm (PMGA) and CST Microwave Studio are combined to achieve an automated parallel design process. The method is applied to optimize the UWB power divider. The optimized power divider is fabricated and measured. The measured results show relatively low insertion loss, good return loss, and high isolation between the output ports across the whole UWB (3.1{10.6GHz).

Compressive sensing for multi-static scattering analysis

Compressive sensing (CS) is a framework in which one attempts to measure a signal in a compressive mode, implying that fewer total measurements are required vis a vis direct sampling methods. Compressive sensing exploits the fact that the signal of interest is compressible in some basis, and the CS measurements correspond to projections (typically random projections) performed on the basis function coefficients. In this paper, we demonstrate that ideas from compressive sensing may be exploited in the context of electromagnetic modeling, here multi-static scattering from an arbitrary target. In this context, the computational analysis may be viewed as a numerical experiment, and ideas from compressive sensing may be used to reduce the number of computations required for target characterization. It is demonstrated that the compressive sensing framework may be applied with relatively minor modifications to many existing numerical models, with examples presented here for a fast-multipole computational engine.

Numerical studies of the high power microwave breakdown in gas using the fluid model with a modified electron energy distribution function

A modified electron energy distribution function (EEDF) is introduced into the fluid model and its effects on the high power microwave (HPM) breakdown in air and argon are investigated. A proper numerical scheme for the finite-difference time-domain method is employed to solve the fluid model. Numerical simulations show that the HPM breakdown time in argon predicted by the fluid model with the modified EEDF agrees well with the results of Particle-in-cell-Monte Carlo collision simulations, while the Maxwellian EEDF results in faster HPM breakdown when the mean electron energy is less than 20 eV. We also confirm that the Maxwellian EEDF can be used in the fluid model for simulating the air breakdown at the low frequencies based on the reported experiments.

Pseudospectral method based on prolate spheroidal wave functions for semiconductor nanodevice simulation

We solve Schrodingers equation for semiconductor nanodevices by applying prolate spheroidal wave functions of order zero as basis functions in the pseudospectral method. When the functions involved in the problem are bandlimited, the prolate pseudospectral method outperforms the conventional pseudospectral methods based on trigonometric and orthogonal polynomials and related functions, asymptotically achieving similar accuracy using a factor of π/2 less unknowns than the latter. The prolate pseudospectral method also employs a more uniform spatial grid, achieving better resolution near the center of the domain.

A Hybrid Method Based on Differential Evolution and Continuous Ant Colony Optimization and its Application on Wideband Antenna Design

An evolutionary learning algorithm based on difierential evolution strategy (DES) and continuous ant colony optimization (CACO) for wideband antenna design is proposed. The advantages of this hybrid method are demonstrated with several mathematical functions and a linear array pattern synthesis. This method is applied to design an E-shaped wideband patch antenna, which achieves the impedance bandwidth 4:8 » 6:53GHz. We compare the hybrid method with the traditional DES and CACO optimization algorithms, and the advantage of this hybrid method over the DES and the CACO is also demonstrated.

Pseudospectral method based on prolate spheroidal wave functions for frequency-domain electromagnetic simulations

We apply prolate spheroidal wave functions of order zero as basis functions in the pseudospectral method for frequency-domain electromagnetic simulation problems. Like the traditional pseudospectral frequency-domain (PSFD) methods based on Chebyshev and Legendre polynomial series, the prolate PSFD method yields exponential order of accuracy. In terms of the number of samples utilized per wavelength, the prolate expansion is superior to the Chebyshev and Legendre polynomial series by a factor of /spl pi//2. In addition, the prolate PSFD method employs a more uniform spatial grid, achieving better resolution near the center of the domain.

A Compact MIMO Antenna System Design with Low Correlation from 1710 MHz to 2690 MHz

A compact and low-correlation multiple input multiple output antenna system covering 1710{2690MHz band for wireless communication standards is proposed. It comprises two identical elements with coupled feeding plate and radiating strip, and each element has a volume of 24:5 £ 15 £ 1:2mm 3 . Simulated and measured results show that it has good potentials for high-band-only mobile phone. 45% bandwidth (based on S11 < i6dB), i12dB isolation, over 49% e-ciency and less than 0.15 correlation coe-cient are achieved in the frequency ranging from 1710MHz to 2690MHz. Several key parameters are also discussed in this study to better understand the antenna principles.

Adiabatic gravitational perturbation during reheating

We study the possibilities of parametric amplification of the gravitational perturbation during reheating in single-held inflation models. Our result shows that there is no additional growth of the super-horizon modes beyond the usual predictions.

Rapid Prolate Pseudospectral Differentiation and Interpolation with the Fast Multipole Method

Pseudospectral methods utilizing prolate spheroidal wave functions as basis functions have been shown to possess advantages over the conventional pseudospectral methods based on trigonometric and orthogonal polynomials. However, the spectral differentiation and interpolation steps of the prolate pseudospectral method involve matrix-vector products, which, if evaluated directly, entail O(N2) memory requirement and computational complexity (where N is the number of unknowns utilized for discretization and interpolation). In this work we show that the fast multipole method (FMM) can be used to reduce the memory requirement and computational complexity of the prolate pseudospectral method to O(N). Example simulation results demonstrate the speed and accuracy of the resulting fast prolate pseudospectral solver.