小型氢激射器的性能

小型氢激射器虽已制成,然而研制工作仍在几个研究机构里继续进行。美国喷气推进实验室(JPL)设有专门实验室,致力于新型频率标准的检验、测定、改进和维修等工作。本文发表了两种不同类型的小型氢激射器的一系列测试结果。这些测试工作是由JPL的频率标准实验室完成的。在这些小型激射器中,有一台是样机,是加州Malibu的休斯(Hughes)研究实验室专为美国海军研究实验室研制的。这种激射器用增强Q的办法激起腔体振荡,并备有腔体调谐伺服系统;该伺服系统是由科罗拉多州波尔德的美国国家标准局研制的。本文给出了如下一些数据:输出频率与某些环境参数的关系、阿仑方差和漂移。休斯研究实验室所研制的激射器,是美国海军研究实验室(NRL)资助的。NBS研制的无源激射器。则由美国国家航空与宇宙航行局(NASA)及NRL共同资助。     

利用滤波技术和有效值电压表进行频率稳定度的时域测量

传统的频率稳定度的时域测量都是用计数式频率计采集数据,然后用统计方法处理的。本文介绍利用滤波技术和有效值电压表进行频率稳定度的时域测量技术。这种测试技术具有测试设备简单、测试方法简便,既能给出阿仑方差、滤波方差,又能给出相位噪声谱密度的优点。目前国内外正在推广这种测试短期频率稳定度的方法。     

8mm可调稳幅源

一、引言近年来,随着我国毫米波技术的迅速发展,需要建立毫米波计量标准。一个幅度稳定的信号源,无论是在建标工作中,还是在毫米波日常测试工作中,都是必不可少的。目前,我国的毫米波固态源大多是固定频率的,即使有频率可调的,频率稳定度和幅度稳定度也都较差,难以满足计量测试工作的要求。而速调管毫米波源,调节频率麻烦,体积大,价格贵,频率和幅度稳定度也不太理想。又由于一些国家在毫米波技术、设备方面对我国实行严格的禁运政策,即使能引进某些产品,也只能是小功率的源,且价格十分昂贵。这种小功率的源在大多数计量测试工作中都不能满足工作需要。为此,我们和中国科技大学协作研制了     

脉冲雷达频率稳定度的测试方法

雷达发射机频率稳定度严重地限制雷达改善因子和可见度。现在连续波频率稳定度测量技术已达很高水平,但现代雷达多半采用脉冲调制体制,射频脉冲序列稳定度是一个新问题。本文给出测试系统原理图、脉间频率和相位稳定度测量方法以及时域、频域测量结果。     

利用Q倍增谐振器改善信号源稳定度的自动频率控制系统

本文实现了一种利用有源Q倍增谐振器稳定振荡器的自动频率控制系统。分析了该锁频环路的静态响应和动态响应,给出了环路传递函数和噪声模型。理论和实验结果都证明该系统可以明显地改善信号源的短期频率稳定度。     

影响氢原子钟长期稳定度的系统效应分析

进一步改进氢钟的频率稳定度，需要了解影响频率稳定度的各个过程及其影响的相对大小。讨论影响氢钟频率稳定度的各种系统过程。这种分析能够使钟的研制者和用户去识别稳定度的限制因素并加以选择以便改进。     

一种用于测量谐振频率和Q值的数字系统

最近研制和测试了一种用于准确测量射频和微波谐振电路的谐振频率和Q值的计算机控制的自动系统。在此系统中,谐振曲线的拐点利用联机数字计算机通过数学计算过程进行识别,并利用它计算谐振频率和Q值。这个系统的主重优点是它的结果与谐振电路谐振频率的性能无关,与信号源的3dB点、幅度、频率漂移无关。也与检测器的不稳定性无关。设计、安装并测试了一种在1—2千兆赫之间工作的试验系统,用实验室小型计算机PDP11/34进行数据采集与处理以及对整个系统的控制。Q值的测量范围在100—1500之间,估计的谐振频率的不确定度为40ppm,Q值为1000的谐振电路的Q值的不确定度为百分之七。     

超导腔稳频红宝石激射器振荡器

本文介绍一种新型全致冷频率源——超导腔稳频激射器振荡器(SCSMD)的稳定度分析、性能及设计。文中还将介绍该振荡器的各个部件的实验研究结果。它将充分证明采用这一技术方案有可能使频率稳定度达到优于△f/f=10~(-17)的水平。以前设计超导腔稳频振荡器,采用低温下的铌腔与室温下的微波电子器件相结合的方案。但据报道这种方案所能达到的最佳频率稳定度仅为△f/f=3×10~(-16)。这主要是因电子系统与腔体互连的部分不稳定影响了长期性能。但全致冷的方案却不存在这一问题,因为热膨胀系数被冻结(freezing-out)以及完全没有热梯度。红宝石激射器在目前所有微波放大器中噪声温度最低(1.5k),看来它是用于全致冷振荡器的较理想的器件。它的有效输出功率(～10~(-7)W/cm~3在4.2k)足以用来进行时间长于一秒的测量。另外,它具有相当高的增益(Q_m=-100)使得仅需振荡器与稳定腔之间保持很弱的耦合即可。此外,它本身的功耗也很小,即使在低至1k时也能与腔体工作在同一温度环境条件下。但激射器的工作和调谐却需外加磁场,这就产生了一个很重要的技术问题,即需要对超导腔加以磁屏蔽,同时磁场的变化还将产生频率的牵引。为此,文中介绍了采用多腔方案来解决这一问题。该方案将红宝石部件与超导腔隔开一段距离。我们还将谈到具有较大频率牵引效应的低Q腔稳定的激射器振荡器的频率稳定度。最后,本文将介绍有关研制结构最坚固的超导腔的结果。兰宝石的热膨胀系数仅为铌的百分之一,其微波损耗在1.5k时,目前已达的最低值为7×10~(-10)。在兰宝石上镀超导体的谐振腔可达(按设计要求值)Q=10~8。     

氢脉泽谐振腔频率-温度效应分析

进一步改进氢钟的频率稳定度,要求了解影响频率稳定度的各个过程及其影响的相对大小。讨论影响氢钟谐振腔频率的各种热动力学过程。这种分析能够使钟的研制者和用户去识别稳定度的限制因素并因此加以选择以便改进频率稳定度。     

基于EMD的氢钟频率预报方法研究CSCD

目前用于时间保持的主要频率源类型是铯钟和氢钟。氢钟具有优良的短期稳定度和良好的低噪声特性,非常适合做主钟频率源。然而绝大数氢钟具有明显的频率漂移,使氢钟的长期稳定度随着时间的推移不断降低。因此在时间保持应用中,必须对氢钟的频率偏差和漂移进行准确的预测,才能充分发挥其作用。本文利用经验模态分解方法提取氢钟频率变化的趋势项,实现氢钟频率预报,并利用该预报频率对其输出信号进行模拟驾驭。结果表明:该方法正确、有效,不仅保持氢钟短稳优秀的特性,而且改善了其长期稳定度。     

Scaling analysis and model estimation of solar corona index

A monthly average solar green coronal index time series for the period from January 1939 to December 2008 collected from NOAA (The National Oceanic and Atmospheric Administration) has been analysed in this paper in perspective of scaling analysis and modelling. Smoothed and de-noising have been done using suitable mother wavelet as a pre-requisite. The Finite Variance Scaling Method (FVSM), Higuchi method, rescaled range (R/S) and a generalized method have been applied to calculate the scaling exponents and fractal dimensions of the time series. Autocorrelation function (ACF) is used to find autoregressive (AR) process and Partial autocorrelation function (PACF) has been used to get the order of AR model. Finally a best fit model has been proposed using Yule-Walker Method with supporting results of goodness of fit and wavelet spectrum. The results reveal an anti-persistent, Short Range Dependent (SRD), self-similar property with signatures of non-causality, non-stationarity and nonlinearity in the data series. The model shows the best fit to the data under observation.     

Stochastic particle acceleration by the forced interaction of relativistic current sheets

We address the problem of interacting relativistic current sheets in self-consistent kinetic plasma simulations within the framework of the Particle-In-Cell model. The interaction is enforced in head-on collisions of up to 10 current sheets at relativistic bulk speeds. The simulations are motivated by the general problem of Poynting flux dissipation in ‘striped wind’ configurations presumably governing the relativistic outflows pervasive in pulsar winds and gamma-ray bursts. We identify the generation of non-thermal particles and formation of a stable power-law shape in the particle energy distributions f(γ) dγ ∝ γ^−s dγ. In 1D, a spectral index s ∼ 2 is observed and attributed to a stochastic Fermi-type acceleration mechanism. In 2D, the generic index of s ∼ 3–4 is retained as in previous simulations of individual current sheets. Whereas in 2D the high energy cut-off is constrained by the limited dissipation of magnetic energy, in 1D the process converts the bulk motion of current sheets towards directed particle momentum of an exclusive class of non-thermal particles.     

Development of a gas chromatography compatible Sample Processing System (SPS) for the in-situ analysis of refractory organic matter in martian soil: preliminary results

In the frame of the 2009 Mars Science Laboratory (MSL) mission a new sample preparation system (SPS) compatible with gas chromatography–mass spectrometry (GC–MS) has been developed for the in situ analysis of complex organic molecules in the Martian soil. The goal is to detect, if they exist, some of the key compounds that play an important role in life on Earth including carboxylic acids, amino acids and nucleobases.     

Concept options for the aerial survey of Titan

Various aerial platforms intended for long endurance survey of the Titan surface are presented. A few novel concepts are introduced, including a heated methane balloon and a balloon with a tethered wind turbine. All the concept options are predicted to have lower scientific payload fractions than the Huygens probe. It is concluded that the selection of the best aerial platform option depends on more accurate mass estimates and a clear decision on whether, or not, in situ surface composition measurements are required in conjunction with aerial remote sensing.     

Heterogeneous solid/gas chemistry of organic compounds related to comets,meteorites, Titan,and Mars: Laboratory and in lower Earth orbit experiments

To understand the evolution of organic molecules involved in extraterrestrial environments and with exobiological implications, many experimental programs in the laboratory are devoted to photochemical studies in the gaseous phase as well as in the solid state. The validity of such studies and their applications to extraterrestrial environments can be questioned as long as experiments conducted in space conditions, with the full solar spectrum, especially in the short wavelength domain, have not been implemented. The experiments that are described here will be carried out on a FOTON capsule, using the BIOPAN facility, and on the International Space Station, using the EXPOSE facility. Vented and sealed exposition cells will be used, which will allow us to study the chemical evolution in the gaseous phase as well as heterogeneous processes, such as the degradation of solid compounds and the release of gaseous fragments.     

基于LuGre模型的电液加载系统摩擦补偿

为了提高电液加载系统控制精度,针对摩擦问题提出了基于LuGre摩擦模型的前馈补偿方法.建立了用于摩擦仿真分析和补偿器设计的电液加载系统数学模型;通过实验获取并分析了相关的摩擦数据;基于实验数据进行LuGre模型参数辨识,把LuGre模型和辨识结果引入电液加载系统数学模型,并进行仿真结果与实际摩擦数据的对照,证明了LuGre摩擦模型的准确性.设计前馈补偿器,进行了实验对比,实验结果表明前馈补偿器可将摩擦产生的控制误差有效地降至未补偿时的30%左右.      

CoRoT and Kepler results: Solar-like oscillators

The space-borne observatories CoRoT (Convection Rotation and planetary Transits) and Kepler have provided photometric time series data of unprecedented precision for large numbers of stars. These data have revolutionized the fields of transiting exoplanets and asteroseismology. In this review some important asteroseismic results obtained using data from the CoRoT and Kepler space missions concerning stars that show solar-like oscillations are discussed. These results comprise, among others, measurements of the location of the base of the convection zone and helium second-ionization zone in main-sequence stars, the presence (or not) of core-helium burning in red-giant stars, as well as differential rotation in these stars.     

The ESA scientific programme

The ESA scientific programme has, so far, provided several significant astrophysics experiments and further important missions are scheduled for execution during the next decade. These missions are briefly summarised together with several astrophysics investigations presently under study.     

Return to Europa: Overview of the Jupiter Europa orbiter mission

Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Subsequently, the Galileo spacecraft supplied fascinating new insights into this satellite of Jupiter. Now, an international team is proposing a return to the Jupiter system and Europa with the Europa Jupiter System Mission (EJSM). Currently, NASA and ESA are designing two orbiters that would explore the Jovian system and then each would settle into orbit around one of Jupiter’s icy satellites, Europa and Ganymede. In addition, the Japanese Aerospace eXploration Agency (JAXA) is considering a Jupiter magnetospheric orbiter and the Russian Space Agency is investigating a Europa lander.     

HELIO: The Heliophysics Integrated Observatory

Heliophysics is a new research field that explores the Sun–Solar System Connection; it requires the joint exploitation of solar, heliospheric, magnetospheric and ionospheric observations.