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直供电系统中电网调频方式研究 预览
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作者 唐长忠 《电工技术》 2019年第6期38-39,41共3页
本文分析了直供电系统网内频率变化特性,在优化网内机组调频的基础上,提出配置负荷调频措施来抑制电网的频率劣化,既能减轻机组调频装置频繁动作对调速系统的损坏,又有利于电网频率的快速恢复。
关键词 直供电系统 频率 机组调频方式 负荷调频
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Joint timing and frequency synchronization in coherent optical OFDM systems
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作者 Xinwei DU Pooi-Yuen KAM Changyuan YU 《中国光电子学前沿:英文版》 EI CSCD 2019年第1期4-14,共11页
In this paper,we review our joint timing and frequency synchronization algorithms in coherent optical orthogonal frequency division multiplexing(CO-OFDM)systems.We first present a timing estimation method by designing... In this paper,we review our joint timing and frequency synchronization algorithms in coherent optical orthogonal frequency division multiplexing(CO-OFDM)systems.We first present a timing estimation method by designing the pattern of the training symbol,whose timing metric has a sharp and clear peak,to ensure accurate timing offset(TO)estimation performance.Then we provide both data-aided(DA)and blind(BL)approaches to estimate the carrier frequency offset(CFO).For the DA algorithm,we utilize the same training symbol structure as the timing estimation does,while for the BL algorithm,we utilize the zero-subcarrier power(ZSP)to achieve full-range CFO estimation.Note that our joint timing and frequency synchronization approaches require only one OFDM symbol,which ensure not only the data transmission efficiency,but also the TO and CFO estimation performance.A modified BL ZSP algorithm is proposed to further improve the CFO estimation performance by taking the power average over a series of OFDM symbols.The effectiveness of the TO estimation algorithm,and both the DA and BL CFO estimation algorithms are verified and demonstrated in both simulations and experiments. 展开更多
关键词 timing OFFSET (TO) carrier FREQUENCY OFFSET (CFO) COHERENT OPTICAL orthogonal FREQUENCY division multiplexing (CO-OFDM) zero-subcarrier power (ZSP)
Photonic Vector Signal Generation by Frequency Sextupling for Radio Over Fiber Systems 预览
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作者 Siyang Wang Nai’an Liu 《哈尔滨工业大学学报:英文版》 EI CAS 2019年第4期18-24,共7页
To generate high-frequency radio frequency(RF) vector signals, a vector signal generation method by optical frequency sextupling using a dual-parallel modulator is proposed. The method modulates vector signal on +3 rd... To generate high-frequency radio frequency(RF) vector signals, a vector signal generation method by optical frequency sextupling using a dual-parallel modulator is proposed. The method modulates vector signal on +3 rd order optical sideband and local oscillator(LO) signal on-3 rd order sideband using the intermodulation process in the DPMZM. After suppressing of the optical carrier and other sidebands through proper adjustment for modulator biases and modulation index, a frequency sextupled millimeter-wave vector signal can be generated after photodetection. The frequency sextupling will lower the bandwidth of the modulator, the local oscillator and the driving circuits. In addition, the phase of generated signal is not distorted after detection, and the power fading after fiber transmission can be avoided. In the simulation, a 500-MSym/s QPSK signal at 60 GHz is generated by 10-GHz drive signal. After travelling over fiber with length of 20/30/40-km, receiver power penalty keeps below 2.5 dB. 展开更多
关键词 dual-parallel MACH-ZEHNDER modulator(DPMZM) FREQUENCY MULTIPLICATION FREQUENCY UP-CONVERSION VECTOR signal radio over fiber(RoF)
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Quantitative SNR analysis of QFM signals in the LPFT domain with Gaussian windows
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作者 Yan-Na ZHANG Bing-Zhao LI +1 位作者 Navdeep GOEL Salvador GABARDA 《中国科学:信息科学(英文版)》 SCIE EI CSCD 2019年第2期127-141,共15页
The purpose of this paper is to present a quantitative SNR analysis of quadratic frequency modulated(QFM)signals.This analysis is located in the continuous-time local polynomial Fourier transform(LPFT)domain using a G... The purpose of this paper is to present a quantitative SNR analysis of quadratic frequency modulated(QFM)signals.This analysis is located in the continuous-time local polynomial Fourier transform(LPFT)domain using a Gaussian window function based on the definition of 3 dB signal-to-noise ratio(SNR).First,the maximum value of the local polynomial periodogram(LPP),and the 3 dB bandwidth in the LPFT domain for a QFM signal is derived,respectively.Then,based on these results,the 3 dB SNR of a QFM signal with Gaussian window function is given in the LPFT domain with one novel idea highlighted:the relationship among standard SNR,parameters of QFM signals and Gaussian window function is clear,and the potential application is demonstrated in the parameter estimation of a QFM signal using the LPFT.Moreover,the 3 dB SNR in the LPFT domain is compared with that in the linear canonical transform(LCT)domain.The validity of theoretical derivations is confirmed via simulation results.It is shown that,in terms of SNR,QFM signals in the LPFT domain can achieve a significantly better performance than those in the LCT domain. 展开更多
关键词 SNR ANALYSIS quadratic FREQUENCY modulated signal local polynomial Fourier TRANSFORM linear CANONICAL TRANSFORM time FREQUENCY representations
Formulation of Determining the Gravity Potential Difference Using Ultra-High Precise Clocks via Optical Fiber Frequency Transfer Technique
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作者 Ziyu Shen Wen-Bin Shen +3 位作者 Zhao Peng Tao Liu Shougang Zhang Dingbo Chao 《地球科学学刊:英文版》 SCIE CAS CSCD 2019年第2期422-428,共7页
Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential(geopotential) difference between arbitrary two points P and Q by r... Based on gravity frequency shift effect predicted by general relativity theory, this study discusses an approach for determining the gravity potential(geopotential) difference between arbitrary two points P and Q by remote comparison of two precise optical clocks via optical fiber frequency transfer. After synchronization, by measuring the signal’s frequency shift based upon the comparison of bidirectional frequency signals from P and Q oscillators connected with two optical atomic clocks via remote optical fiber frequency transfer technique, the geopotential difference between the two points could be determined, and its accuracy depends on the stabilities of the optical clocks and the frequency transfer comparison technique. Due to the fact that the present stability of optical clocks achieves 1.6×10-18 and the present frequency transfer comparison via optical fiber provides stabilities as high as 10-19 level, this approach is prospective to determine geopotential difference with an equivalent accuracy of 1.5 cm. In addition, since points P and Q are quite arbitrary, this approach may provide an alternative way to determine the geopotential over a continent, and prospective potential to unify a regional height datum system. 展开更多
关键词 GRAVITY FREQUENCY shift OPTICAL fiber FREQUENCY TRANSFER OPTICAL CLOCK GRAVITY potential
联轴部分对转轮自振频率计算结果的影响分析 预览
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作者 卜良峰 吉敏刚 《大电机技术》 2019年第1期51-56,共6页
本文以一个电厂结构为分析基础,选取了一个完整的转轮和不同轴身长度作为计算模型,计算出了转轮不同节径的自振频率。对计算结果和不同振型进行了分析,对转轮自振频率与激振频率的关系进行了分析,给出了设计时对转轮自振频率计算模型的... 本文以一个电厂结构为分析基础,选取了一个完整的转轮和不同轴身长度作为计算模型,计算出了转轮不同节径的自振频率。对计算结果和不同振型进行了分析,对转轮自振频率与激振频率的关系进行了分析,给出了设计时对转轮自振频率计算模型的选取及对计算结果的影响和取值方面的指导性意见,对结构优化调整转轮自振频率具有一定的指导意义。 展开更多
关键词 混流式水轮机 转轮 计算模型 频率 节径 结构优化 有限元分析 自振频率
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Using irrigation intervals to optimize water-use efficiency and maize yield in Xinjiang,northwest China 预览
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作者 Guoqiang Zhang Dongping Shen +10 位作者 Bo Ming Ruizhi Xie Xiuliang Jin Chaowei Liu Peng Hou Jun Xue Jianglu Chen Wanxu Zhang Wanmao Liu Keru Wang Shaokun Li 《作物学报:英文版》 CAS CSCD 2019年第3期322-334,共13页
Worldwide, scarce water resources and substantial food demands require efficient water use and high yield.This study investigated whether irrigation frequency can be used to adjust soil moisture to increase grain yiel... Worldwide, scarce water resources and substantial food demands require efficient water use and high yield.This study investigated whether irrigation frequency can be used to adjust soil moisture to increase grain yield and water use efficiency(WUE) of high-yield maize under conditions of mulching and drip irrigation.A field experiment was conducted using three irrigation intervals in 2016: 6, 9, and 12 days(labeled D6, D9, and D12) and five irrigation intervals in 2017: 3, 6, 9, 12, and 15 days(D3, D6, D9, D12, and D15).In Xinjiang, an optimal irrigation quota is 540 mm for high-yield maize.The D3, D6, D9, D12, and D15 irrigation intervals gave grain yields of 19.7, 19.1–21.0, 18.8–20.0, 18.2–19.2, and 17.2 Mg ha^-1 and a WUE of 2.48, 2.53–2.80, 2.47–2.63, 2.34–2.45, and 2.08 kg m-3, respectively.Treatment D6 led to the highest soil water storage, but evapotranspiration and soil-water evaporation were lower than other treatments.These results show that irrigation interval D6 can help maintain a favorable soil-moisture environment in the upper-60-cm soil layer, reduce soilwater evaporation and evapotranspiration, and produce the highest yield and WUE.In this arid region and in other regions with similar soil and climate conditions, a similar irrigation interval would thus be beneficial for adjusting soil moisture to increase maize yield and WUE under conditions of mulching and drip irrigation. 展开更多
关键词 IRRIGATION frequency Soil MOISTURE MAIZE High YIELD (>15 Mg ha^?1) Water use efficiency
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Genome-wide dissection of segregation distortion using multiple inter-subspecific crosses in rice
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作者 Guangwei Li Jiye Jin +5 位作者 Yan Zhou Xufeng Bai Donghai Mao Cong Tan Gongwei Wang Yidan Ouyang 《中国科学:生命科学英文版》 SCIE CAS CSCD 2019年第4期507-516,共10页
Mendelian inheritance can ensure equal segregation of alleles from parents to offspring,which provides fundamental basis for genetics and molecular biology.Segregation distortion(SD)leads to preferential transmission ... Mendelian inheritance can ensure equal segregation of alleles from parents to offspring,which provides fundamental basis for genetics and molecular biology.Segregation distortion(SD)leads to preferential transmission of certain alleles from generation to generation.Such violation of Mendelian genetic principle is often accompanied by reproductive isolation and eventually speciation.Although SD is observed in a wide range of species from plants to animals,genome-wide dissection of such biased transmission of gametes is rare.Using nine inter-subspecific rice crosses,a genome-wide screen for SD loci is performed,which reveals 61 single-locus quantitative trait loci and 194 digenic interactions showing distorted transmission ratio,among which 24 new SD loci are identified.Biased transmission of alleles is observed in all nine crosses,suggesting that SD exists extensively in rice populations.72.13% distorted regions are repeatedly detected in multiple populations,and the most prevalent SD hotspot that observed in eight populations is mapped to chromosome 3.Xian alleles are transmitted at higher frequencies than geng alleles in inter-subspecific crosses,which change the genetic composition of the rice populations.Epistatic interaction contributes significantly to the deviation of Mendelian segregation at the whole-genome level in rice,which is distinct from that in animals.These results provide an extensive archive for investigating the genetic basis of SD in rice,which have significant implications in understanding the reproductive isolation and formation of inter-subspecific barriers during the evolution. 展开更多
关键词 Oryza sativa xian/indica geng/japonica segregation distortion REPRODUCTIVE isolation ALLELE frequency
Frequency control of future power systems: reviewing and evaluating challenges and new control methods 预览
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作者 Zeyad Assi OBAID Liana M.CIPCIGAN +1 位作者 Lahieb ABRAHIM Mazin T.MUHSSIN 《现代电力系统与清洁能源学报(英文)》 CSCD 2019年第1期9-25,共17页
Integration of more renewable energy resources introduces a challenge in frequency control of future power systems.This paper reviews and evaluates the possible challenges and the new control methods of frequency in f... Integration of more renewable energy resources introduces a challenge in frequency control of future power systems.This paper reviews and evaluates the possible challenges and the new control methods of frequency in future power systems.Different types of loads and distributed energy resources(DERs) are reviewed.A model representation of a population of the water heater devices for the demand side frequency response is considered.A model representation of a population of battery energy storage system(BESS)-based DERs such as smart electric vehicles(EVs) charging, large-scale BESSs, and residential and non-residential BESSs, are highlighted.The simplified Great Britain power system and the 14-machine South-East Australian power system were used to demonstrate the effectiveness of the new methods in controlling power system frequency following a disturbance.These new methods are effective in recovering the fallen frequency response and present a great potential in controlling the frequency in future power systems. 展开更多
关键词 Battery ENERGY storage system(BESS) Distributed ENERGY resource(DER) Electric vehicle HOME-BASED BESS Large-scale BESS AGGREGATION Demand side response Frequency control Markov chain
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不同面密度吸声板对声屏障结构影响研究 预览
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作者 黄涛 常婷婷 于梦阁 《中国铁路》 2019年第4期85-91,共7页
针对吸声板面密度为40 kg/m~2和60 kg/m~2的2种直立插板式声屏障,基于有限元法计算声屏障结构的模态,并在台风荷载为8 000 Pa条件下,研究声屏障的稳态结构响应。研究结果表明:(1)这2种声屏障的1阶振动频率和2阶振动频率各不相同,3阶到6... 针对吸声板面密度为40 kg/m~2和60 kg/m~2的2种直立插板式声屏障,基于有限元法计算声屏障结构的模态,并在台风荷载为8 000 Pa条件下,研究声屏障的稳态结构响应。研究结果表明:(1)这2种声屏障的1阶振动频率和2阶振动频率各不相同,3阶到6阶模态相差不大;(2)当吸声板面密度为60 kg/m~2时,声屏障结构具有较好的稳定性。并提出建议:(1)在保证吸声降噪效果的前提下,建议吸声板面密度选择60 kg/m~2;(2)在台风载荷作用下,声屏障结构会发生破坏,建议结构设计时在H形支座底部合理布置一定数量的加强筋板或将H形支座材料改为Q275钢。 展开更多
关键词 声屏障 面密度 结构 模态 频率 应力
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基于振动减阻原理的吸力式基础纠偏试验研究 预览
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作者 谢立全 林田田 《高技术通讯》 CAS 北大核心 2019年第1期53-59,共7页
吸力式基础作为一种新兴的海洋锚泊基础,具有运输便捷、施工快、节省材料等优点。但吸力式基础在复杂多变的海洋环境以及海床地质条件贯入过程中易发生倾斜,严重影响自身的承载功能。本文针对海底吸力式基础的沉贯倾斜问题,展开一系列试... 吸力式基础作为一种新兴的海洋锚泊基础,具有运输便捷、施工快、节省材料等优点。但吸力式基础在复杂多变的海洋环境以及海床地质条件贯入过程中易发生倾斜,严重影响自身的承载功能。本文针对海底吸力式基础的沉贯倾斜问题,展开一系列试验,采用振动减阻原理达到纠偏效果。试验分别研究了在不同振动频率、不同初始倾斜角、不同静压荷载作用下的基础纠偏过程。结果表明,倾斜的吸力式基础可通过侧壁振动方式实现有效纠偏,其纠偏效果与振动频率、初始倾斜角以及静压荷载息息相关:相同激振力作用下,激振频率越大(25~75Hz),所需纠偏时间越短;桶基的初始倾斜角越大,则完全纠偏所需时间越长,且呈非线性增加;贯入荷载(或贯入深度)越大,纠偏难度越大,纠偏越早(贯入深度越小)越容易。 展开更多
关键词 吸力式基础 倾斜 振动 纠偏 频率
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黑龙江省水稻生育阶段极端降水事件时空特征 预览
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作者 朱海霞 姜丽霞 +3 位作者 曲辉辉 王晾晾 纪仰慧 闫平 《气象》 CSCD 北大核心 2019年第4期522-532,共11页
利用百分位相对指数法,基于1971-2016年历史长序列的日降水资料,分析研究黑龙江省水稻生长季极端降水事件的阈值、频数和强度的时空特征。结果表明:黑龙江省极端降水事件阈值的高值区主要位于松嫩平原中南大部;极端降水事件主要集中于... 利用百分位相对指数法,基于1971-2016年历史长序列的日降水资料,分析研究黑龙江省水稻生长季极端降水事件的阈值、频数和强度的时空特征。结果表明:黑龙江省极端降水事件阈值的高值区主要位于松嫩平原中南大部;极端降水事件主要集中于水稻生长季的5-9月,尤其是在水稻生殖生长的关键阶段,发生了大部分的极端降水事件;46年中,水稻种植区极端降水事件频数在18~72 d,极端降水事件频数总体呈经向分布特征,自东向西逐渐减少。7月下旬为中西部地区极端降水事件发生的高频时段,东部地区极端降水事件发生的高频时间段为5和9月,6月为极端降水事件发生的低频时段。近6年为极端降水事件频数最高、强度最大的一段时期,20世纪70年代则反之;极端降水事件频数和强度存在高度的相关性;在有雨量观测的小区域内,洪涝灾害事件基本可以被极端降水事件捕捉到,同时极端降水事件对洪涝灾害的指示性也较高。 展开更多
关键词 水稻 洪涝 极端降水事件 频数 强度
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Effects of secondary electron emission on plasma characteristics in dual-frequency atmospheric pressure helium discharge by fluid modeling
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作者 王一男 李帅星 +1 位作者 刘悦 王莉 《中国物理B:英文版》 SCIE EI CAS CSCD 2019年第2期346-351,共6页
A one-dimensional(1D) fluid simulation of dual frequency discharge in helium gas at atmospheric pressure is carried out to investigate the role of the secondary electron emission on the surfaces of the electrodes. In ... A one-dimensional(1D) fluid simulation of dual frequency discharge in helium gas at atmospheric pressure is carried out to investigate the role of the secondary electron emission on the surfaces of the electrodes. In the simulation, electrons,ions of He+ and He2+, metastable atoms of He*and metastable molecules of He*2 are included. It is found that the secondary electron emission coefficient significantly influences plasma density and electric field as well as electron heating mechanisms and ionization rate. The particle densities increase with increasing SEE coefficient from 0 to 0.3 as well as the sheath’s electric field and electron source. Moreover, the SEE coefficient also influences the electron heating mechanism and electron power dissipation in the plasma and both of them increase with increasing SEE coefficient within the range from 0 to 0.3 as a result of increasing of electron density. 展开更多
关键词 dual frequency SECONDARY electron emission ATMOSPHERIC pressure DISCHARGE
超声强化小型溴化锂吸收式制冷机性能实验研究 预览
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作者 朱茂川 周国兵 《化工进展》 EI CAS CSCD 北大核心 2019年第3期1316-1323,共8页
在10kW溴化锂吸收式制冷机发生器侧壁(厚度4mm)粘贴超声波振子,实验研究了频率为28kHz超声波在溴化锂溶液两种不同液位高度下对机组性能的影响,并对超声波强化溴化锂溶液沸腾传热传质机理进行了分析。实验结果表明:无超声波作用时,溶液... 在10kW溴化锂吸收式制冷机发生器侧壁(厚度4mm)粘贴超声波振子,实验研究了频率为28kHz超声波在溴化锂溶液两种不同液位高度下对机组性能的影响,并对超声波强化溴化锂溶液沸腾传热传质机理进行了分析。实验结果表明:无超声波作用时,溶液泵转速控制电机运行频率从17Hz升高到18Hz,溶液泵流量升高,发生器中溴化锂溶液液位升高5cm,制冷量增加16.8%,但性能系数(COP)降低44.3%;而施加超声波作用可以强化溴化锂吸收式制冷机性能,且强化效果与溶液液位有关,当机组发生器内液位高于超声波换能器中心线8~10cm时,机组制冷量升高19.6%,COP提高13.8%;而当液位与换能器中心线相差3~5cm时,制冷量提升并不明显,仅为4.7%,COP提升5.4%。实验结果为超声波作用提升小型太阳能溴化锂吸收式制冷系统性能提供指导和依据。 展开更多
关键词 吸收式制冷 传热传质 溶液泵 超声波 频率
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杭州市主城区道路绿化树种资源调查及多样性分析 预览
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作者 吴渝 吕楚 +5 位作者 魏伟 丰睿 罗坤 吴晓华 申亚梅 严少君 《林业资源管理》 北大核心 2019年第3期74-79,93共7页
采用抽样调查方法,对杭州主城区19条道路绿化树种资源进行调查,分析了杭州市城市道路绿化的树种结构及频度,计算了各道路物种丰富度指数、Shannon-Weiner指数、Simpson指数及Pielou指数。结果表明:杭州市主城区道路绿化树种资源共33种,... 采用抽样调查方法,对杭州主城区19条道路绿化树种资源进行调查,分析了杭州市城市道路绿化的树种结构及频度,计算了各道路物种丰富度指数、Shannon-Weiner指数、Simpson指数及Pielou指数。结果表明:杭州市主城区道路绿化树种资源共33种,隶属于24科31属。其中乔木12种,灌木为21种,樟(Cinnamomum camphora)与红花檵木(Loropetalum chinense var.rubrum)的出现频率超过50%,分别达60.71%与53.57%。各道路绿化树种多样性指数相差各异,多样性综合指数排名前10的为庆春东路(0.9570)>环城北路1(0.9428)>艮山西路1(0.9373)>天目山路1(0.9218)>文一西路(0.9162)>凤起东路2(0.9113)>天目山路2(0.8958)>环城北路4(0.8462)>环城北路3(0.8288)>环城北路2(0.8082)。 展开更多
关键词 道路绿化 频度 丰富度指数 多样性指数 杭州市
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Femtosecond enhancement cavity with kilowatt average power
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作者 张津 华林强 +2 位作者 余少刚 陈忠 柳晓军 《中国物理B:英文版》 SCIE EI CAS CSCD 2019年第4期211-215,共5页
Femtosecond enhancement cavity(fsEC) has been proved to be a powerful tool in a diverse range of applications.Here, we report the recent progresses in building an fsEC on kilowatt level average power, with the aim of ... Femtosecond enhancement cavity(fsEC) has been proved to be a powerful tool in a diverse range of applications.Here, we report the recent progresses in building an fsEC on kilowatt level average power, with the aim of realization of intracavity high harmonic generation(HHG) and extension of the wavelength of femtosecond optical frequency comb from infrared(IR) to extreme ultraviolet(XUV). Upon mode-matching optimization and cavity length locking, an intracavity average power of 6.08 kW is reached and the corresponding buildup is 225. After introducing noble gas of Xe into the focus region, clear sign of plasma has been observed. The generated HHG is also coupled out by a sapphire plate placed at Brewster’s angle for the fundamental laser. Our work paves the way for the realization of an XUV comb. 展开更多
关键词 FEMTOSECOND ENHANCEMENT CAVITY extreme ultraviolet optical frequency COMB HIGH-ORDER harmonic generation
福建省医用X射线诊断应用频度调查
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作者 张燕 郑琪珊 +3 位作者 黄丽华 郑森兴 李津 魏伟奇 《中华放射医学与防护杂志》 CAS CSCD 北大核心 2019年第5期376-380,共5页
目的通过对2016年度福建省9个地市的抽样医院进行X射线诊断检查人次的调查,估算福建省2016年度X射线诊断的应用频度。方法采用全国统一的调查表,通过分层典型抽样的方法,抽取福建省9个地市43家医院开展调查。调查信息主要包括样本医院... 目的通过对2016年度福建省9个地市的抽样医院进行X射线诊断检查人次的调查,估算福建省2016年度X射线诊断的应用频度。方法采用全国统一的调查表,通过分层典型抽样的方法,抽取福建省9个地市43家医院开展调查。调查信息主要包括样本医院的门急诊量、住院量、X射线诊断设备和放射诊断工作人员的基本情况,以及X射线诊断的检查类型、受检者性别、年龄、检查部位等信息,最后根据调查结果建立全省X射线诊断应用频度的预测模型并进行估算。结果 43家医院共有X射线影像诊断设备418台,放射诊断工作人员1 230名,年度门急诊和住院量30 919 162人次,X射线诊断检查的总人次为4 277 518。基于该调查结果所估算的2016年度福建省X射线诊断应用频度为695人次/千人口,普通X射线摄影检查应用频度为373人次/千人口,CT应用频度为293人次/千人口。结论通过本次调查能够大体掌握2016年度福建省X射线诊断应用的基本情况,同时也为完善放射卫生防护的法规和标准体系,加强福建省X射线诊断设备应用情况的监督管理提供数据资料。 展开更多
关键词 X射线诊断 X射线摄影 CT 频度
广东省医用X射线诊断应用频度调查
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作者 谭展 邹剑明 +2 位作者 黄伟旭 余宏伟 瞿鹏翔 《中华放射医学与防护杂志》 CAS CSCD 北大核心 2019年第5期381-387,共7页
目的了解广东省医用X射线诊断医疗照射频度水平,为放射诊断的合理应用提供参考依据。方法采用分层典型抽样的方法,选取广东省广州市、惠州市、江门市、茂名市、梅州市、潮州市和肇庆市等7个地区的56家医疗机构进行X射线诊断医疗照射频... 目的了解广东省医用X射线诊断医疗照射频度水平,为放射诊断的合理应用提供参考依据。方法采用分层典型抽样的方法,选取广东省广州市、惠州市、江门市、茂名市、梅州市、潮州市和肇庆市等7个地区的56家医疗机构进行X射线诊断医疗照射频度情况调查,用调查样本的相关变量(医院总人次数、门急诊量、住院人数、放射工作人员数、放射诊断设备数)做多重线性回归分析,将有相关性的自变量拟合线性回归方程,以此来估算广东省2016年普通X射线诊断检查、CT诊断检查频度和医用X射线诊断检查的总频度。结果胸部摄影在普通X射线诊断检查中样本例数最多,占比为60.84%;颅脑检查在CT诊断检查中的样本例数最多,占比为28.03%;心血管介入在介入放射学检查中的样本例数最多,占比为48.83%;普通X射线诊断检查、CT诊断检查和介入放射学检查均以"40~"年龄组例数所占比例最高,占比分别为47.95%、65.41%和75.64%;各类型的医用X射线诊断检查均集中在三级医院和二级医院。以此调查结果估算广东省2016年普通X射线诊断检查频度为699次/千人口,CT诊断检查频度为293次/千人口。广东省2016年医用X射线诊断检查的总频度为992次/千人口。结论此次频度估算的结果有一定的局限性,但以较少的投入完成了此次调查。调查结果显示,广东省医用X射线诊断的频度水平发展较快,三级医院和二级医院的医用X射线诊断检查类型较多,有关部门应有针对性地加强受检者所受医疗照射的放射防护,积极引导正确合理应用放射诊断,努力实现趋利避害。 展开更多
关键词 放射诊断 医疗照射 频度
基于介电特性的青萝卜干燥品质预测模型 预览
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作者 高亚平 安峰 +4 位作者 赵东海 吴龙 徐庆 郑兆启 王瑞芳 《天津科技大学学报》 CAS 2019年第4期63-71,共9页
以青萝卜为实验物料,利用网络分析仪,采用同轴探头法测量了在16~3500MHz的频率范围内,湿基含水率为15%~93.20%,物料温度为(36±1)℃的青萝卜的介电常数ε'和介质损耗因子ε".分析了频率、含水率、糖度、密度与介电特性的关... 以青萝卜为实验物料,利用网络分析仪,采用同轴探头法测量了在16~3500MHz的频率范围内,湿基含水率为15%~93.20%,物料温度为(36±1)℃的青萝卜的介电常数ε'和介质损耗因子ε".分析了频率、含水率、糖度、密度与介电特性的关系,建立了915MHz和2450MHz下介电特性与含水率、糖度、密度的关联模型,检验了基于介电特性预测含水率、糖度、密度的可行性.结果表明:在16~3500MHz频率段内,介电常数ε'和介质损耗因子ε"随着频率的增大而减小.介电常数ε'随着含水率的增大而增大,介质损耗因子ε"在湿基含水率为15%~60%时,随着含水率的增大而增大,之后随着含水率的增大而降低.糖度与含水率呈负相关.可用二元一次方程描述含水率、糖度、密度与介电特性的关系,对模型进行方差分析,各模型的决定系数均大于0.92,P值小于0.05,表明所得基于介电特性预测干燥过程中青萝卜的含水率、糖度、密度的模型是准确的. 展开更多
关键词 青萝卜 介电特性 频率 湿基含水率 糖度 密度
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Phase–amplitude model for doubly fed induction generators 预览
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作者 Hua HUANG Ping JU +3 位作者 Xueping PAN Yuqing JIN Xiaoming YUAN Yuan GAO 《现代电力系统与清洁能源学报(英文)》 CSCD 2019年第2期369-379,共11页
The doubly fed induction generator(DFIG) is major type of wind turbine generator used in grid-connected wind farms. Practical models of DFIG have been built to study the influence of wind power generation on power sys... The doubly fed induction generator(DFIG) is major type of wind turbine generator used in grid-connected wind farms. Practical models of DFIG have been built to study the influence of wind power generation on power system dynamics. However, most existing practical models of the DFIG are based on rectangular coordinates,in which frequency variation is neglected. In this paper, a phase-amplitude(P-A) model is proposed for a DFIG based on phase and amplitude of the internal voltage. The model structure is much like that of the synchronous generator, and the rotor voltage can manipulate both the amplitude and the phase of the internal voltage.Comparisons have been made between the new P-A model of the DFIG and the synchronous generator model,as well as the asynchronous motor model.The contributions of the new P-A model of the DFIG are discussed and it is demonstrated that the proposed model has better ability in describing power system dynamic phenomena such as voltage dynamics and structural dynamics. Simulation results and a field test validate these contributions. 展开更多
关键词 DOUBLY fed induction generator(DFIG) MODEL Internal VOLTAGE VOLTAGE DYNAMICS Frequency DYNAMICS
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