Ultrahigh performance passive radiative cooling by hybrid polar dielectric metasurface thermal emitters

Real-world passive radiative cooling requires highly emissive,selective,and omnidirectional thermal emitters to maintain the radiative cooler at a certain temperature below the ambient temperature while maximizing the net cooling power.Despite various selective thermal emitters have been demonstrated,it is still challenging to achieve these conditions sim-ultaneously because of the extreme difficulty in controlling thermal emission of photonic structures in multidimension.Here we demonstrated hybrid polar dielectric metasurface thermal emitters with machine learning inverse design,en-abling a high emissivity of~0.92 within the atmospheric transparency window 8-13μm,a large spectral selectivity of~1.8 and a wide emission angle up to 80 degrees,simultaneously.This selective and omnidirectional thermal emitter has led to a new record of temperature reduction as large as~15.4°C under strong solar irradiation of~800 W/m2,signific-antly surpassing the state-of-the-art results.The designed structures also show great potential in tackling the urban heat island effect,with modelling results suggesting a large energy saving and deployment area reduction.This research will make significant impact on passive radiative cooling,thermal energy photonics and tackling global climate change.

RFFsNet-SEI:a multidimensional balanced-RFFs deep neural network framework for specific emitter identification

Existing specific emitter identification(SEI)methods based on hand-crafted features have drawbacks of losing feature information and involving multiple processing stages,which reduce the identification accuracy of emitters and complicate the procedures of identification.In this paper,we propose a deep SEI approach via multidimensional feature extraction for radio frequency fingerprints(RFFs),namely,RFFsNet-SEI.Particularly,we extract multidimensional physical RFFs from the received signal by virtue of variational mode decomposition(VMD)and Hilbert transform(HT).The physical RFFs and I-Q data are formed into the balanced-RFFs,which are then used to train RFFsNet-SEI.As introducing model-aided RFFs into neural network,the hybrid-driven scheme including physical features and I-Q data is constructed.It improves physical interpretability of RFFsNet-SEI.Meanwhile,since RFFsNet-SEI identifies individual of emitters from received raw data in end-to-end,it accelerates SEI implementation and simplifies procedures of identification.Moreover,as the temporal features and spectral features of the received signal are both extracted by RFFsNet-SEI,identification accuracy is improved.Finally,we compare RFFsNet-SEI with the counterparts in terms of identification accuracy,computational complexity,and prediction speed.Experimental results illustrate that the proposed method outperforms the counterparts on the basis of simulation dataset and real dataset collected in the anechoic chamber.

Transverse emittance measurement for the heavy ion medical machine cyclotron

The transverse emittance of the extracted beam from the heavy ion medical machine cyclotron is measured and then optimized for injection into the synchrotron.For the purposes of cross-validation,three methods,i.e.,slit-grid,Q-scan,and 3-grid,are used to measure the emittance.In the slit-grid technique,an automatic selection of the region of interest is adopted to isolate the major noise from the beam phase space,which is an improvement over the traditional technique.After iterating over the contour level,an unbiased measurement of the emittance can be obtained.An improvement in the thin lens technique is implemented in the Q-scan method.The results of these measurements are presented.

Slice emittance and projected emittance

This paper presents an analytical description for the growth of the projected emittanee （here just referred to the transverse emittance） based on the concept of the slice emittance in an RF photoinjector. In the RF photoinjector, the slice emittance undergoes small changes, but the projected emittance changes significantly even in the drift space after the injector. Carefully adjusting the parameters of the RF photoinjector, which usually means emittance compensation, the projected emittance can be minimized to the value of the slice emittance. The relation between slice emittance and projected emittance is explained in this paper. An emittance function, which shows such a relation, is also introduced. A model about the emittance growth in the RF photoinjector is established, which accords with the particle simulation results by using the code ASTRA. The condition to minimize the emittance is also given by using the emittance function, which means the emittance compensation in the RF photoinjector.

Low emittance lattice design with Robinson wiggler in the arc section

Beam emittance reduction is an effective method to increase the brightness of a synchrotron light source.Robinson wiggler can play a role in the beam emittance reduction by increasing the horizontal damping partition number.A replacement of the quadrupoles in the arc section with short combined function dipoles will construct a single-periodic Robinson wiggler in the SSRF storage ring.This scheme provides a lower beam emittance,without occupying any straight section.Detailed analysis is presented in this paper.

A method to measure the nonlinear force caused emittance growth in a RF photoinjector

Based on the multi-slit method, a new method is introduced to measure the nonlinear force caused emittance growth in a RF photoinjector. It is possible to reconstruct the phase space of a beam under some conditions by the multi-slit method. Based on the reconstructed phase space, besides the emittance, the emittance growth from the distortion of the phase space can also be measured. The emittance growth results from the effects of nonlinear force acting on electron, which is very important for the high quality beam in a RF photoinjector.

Emittance Growth Caused by the Transverse Nonuniform Space Charge Distributions of Bunched Beam in Linac

The nonlinear space charge effect of bunched beam in linac is one of the im-portant factors that induces the emittance growth due to the conversion of the field en-ergy to kinetic energy.In this paper,the internal field energies associated with sometransverse nonuniform space charge distributions of a bunched beam,such as Gaussiandistribution,waterbag distribution and parabolic distribution,are worked out.And theemittance growth caused by these nonuniformities are obtained.

A fast emittance measurement unit for high intensity DC beam

A combined unit, which has the ability to measure the current and emittance of the high intensity direct current（DC）ion beam, is developed at Peking University（PKU）. It is a multi-slit single-wire（MSSW）-type beam emittance meter combined with a water-cooled Faraday Cup, named high intensity beam emittance measurement unit-6（HIBEMU-6）. It takes about 15 seconds to complete one measurement of the beam current and its emittance. The emittance of a 50-mA@50-kV DC proton beam is measured.

Emittance Growth Due to the Nonlinear RF Field Near the Cathode of RF Electron Gun

RF effect and space charge effect are the main sources of the emittance growth in the RF elec-tron gun.The single particle dynamics near the cathode is discussed,and the emittance growth due to RFnonlinear force and time-dependent force are studied in detail.Comparison with the emittance at the gunexit shows that the emittanca growth due to nonlinear RF effect near the cathode can not be neglected.

Influence of a Static Magnetic Field on Beam Emittance in Laser Wakefield Acceleration

The enhancement of trapping and the optimization of beam quality are two key issues of Laser Wakefield Acceleration (LWFA). The effect of a homogenous constant magnetic field B0, parallel to the direction of propagation of the pump pulse, is studied in the blowout regime via 2Dx3Dv Particle-In-Cell simulations. Electrons are injected into the wake using a counter-propagating low amplitude laser. Transverse currents are generated at the rim of the bubble, which results in the amplification of the B0 field at the rear of the bubble. Therefore the dynamics of the beam is modified, the main effect is the reduction of the transverse emittance when B0 is raised. Depending on beam loading effects the low energy tail, observed in the non-magnetized case, can be suppressed when B0 is applied, which provides a mono-energetic beam.

Free-Energy and Emittance Growth Caused by Bunched Beam with Nonuniform Distributions in Both Longitudinal and Transverse Directions in Linac

The nonlinear space charge effect of bunched beam in linac is one of theimportant factors that induces the emittance growth due to the conversion of thefield energy to kinetic energy. Using a cylinder model of space charge in a linac,we derive the internal energy for a bunched beam with some nonuniform spacecharge distributions, such as Gaussian, waterbag and parabolic distributions inboth the longitudinal and transverse directions. And the emittance growth causedby these nonuniformities is worked out.

Free Energy and Emittance Growth Caused by Bunched Beam With Gaussian Distribution in Longitudinal Direction in Linac

The nonlinear space charge effect of bunched beam in linac is one of the important factors thatinduces the emittance growth due to the conversion of the field energy to kinetic energy.In this paper,us-ing a cylinder model of space charge in linac,we derived the internal energy for a bunched beam withsome nonuniform space charge distributions,such as longitudinal Gaussian distribution combined withKapchinskij-Vladimirski1,waterbag,parabolic or Gaussian transverse distribution.And the emittancegrowth caused by the above distributions is worked out.

Pulsed intense electron beam emittance measurement

Recently we measured with the Modified Three Gradient Method(MTGM) the beam emittance of an injector constructed in 2012, which was designed to provide a 2.4 kA, 2.6 MeV electron beam. The MTGM is a non-intercept indirect method, which is based on the three gradient type measurements of beam profiles and subsequent data processing which helps to get the least square solution to the beam emittance. Beam profiles under different currents of guiding coil are measured using Cerenkov radiation given off by a piece of quartz glass in the beam tube, which is recorded with a CCD camera. MTGM Code is developed to realize the data fitting as well as beam transport simulation, in which both the σ matrix method and the numerical solution of root-mean-square beam envelope equation are used. The error is also analyzed.

Transverse Emittance Reconstruction with Space Charge Effect

Emittance is a key figure of merit for charged particle beams,and various diagnostic techniques have been developed to try to measure it accurately.To measure the transverse emittance,a traditional emittance measurement device,which consists of slits and wire scanners,is installed at the MEBT section of the Ci-ADS injector Ⅱ.

Phonon-assisted upconversion photoluminescence of quantum emitters

Quantum emitters are widely used in quantum networks,quantum information processing,and quantum sensing due to their excellent optical properties.Compared with Stokes excitation,quantum emitters under anti-Stokes excitation exhibit better performance.In addition to laser cooling and nanoscale thermometry,anti-Stokes excitation can improve the coherence of single-photon sources for advanced quantum technologies.In this review,we follow the recent advances in phononassisted upconversion photoluminescence of quantum emitters and discuss the upconversion mechanisms,applications,and prospects for quantum emitters with anti-Stokes excitation.

Emitter discharge characteristics of vertical tube irrigation affected by various factors

To examine the working principle of vertical tube irrigation, variations in vertical tube emitter discharge and their causes were analyzed in the laboratory experiment. The effects of the pressure head, initial soil water content, and tube diameter on the emitter discharge of the vertical tube were studied. The results show that quantitative relationship between the time and cumulative infiltration and emitter discharge of the vertical tube is obtained, and R 2 is more than 0.98. Emitter discharge exhibits a positive and negative correlation with the pressure head and soil water content, respectively. Tube dia- meter has a nonsignificant effect on the emitter discharge. Changes of the soil water content around the emitter water outlet are the main causes of emitter discharge variations. In the experiments, the range of vertical tube emitter discharge is 0.056-1.102 L/h. The emitter of vertical tube irrigation automatically adjusts the soil water content and maintains the root zone soil water content within an appropriate range, which achieves continuous irrigation, and further achieves the effect of water-saving.

Radar emitter signal recognition method based on improved collaborative semi-supervised learning

Rare labeled data are difficult to recognize by using conventional methods in the process of radar emitter recogni-tion.To solve this problem,an optimized cooperative semi-supervised learning radar emitter recognition method based on a small amount of labeled data is developed.First,a small amount of labeled data are randomly sampled by using the bootstrap method,loss functions for three common deep learning net-works are improved,the uniform distribution and cross-entropy function are combined to reduce the overconfidence of softmax classification.Subsequently,the dataset obtained after sam-pling is adopted to train three improved networks so as to build the initial model.In addition,the unlabeled data are preliminarily screened through dynamic time warping(DTW)and then input into the initial model trained previously for judgment.If the judg-ment results of two or more networks are consistent,the unla-beled data are labeled and put into the labeled data set.Lastly,the three network models are input into the labeled dataset for training,and the final model is built.As revealed by the simula-tion results,the semi-supervised learning method adopted in this paper is capable of exploiting a small amount of labeled data and basically achieving the accuracy of labeled data recognition.

高含沙水滴灌灌水器堵塞机制及防堵技术研究进展

为解决黄河水中粒径小于0.10 mm细小泥沙颗粒引发的滴灌灌水器堵塞问题。通过文献调研,回顾了高含沙水滴灌条件下灌水器物理堵塞机制相关研究进展,并提出了进一步研究方向。细小泥沙颗粒含量较高的浑水滴灌时,滴头堵塞主要是含沙量、泥沙粒径与颗粒级配耦合作用的结果。滴灌系统工作条件如工作压力的动态变化有助于移除流道内黏、粉等细颗粒堵塞物质,促进较大泥沙颗粒排出流道;灌溉水温越高,滴头抗物理堵塞性能越强;对浑水加气和磁化处理可改变毛管内水流水力特性及悬浮泥沙运动规律,增强水流拖拽力,减小管道内泥沙淤积量。此外,施肥增强了水体中泥沙颗粒间的絮凝作用,对浑水水肥一体化滴灌滴头堵塞具有明显加速作用。浑水含沙量、粒径和颗粒级配是引发滴头物理堵塞的重要因素,确定易引发滴头堵塞的敏感含沙量、颗粒粒径段,选用适宜肥料种类和施肥浓度阈值,优化滴灌系统工作条件参数是改变毛管内泥沙颗粒运移和沉积规律、延缓滴头堵塞进程、提高水肥一体化滴灌水肥利用效率的有力措施;采用统一的灌水试验方法,充分利用现代测试手段,结合生产实际有针对性地改进工程技术处理措施是解决滴头堵塞问题的必要途径。

基于电介质超材料的中红外超窄带吸波器

电介质超材料因具有极低欧姆损耗受到广泛关注.提出一种基于电介质超材料的中红外超窄带吸波器,该吸波器由顶层电介质微结构、中间层电介质膜和基底金属构成.研究结果表明:该吸波器在中红外波段存在带宽为2.40nm的吸收峰;电场主要分布于顶层硅微结构之间的间隙,导致吸收损耗功率显著降低,进而实现吸收带宽压缩;作为传感器时,在中红外波段该吸波器的品质因数可达80.可见,该吸波器可应用于中红外热辐射光源和高性能传感器.

空间热光伏发电技术进展与展望

空间同位素电源是通过热电转换技术将衰变热能转换为电能的空间电源装置,广泛应用于深空探测任务中太阳能电池无法使用的任务场景,其热电转换方式包括温差、热光伏等静态转换方式以及斯特林、布雷顿等动态转换方式。空间同位素电源普遍使用钚238核源,其制备生产困难,价格高昂,在全球范围内产能受限,同时,长周期的深空探测任务对同位素电源寿命也提出较高要求,因此热电转换效率高、易于实现长寿命的空间热光伏发电技术成为开展深空探测任务的优选技术路线。近年来,近场热光伏、基于光谱调控的热光伏技术是研究重点,结构热控、辐射器、滤光器、电池晶元等关键器件的理论研究与技术验证也取得了显著的进展。对热光伏空间同位素电源技术方案、关键器件研究情况进行了综述,从电源系统总体设计的角度对各器件的发展现状与未来技术的发展重点方向进行了研判,可为进一步推动热光伏同位素电源系统的工程化研制提供参考。