紧凑型L波段同轴相对论返波振荡器的粒子模拟

 设计了紧凑型L波段同轴相对论返波振荡器，通过粒子模拟研究了L波段同轴相对论返波振荡器相互作用的物理过程，并对器件的电磁结构进行了优化和改进。分析表明，采用同轴慢波结构可以在较低的外加磁场下实现L波段返波振荡器的微波输出，同时可以大大减小微波器件的径向尺寸。这是因为同轴慢波结构的TM01模式有类似于TEM模的性质，没有截止频率，但纵向电场不为零，电子束能够与它发生强相互作用过程。粒子模拟优化结果表明，在器件半径仅为4.0 cm，电子束能量240 keV，电子束流1.8 kA，导引磁场仅为0.75 T时，返波振荡器可以在频率1.60 GHz处获得较大功率的微波输出， 平均峰值功率达140 MW，平均峰值功率效率约为32%。     

压裂液返排冲刷支撑剂颗粒的DEM-CFD模拟研究

支撑剂回流是压裂液返排过程中重点问题之一。目前普遍认为压裂液返排流速对于支撑剂颗粒堆的稳定性具有重要影响,并有研究明确了压裂液返排的临界流速条件。本文通过计算流体力学和离散颗粒元(CFD-DEM)的耦合仿真,对目前已有的支撑剂回流临界返排流速计算模型进行验证,研究了支撑剂颗粒参数(粒径和密度)和流道次缝对于临界返排流速的影响并揭示了相应的流动机理。研究结果可为优化压裂液返排流程提供理论指导。     

压力对返料器性能影响的实验研究

为研究加压循环流化床返料器的返料特性,在下降段和上升段直径均为35 mm的小型高压实验台上研究压力、返料风流化数和颗粒粒径对返料流率的影响.返料器流化风流化数N1和松动风流化数N2不变时,压力和颗粒粒径对返料流率没有明显影响;当松动风流化数N2<3时,返料流率随流化风流化数N1的增大而增大;当松动风流化数N2≥3时,返料器达到最大返料流率,增大流化风流化数N1对返料流率没有影响.研究结果可以为高压返料器的设计和运行提供指导.     

真空系统返油红外光谱测定法

真空系统返油是使用有油真空抽气机组一直存在的问题.返油来源于真空机械泵的密封油及真空油扩散泵的工作泵油.返油直接影响真空泵的极限真空和真空系统的污染程度.近年来由于工业生产和研究部门要求真空泵返油率极低和真空系统的超净.这一问题就显得更为突出了. 由于真空油扩散泵价廉,使用也方便.已成为大量使用的获得真空的为设备,而返油率是它重要的性能指标之一.英国、联邦德国、美国和日本都有测量标准,但这些标准都只对测量高返油率适用.目前真空油扩散泵制造已向低返油率方向突进,低返油率的测量已成为需要解决的问题。例如,英国Edw…     

回旋行波放大器输出端反射对其稳定性的影响

 基于回旋行波放大器的线性理论和反射边界条件,建立了渐变输出端存在反射时回旋行波放大器模型,分析了TE₀₁圆电模基波回旋行波放大器输出端反射对其增益、返波起振长度、返波起振电流和返波起振频率的影响。结果表明,随着反射增强,线性增益降低,返波起振长度和起振电流减小,返波起振频率的变化较小;反射增强将在一定程度内影响放大器的稳定性。     

美陆军封杀最大武器项目"科曼奇"直升机

2月 2 3日 ,美国陆军宣布取消总预算高达 390亿美元的“科曼奇”武装侦察直升机研发项目 ,这是美军继取消耗资 2 0亿美元的“十字军”火炮计划之后取消的又一个大型项目。“科曼奇”计划从 1 983年开始实施以来 ,历时 2 1年 ,耗资 69亿美元。已经研制成 RAH- 66“科曼奇”和“科曼奇长弓”两种型号 ,采用了直升机中前所未有的全面隐身设计、先进的导航、目标瞄准系统和计算机辅助侦察系统等一系列先进技术。原计划用它代替 JH- 5 8“基奥瓦”和 AH- 1“眼镜蛇”,并协助 AH- 64“阿帕奇”发现目标 ,也一度希望能与“阿帕奇”一起形成 2 1…     

螺旋线行波管三维返波互作用理论与数值模拟

建立了三维非线性返波互作用模型,用于精确分析大功率螺旋线行波管中返波振荡非线性过程问题,并提出了计算返波振荡功率的方法及磁场抑制手段.该理论模型包括三维线路场方程、三维运动方程以及三维空间电荷场.首先比较三维模型与原有一维模型之间的差异,发现一维空间电荷场的径向交流电流分布模型与三维模型的差异是导致振荡频率偏大及起振长度缩短的主要原因.然后计算返波饱和输出功率大小并揭示返波饱和功率和振荡频率与互作用长度的关系,并探讨了磁场对返波振荡的抑制影响.最后以某一毫米波行波管为例,实验对比了一维与三维模型计算的振荡频率与热测的差异,其中三维模型的相对误差小于4.8%.     

大曲率半径球形光学列阵的菲涅耳成象与赝位相共轭特性

本文利用菲涅耳衍射积分公式,计算了大曲率半径球形光学列阵的成象特性,获得了成象公式及其成立的条件。文中证明了,自返平面列阵具有补偿畸变的能力,不是由于产生位相共轭反射波,而是由于自返光线的叠加和自返元件的特性。 关键词：     

返波型行波电子直线加速结构的理论研究

比较了行波与驻波两种工作方式以及前向波与返波型行波的不同特点;研究了返波型行波电子直线加速结构的工作特性;并针对返波型行波电子直线加速结构的特点,进行工作模式的选择;对工作频率为2856MHz,3π/4模的返波加速结构进行了优化设计;研究了带聚束段的返波型行波加速管的纵向与横向粒子动力学问题;为海关大型集装箱在线检测用加速器设计了一根9MeV返波型行波加速管,并与采用盘荷波导结构的加速管进行了对比.研究结果表明,返波型行波电子直线加速结构具有一定的优势,它把鼻锥型加速结构分流阻抗高及盘荷波导行波工作方式填充时间短,工作稳定性好等优点结合起来,性能明显优越.     

钻井气液两相流体溢流与分布特征研究

常规钻井返出管线敞口,钻井液在返出管线中流动为非满管流动,小气侵量时,泥浆池液面变化不明显,检测装置无法测量液面变化,钻井液返出流量无法测量或者测量误差较大,无法感知小流量差的变化。本文基于气液两相流理论建立了井筒气液两相流参数与井口溢流速度的关系模型,计算表明气侵速度越大,气泡运移距井口越近,井口溢流速度越大。并以空气和钻井液(非牛顿流体)为介质,基于VOF多相流模型,模拟了钻井液返出流量变化时气液界面变化情况,分析了纯钻井液和不同含气率钻井液在返出管线流动时液面变化特征,得出纯钻井液流动时喇叭口处液面变化最明显,优化液面检测装置安装在喇叭上,更能准确及时地检测液面变化早期发现溢流,进气速度影响返出管线钻井液液面稳定性。     

虚阴极振荡器“硬管化”实验研究

 采用锆泵作为吸气泵，设计出静态保真空模拟实验装置。在该实验装置上进行了保真空可行性实验，装置在经过10 d的保真空后，真空度为1.4×10^-3 Pa 。在模拟保真空实验成功实现的基础上，用常规微波管技术设计加工了“硬管”器件，并进行保真空实验，静态下真空度经过15 d仍能维持在2×10^-2 Pa。该器件在峰值电压为330 kV的条件下，输出微波功率达到650 MW，脉宽为40 ns，主频为3.68 GHz，性能比普通器件有很大改善，微波脉宽更宽，频谱更单一，而且稳定性很高。     

Listeners' sensitivity to "onset/offset" and "ongoing" interaural delays in high-frequency, sinusoidally amplitude-modulated tones

The relative potency of onset/offset and envelope-based ongoing interaural time delays (ITDs) was assessed using high-frequency stimuli. A two-cue, two-alternative, forced-choice adaptive task was employed to measure threshold ITDs with 100% sinusoidally amplitude-modulated tones centered at 4 kHz. Modulation rates of 125, 250, and 350 Hz were tested with durations of 32, 90, or 240 ms. In the first experiment, ITDs to be detected were imposed only at the onset/offset, only within the ongoing portion, or within both the onset/offset and ongoing portions of the stimuli. Results indicated that ongoing ITDs dominated onset/offset ITDs. The relative potency of ongoing ITDs was directly proportional to duration and inversely proportional to modulation rate. Quantitative analysis suggested that listeners effectively combine onset/offset and ongoing ITDs. Furthermore, the data could be largely accounted for by assuming that listeners attend to the interaural decorrelation of the stimulus resulting from onset/offset and/or ongoing ITDs. A second experiment showed that, (1) overall, an ongoing ITD of one-half period of the envelope had little impact on listeners' sensitivity to delays of the onset/offset and (2) sensitivity to delays within the onset/offset portion of the waveform was reduced by roving the delay within the ongoing portion of the waveform.     

Comment on "Optical-fiber-based Mueller optical coherence tomography"

We comment on the recent Letter by Jiao et al. [Opt. Lett. 28, 1206 (2003)] in which a polarization-sensitive optical coherence tomography system was presented. Interrogating a sample with two orthogonal incident polarization states cannot always recover birefringence correctly. A previously presented fiber-based polarization-sensitive system was inaccurately characterized, and its method of eliminating the polarization distortion caused by single-mode optical fiber was presented earlier by Saxer et al. [Opt. Lett. 25, 1355 (2000)].     

Comments on "Speech Understanding and Aging"

The hypothesis advanced by the CHABA Working Group on Speech Understanding and Aging [J. Acoust. Soc. Am. 83, 859-895 (1986)] that the systematic decline in speech understanding with age might be explained by concomitant decline in extra-auditory cognitive factors was tested by examining speech audiometric findings in patients with dementia. The fact that performance was consistent with normal central auditory function in 12 of 23 such patients, in spite of deficits in immediate memory for spoken material, tolerance of distraction, mental tracking and sequencing, cognitive flexibility, and set shifting argues against the hypothesis that speech understanding deficits in the elderly can be explained as the simple consequence of cognitive decline.     

Comment on "microstructured polymer fiber laser"

We comment on the recent Letter by Argyros et al. [Opt. Lett. 29, 1882 (2004)] in which a microstructured polymer fiber doped with the dye Rhodamine 6G was discussed as a possible fiber laser source. We suggest that the lasing action at 632 nm was due to stimulated Raman scattering in the poly(methyl methacrylate) host material.     

"Safe" Coulomb excitation of 30Mg

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation gamma-ray yields the B(E2;0(+)gs-->2(+)1) value of 30Mg was determined to be 241(31)e2 fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg resides outside the "island of inversion."     

Resonant "forbidden" reflections in magnetite

Resonant x-ray scattering was used to investigate electronic fluctuations of the octahedral iron atoms in magnetite. We measured the (002) and (006) "forbidden" x-ray diffraction reflections permitted by the anisotropy of the iron anomalous scattering factor. The energy and azimuthal angle dependencies of these reflections, and the polarization analysis, are shown and discussed. The results clearly show p and d iron empty states ordering in magnetite at room temperature. Moreover, the octahedral iron atoms are electronically equivalent in a time scale lower than 10(-16) sec. Therefore, magnetite should be considered as an itinerant magnet and not as a fluctuating mixed valence material.     

"Work-Hardenable" ductile bulk metallic glass

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (< 1%) at room temperature. We present a new class of bulk metallic glass, which exhibits high strength of up to 2265 MPa together with extensive "work hardening" and large ductility of 18%. Significant increase in the flow stress was observed during deformation. The "work-hardening" capability and ductility of this class of metallic glass is attributed to a unique structure correlated with atomic-scale inhomogeneity, leading to an inherent capability of extensive shear band formation, interactions, and multiplication of shear bands.     

Switching from "absorption within transparency" to "transparency within transparency" in an electromagnetically induced absorption dominated transition

The absorption of a resonant coupling laser driving a closed degenerate two-level system in an atomic cesium beam was investigated as a function of the detuning of a second laser probing the same transition. The measurements were performed for four different polarization combinations of the two laser beams. Except for the beams of counterrotating polarizations all coupling-laser absorption profiles showed "absorption within transparency," i.e., the absorption in the region around the two-photon resonance was smaller than the absorption corresponding to the one-photon transition induced by the coupling laser, and an extra absorption peak was observed on this curve at the two-photon resonance. With regard to the beams of counterrotating polarizations we observed a switch from absorption within transparency to "transparency within transparency" when the probe-laser power exceeded the constant coupling-laser power. In other words, the cesium ensemble became mostly transparent to the coupling-laser beam at the two-photon resonance.     

Comment on "Phase-shifting shearing interferometer"

The role of the liquid-crystal layer in the phase-shifting shearing interferometer that was proposed by Griffin [Opt. Lett. 26, 140 (2001)] is analyzed. An extra voltage-dependent phase difference is produced when the phase retarder is oriented at an angle from the optical axis of the laser beam such that the interferogram will not repeat itself after a 2pi phase change of the liquid-crystal retarder in either the common Mach-Zehnder interferometer or the phase-shifting shearing interferometer.