We numerically investigate the properties of coherent femtosecond single electron wave packets photoemitted from nanotips in view of their application in ultrafast electron diffraction and non-destructive imaging with low-energy electrons. For two different geometries, we analyze the temporal and spatial broadening during propagation from the needle emitter to an anode, identifying the experimental parameters and challenges for realizing femtosecond time resolution. The simple tip-anode geometry is most versatile and allows for electron pulses of several ten of femtosecond duration using a very compact experimental design, however, providing very limited control over the electron beam collimation. A more sophisticated geometry comprising a suppressor-extractor electrostatic unit and a lens, similar to typical field emission electron microscope optics, is also investigated, allowing full control over the beam parameters. Using such a design, we find ∼230 fs pulses feasible in a focused electron beam. The main limitation to achieve sub-hundred femtosecond time resolution is the typical size of such a device, and we suggest the implementation of more compact electron optics for optimal performance
Characteristics of liquid-liquid slug flow are investigated in a microchannel with focus on the leakage flow that bypasses droplets through channel gutters. The results show that the leakage flow rate varies in a range of 10.7-53.5% and 8.3-30.9% of the feed flow rate, during the droplet formation (i.e., at T-junction) and downstream flow (i.e., in the main channel), respectively, which highly depends on Ca number and wetting condition. Empirical correlations are proposed to predict them for perfectly and partially wetting conditions. Leakage flow contribution is further used to improve the Garstecki model for size scaling in order to extend its suitability for both squeezing and shearing regimes. The instantaneous flow rates of the immiscible phases are found to fluctuate periodically with the formation cycles, but in opposite behavior. The effect of the presence of leakage flow on such fluctuation are investigated and compared with gas-liquid systems.
A three-stage high conversion efficiency optical parametric amplifier (OPA) with passively stabilized carrier-envelope phase (CEP) is presented. After passing through an argon-filled hollow fiber and the dispersion compensator for pulse compression, CEP stabilized 0.7 mJ, 1.5 cycle laser pulses at 1.75 μm center wavelength are obtained. Terahertz (THz) emission spectroscopy is adopted to determine the value of CEP, indicating an excellent long-term CEP stability.
Fault diagnosis technology is key to the safe and stable operation of wind turbines. An effective fault diagnosis technology for wind turbines can quickly identify fault types to reduce the operation and maintenance costs of wind farms and improve power generation efficiency. Currently, most wind farms obtain operation and maintenance data via supervisory control and data acquisition (SCADA) systems, which contain rich information related to the operation characteristics of wind turbines. However, few SCADA systems provide fault diagnosis functionality. Support vector machines (SVMs) are a popular intelligence method in the fault diagnosis of wind turbines. SVM parameter selection is key for accurate model classification. The sparrow search algorithm (SSA) is a novel and highly efficient optimization method used to optimize the penalty factor and kernel function parameter of SVM in this paper and to construct the SSA-SVM wind turbine fault diagnosis model. Data are acquired from a wind farm SCADA system and form a faulting set after preprocessing and feature selection. Experiments show that the SSA-SVM diagnostic model effectively improves the accuracy of wind turbine fault diagnosis compared with the GS-SVM, GA-SVM and PSO-SVM models and has fast convergence speed and strong optimization ability. Moreover, the SSA-SVM diagnostic model can be used to diagnose faults in practical engineering applications.
Gas-liquid-liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas-liquid-liquid slug flow in microreactors and the relevant mass transfer behavior thereof.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.