Improving millimeter-wave imaging quality using the vortex phase method

This paper investigates a new vortex wave imaging approach to improve the imaging quality of small metal targets of size less than 1.5 mm.Antennas with different spiral phase plates are designed to efficiently transmit vortex beams with orbital angular momentums(OAMs).By analyzing the OAM spectrum of the target,it was discovered that the predominant reflection contains a particular OAM mode that carries abundant azimuthal information.This can be explained by the OAM selectivity of the target and the guidance of the vortex transmitting beam.A simple reflection vortex imaging system was designed to capture the phase information.Measurement results show that the high image contrast reaches 14.9%,which is twice as high as that of the imaging without OAM.Both of simulations and experiments demonstrate that the vortex phase imaging approach proposed in this paper can effectively improve the imaging quality at 80 GHz.This approach is suitable for other millimeter wave imaging systems and is helpful to improve the resolution in anti-terrorism security checks.

Study of the influence of discharge loop parameters on anode side on generation characteristics of metal plasma jet in a pulsed vacuum discharge

In a pulsed vacuum discharge,the ejection performance of a metal plasma jet can be effectively improved by preventing charged particles from moving to the anode.In this paper,the effects of resistance and capacitance on the anode side on the discharge characteristics and the generation characteristics of plasma jet are investigated.Results show that the existence of a resistor on the anode side can increase the anode potential,thereby preventing charged particles from entering the anode and promoting the ejection of charged particles along the axis of the insulating sleeve nozzle.The application of a capacitor on the anode side can not only absorb electrons at the initial stage of discharge,increasing the peak value of the cathode hump potential,but also prevent charged particles from moving to the anode,thereby improving the ejection performance of the plasma jet.In addition,the use of a larger resistance and a smaller capacitance can improve the blocking effect on charged particles and further improve the ejection performance of the plasma jet.Results of this study will provide a reference for the improvement of the ejection performance of plasma jets and their applications.

THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves

We propose a novel scheme for THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves(HSWs).The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity(RC)where low-order Stokes waves(LSWs)are converted to high-order Stokes waves again and again.The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation(OCDFG)where the poling periods of the optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally.Combined with the repeated and continuous frequency conversions,the optical-to-THz energy conversion efficiency(OTECE)exceeds 26%at 300 K and 43%at 100 K with pump intensities of 300 MW/cm^(2).

High-average-power, high-repetition-rate tunable terahertz difference frequency generation with GaSe crystal pumped by 2 μm dual-wavelength intracavity KTP optical parametric oscillator

We have demonstrated a high-average-power,high-repetition-rate optical terahertz(THz)source based on difference frequency generation(DFG)in the GaSe crystal by using a near-degenerate 2μm intracavity KTP optical parametric oscillator as the pump source.The power of the 2μm dual-wavelength laser was up to 12.33 W with continuous tuning ranges of 1988.0–2196.2 nm/2278.4–2065.6 nm for two waves.Different GaSe cystal lengths have been experimentally investigated for the DFG THz source in order to optimize the THz output power,which was in good agreement with the theoretical analysis.Based on an 8 mm long GaSe crystal,the THz wave was continuously tuned from 0.21 to 3 THz.The maximum THz average power of 1.66μW was obtained at repetition rate of 10 kHz under 1.48 THz.The single pulse energy amounted to 166 pJ and the conversion efficiency from 2 μm laser to THz output was 1.68×10^(-6).The signal-to-noise ratio of the detected THz voltage was 23 dB.The acceptance angle of DFG in the GaSe crystal was measured to be 0.16°.

Multi-stage Co-planning Model for Power Distribution System and Hydrogen Energy System Under Uncertainties

The increased deployment of electricity-based hydrogen production strengthens the coupling of power distribution system(PDS)and hydrogen energy system(HES).Considering that power to hydrogen(PtH)has great potential to facilitate the usage of renewable energy sources(RESs),the coordination of PDS and HES is important for planning purposes under high RES penetration.To this end,this paper proposes a multi-stage co-planning model for the PDS and HES.For the PDS,investment decisions on network assets and RES are optimized to supply the growing electric load and PtHs.For the HES,capacities of PtHs and hydrogen storages(HSs)are optimally determined to satisfy hydrogen load considering the hydrogen production,tube trailer transportation,and storage constraints.The overall planning problem is formulated as a multistage stochastic optimization model,in which the investment decisions are sequentially made as the uncertainties of electric and hydrogen load growth states are revealed gradually over periods.Case studies validate that the proposed co-planning model can reduce the total planning cost,promote RES consumption,and obtain more flexible decisions under long-term load growth uncertainties.

Optimal Micro-PMU Placement for Improving State Estimation Accuracy via Mixed-integer Semidefinite Programming

Micro-phasor measurement units(μPMUs)with a micro-second resolution and milli-degree accuracy capability are expected to play an important role in improving the state estimation accuracy in the distribution network with increasing penetration of distributed generations.Therefore,this paper investigates the problem of how to place a limited number ofμPMUs to improve the state estimation accuracy.Combined with pseudo-measurements and supervisory control and data acquisition(SCADA)measurements,an optimalμPMU placement model is proposed based on a two-step state estimation method.The E-optimal experimental criterion is utilized to measure the state estimation accuracy.The nonlinear optimization problem is transformed into a mixed-integer semidefinite programming(MISDP)problem,whose optimal solution can be obtained by using the improved Benders decomposition method.Simulations on several systems are carried out to evaluate the effective performance of the proposed model.

越南民族主义形成中的历史构建

法国、日本和美国的殖民统治是激发越南民族主义形成的外部力量,以儒教为代表的传统文化和文字的统一成为其民族主义形成的“内聚力”及推手,在此过程中越南民族精英则发挥了引领作用。在民族主义促使下,胡志明提出“越南民族是一个整体”,为了迎合这一口号,越南史学界提出民族起源的“本土”说以凸显历史的独特性和自发性,将历史上中越关系构建为“侵略”与“反侵略”以强化民众对国家的认同意识,在民族历史书写中彰显越南古代文明成就以增强民族自信心。在国家主导下,雄王信仰不断高涨,试图将全国各民族纳入到通过雄王符号构建起来的认同体系之中。越南将民族历史的客观性置于民族国家构建需求之下的主观性叙述,也体现出一定的民族主义色彩。