56 research outputs found
Free electron emission in vacuum assisted by photonic time crystals
The Cerenkov radiation and the Smith-Purcell effect state that free electron
emission occurs exclusively in dielectrics when the velocity of the particles
exceeds the speed of light in the medium or in the vicinity of periodic
gratings close to each other within a vacuum. We demonstrate that free
electrons in a vacuum can also emit highly directional monochromatic waves when
they are in close proximity to a medium that is periodically modulated
temporally, suggesting the existence of temporal Smith-Purcell effect. The
momentum band gaps of time-varying media, such as photonic time crystals
(PTCs), create new pathways for the injection of external energy, allowing the
frequency, intensity, and spatial distribution of the electromagnetic fields to
be controlled. Moreover, the PTC substrate enables the conversion of localized
evanescent fields into amplified, highly directional propagating plane waves
that are only sensitive to the velocity of particles and the modulation
frequency, which allows us to observe and utilize Cerenkov-like radiation in
free space. Our work exhibits significant opportunities for the utilization of
time-varying structures in various fields, including particle identification,
ultraweak signal detection, and improved radiation source design
Research on the Influence of Switching Frequency on Low-Frequency Oscillation in the Voltage-Controlled Buck-Boost Converter
The influence of switching frequency on the low-frequency oscillation in the voltage-controlled buck-boost converter is studied in this paper. Firstly, the mathematical model of this system is derived. And then, a glimpse at the influence of switching frequency on the low-frequency oscillation in this system by MATLAB/Simulink is given. The improved averaged model of the system is established, and the corresponding theoretical analysis is presented. It is found that the switching frequency has an important influence on the low-frequency oscillation in the system, that is, the low-frequency oscillation is easy to occur when the switching frequency is low. Finally, the effectiveness of the improved averaged model and the theoretical analysis are confirmed by circuit experiment
Analysis and design of transition radiation in layered uniaxial crystals using Tandem neural networks
With the flourishing development of nanophotonics, Cherenkov radiation
pattern can be designed to achieve superior performance in particle detection
by fine-tuning the properties of metamaterials such as photonic crystals (PCs)
surrounding the swift particle. However, the radiation pattern can be sensitive
to the geometry and material properties of PCs, such as periodicity, unit
thickness, and dielectric fraction, making direct analysis and inverse design
difficult. In this article, we propose a systematic method to analyze and
design PC-based transition radiation, which is assisted by deep learning neural
networks. By matching boundary conditions at the interfaces, Cherenkov-like
radiation of multilayered structures can be resolved analytically using the
cascading scattering matrix method, despite the optical axes not being aligned
with the swift electron trajectory. Once well trained, forward deep learning
neural networks can be utilized to predict the radiation pattern without
further direct electromagnetic simulations; moreover, Tandem neural networks
have been proposed to inversely design the geometry and/or material properties
for desired Cherenkov radiation pattern. Our proposal demonstrates a promising
strategy for dealing with layered-medium-based Cherenkov radiation detectors,
and it can be extended for other emerging metamaterials, such as photonic time
crystals
Average and Small Signal Modeling of Negative-Output KY Boost Converter in CCM Operation
Negative-output KY Boost converter, which can obtain the negative output voltage and could be driven easily, is a good topology to overcome traditional Boost and Buck-Boost converters and it is believed that this converter will be widely used in engineering applications in the future. In this study, by using the averaging method and geometrical technique, the average and small signal model of the negative-output KY Boost converter are established. The DC equilibrium point and transfer functions of the system are derived and analyzed. Finally, the effectiveness of the established model and the correctness of the theoretical analysis are confirmed by the circuit experiment
Space-Time Fresnel Prism
Space-time modulation-based metamaterials have recently spurred considerable
interest, owing to the fundamental addition of the time dimension to the medium
parameters, and resulting novel properties and potential applications. However,
the implementation of most related structures -- e.g., involving step, slab or
gradient discontinuities -- has been hindered by the impossible requirement of
infinitely or prohibitively large device sizes. We provide here a solution to
this issue, consisting in a space-time transposition of the conventional
Fresnel prism, whereby a copy of the target modulation is periodically
re-injected at the input of a Fresnel-reduced finite structure, so as to
provide the same anharmonic and nonreciprocal frequency conversion as the
target space-time interface in the case of a modulation step. This concept,
which may readily extend to slab or gradient modulations, as well as
accelerated profiles for space-time chirping operations, may pave the way for
the practical development of a wide range of novel microwave and optical
space-time systems
Oscillatory Stability and Eigenvalue Sensitivity Analysis of A DFIG Wind Turbine System
2010-2011 > Academic research: refereed > Publication in refereed journa
Wireless Real-Time Capacitance Readout Based on Perturbed Nonlinear Parity-Time Symmetry
In this article, we report a vector-network-analyzer-free and real-time LC
wireless capacitance readout system based on perturbed nonlinear parity-time
(PT) symmetry. The system is composed of two inductively coupled reader-sensor
parallel RLC resonators with gain and loss respectively. By searching for the
real mode that requires the minimum saturation gain, the steady-state frequency
evolution as a function of the sensor capacitance perturbation is analytically
deduced. The proposed system can work in different modes by setting different
perturbation point. In particular, at the exceptional point of PT symmetry, the
system exhibits high sensitivity. Experimental demonstrations revealed the
viability of the proposed readout mechanism by measuring the steady-state
frequency of the reader resonator in response to the change of trimmer
capacitor on the sensor side. Our findings could impact many emerging
applications such as implantable medical device for health monitoring,
parameter detection in harsh environment and sealed food packages, etc
HOPF BIFURCATION AS AN INTERMEDIATE-SCALE INSTABILITY IN SINGLE-STAGE POWER-FACTOR-CORRECTION POWER SUPPLIES: ANALYSIS, SIMULATIONS AND EXPERIMENTAL VERIFICATION
NSGA-II Algorithm with a Local Search Strategy for Multiobjective Optimal Design of Dry-Type Air-Core Reactor
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