HeⅡ的粘度理论分析与计算

在一般的分析与计算中,超流氦的粘度往往成为一个被忽视甚至被省略的一个物理量.但是在某些低温系统中,需要完成超流氦长距离输送,粘度成为确定其流动阻力损失的一个重要物理量.目前各具特色的理论和计算关系式很多,但是迄今为止,还没有任何一种理论体系或计算关系式可以适用于所有的物质或所有的温度和压力范围,因此如何应用和借鉴现有的粘度理论或关系式以及实验结果,实现对HeⅡ这种特殊流体粘度的分析和计算是一项十分重要的工作.     

超流氦浴中的热波传热研究

热波传热机制是超流氦传热非常重要的一个方面.在小热流密度的情况下,超流氦中的热波完全保持加热热流的波形,热量的传输完全靠热波来完成;随着热流密度的增加到一定程度,会在超流氦浴中激发量子涡旋.量子涡旋使热波发展成为热激波.在开放氦浴中,热波的波形不同于狭窄通道里的热波,在热波的尾部会出现一个冷却波;并且随着加热时间的变化,冷却波的形状和幅度会发生很大的变化.运用二流体模型和涡旋方程对超流氦中的热波进行了计算,实验结果与计算结果吻合得较好. 关键词： 超流氦 热波 量子涡旋 热激波     

超流氦相分离器临界速度计算方法与比较

超流氦在空间探测的极低温制冷系统中被广泛应用,在研究超流氦通过圆管或者狭缝时,需要分清超流氦在圆管或者狭缝中的流动状态,从而应用不同的模型来计算超流氦的制冷量。根据查阅已有的相关文献资料,列举分析不同的超流氦临界速度计算方法,然后进行比较。     

超流氦多孔塞相分离器的研究综述

超流氦恒温器是远红外探测器冷却系统的主要设备,气液相分离器则是超流氦恒温器的关键部件,能有效完成超流氦的气液分离,防止泄漏并实现空间液体的综合管理。文中介绍了在空间红外探测中应用较多的多孔塞相分离器的工作原理,描述了多孔塞的流量特性曲线,并对曲线上各工作区域的质量流量与温差(压差)的关系式进行了介绍。文中还对多孔塞的制备工艺进行了分析,并结合国内外研究进展给出了合理选择多孔塞所需要注意的相关参数。     

超流氦中的热波实验研究

在本文中,采用新型的超导传感器对超流氦浴中的热波在不同的热流密度及氦浴温度下进行了测量,同时研究了超流氦浴中的热激波现象,并且与理论分析进行了比较。     

超流氦噪声沸腾中的压力测量

噪声沸腾是涉及超流氦传热和超流氦物理中的一个特殊现象，对它的充分认识对于超导磁体等的应用有重要的意义，在本实验中，对一个大小为２．５ｃｍ×２．５ｃｍ的小试样进行瞬态加热，并借助一个压力传感器，从１．７Ｋ到的点，从饱和压力到几百毫米汞柱压力，详细研究了超流氮噪声沸腾中压力振动，同时对常流态氮Ｉ中的沸腾中也进行了比较研究 。     

稀玻色气体的超流动性

1925年爱因斯坦发表了著名的关于玻色凝聚的文章以来,人们一直期望能在实验上得到证实.为解释超流波体氦(4He)不寻常的性质,1938年他敦(F·London)提出液体氦从正常相到超流相的相变是一种坡色凝聚现象,其主要根据是计算出的凝聚温度3.1K和超流相变温度2.17K很接近,这种看法大体正确.但是严格地说,液体氦是强相互作用的多体系统,并不是理论所讨论的可以忽略相互作用的理想的玻色系统,这反映在某些性质和理论结果有定性上的不同.在压强增高,单位体积的4He原子数n增加时,按照理论,凝聚温度Tc正比于n2/s,Tc应上升,实际上却是下降的.此外,在…     

超流氦传热中的特殊物理问题：静压及范德瓦尔斯压力对传热的影响

本以ＨｅＩＩ传热的基本规律及ＨｅＩＩ的相图为基础，研究了压力对ＨｅＩＩ传热的影响。在压力小于λ点压力的情况下，得出了超流氦浴中的ＨｅＩＩ静压以及范德瓦尔斯压力对临界热负荷的影响关系，并分析了无噪声沸腾，噪声沸腾工况下临界热负荷关系式的特点。在压力大于λ点压力的情况下，得出了超流浴中临界热负荷的一般关系解。     

低温传热中的热波现象及其对超流氦膜沸腾发生的影响

热波是低温传热中的常见现象，对热波现象的研究具有重要理论和实用价值．本文研究了固体和超流氦液体中的热波．对超流氦中的非稳态传热研究表明，热脉冲作用下的膜沸腾形成过程可由超流氨中的第二声（热波）进行很好的描述．     

美国物理年会热议固态氦超流

美国物理学会年会于2007年3月5—9日在科罗拉多州首府丹佛召开．期间固态氦超流再次成为热门话题．2004年，M．Chen和E．Kim构建了一个固态氦扭摆（小罐中的氦始终保持加压状态）．他们发现，当温度降到几十mK，小罐的扭动频率突然增加．这意味着小罐整体的转动惯量突降，罐中的部分固态氦不再参与摆动，而是脱离了主体，并且可以无阻地在主体固态氦之间穿插（见Science，1 July 2005，p38）．这一现象，当时被认为是固态氦超流．不过，一些理论物理学家很快就论证了：上述穿插图像在有序晶体中是完全不可能的．替代的解释是：残存在固态氦中的传统超流液氦以谕渗方式无阻地穿越固态氦晶体中的缺陷（《固态氦也能超流吗？》一文刊登在《物理》，2004，33（6）：468）．     

Structure of metallic nanowires and nanoclusters formed in superfluid helium

It is shown that metallic nanowires (5–8 nm in diameter) that form during laser ablation of Ni, Pb, In, and Sn targets embedded in HeII contain extended single-crystal segments, while spherical clusters (about 2 μm in diameter) that form under these conditions have a regular shape and an atomically smooth surface. Such structures are explained by melting of metal ablation products under their coalescence in HeII. The short-term action of a low-intensity beam of electrons with an energy of 200 keV initiates the explosion in metallic spheres preserved in the vacuum chamber of a transmission electron microscope, which is accompanied with the formation of thousands of clusters with a diameter of a few nanometers. This effect is due to metastability of internal mechanical stresses produced upon sharp cooling of molten spheres by liquid helium. A mechanism of condensation of atoms and nanoparticles in quantized vortices of superfluid helium is proposed.     

On the electric activity of superfluid helium at the excitation of first and second sound waves

We show that the electric activity of superfluid helium (HeII) observed in the experiments [3] during the excitation of standing second sound waves in an acoustic resonator can be described in terms of the phenomenological mechanism of the inertial polarization of atoms in a dielectric, in particular, in HeII, when the polarization field induced in the medium is proportional to the mechanical acceleration, by analogy with the Stewart-Tolman effect. The variable relative velocity w = v _n − v _s of the normal and superfluid HeII components that emerges in the second sound wave determines the mean group velocity of rotons, V _g ≈ w, with the density of the normal component related to their equilibrium number density in the temperature range 1.3 K ≤ T ≤ 2 K. Therefore, the acceleration of the 4He atoms involved in the formation of a roton excitation is proportional to the time derivative of the relative velocity.w. In this case, the linear local relations between the variable values of the electric induction, electric field strength, and polarization vector should be taken into account. As a result, the variable displacement current induced in the bulk of HeII and the corresponding potential difference do not depend on the anomalously low polarizability of liquid helium. This allows the ratio of the amplitudes of the temperature and potential oscillations in the second sound wave, which is almost independent of T in the above temperature range, consistent with experimental data to be obtained. At the same time, the absence of an electric response during the excitation of first sound waves in the linear regime is related to an insufficient power of the sound oscillations. Based on the experimental data on the excitation of first and second sounds, we have obtained estimates for the phenomenological coefficient of proportionality between the polarization vector and acceleration and for the drag coefficient of helium atoms by rotons in the second sound wave. We also show that the presence of a steady heat flow in HeII with nonzero longitudinal velocity and temperature gradients due to finite viscosity and thermal conductivity of the normal component leads to a change in the phase velocities of the first and second sound waves and to an exponential growth of their amplitudes with time, which should cause the amplitudes of the electric signals at the first and second sound frequencies to grow. This instability is analogous to the growth of the amplitude of long gravity waves on a shallow-water surface that propagate in the direction of decreasing basin depth.     

抽真空获取1.8K超流氦方案分析以及真空泵抽速的预测

低温地面支持系统(CGSE)是专为AMS-02国际合作科学实验项目开发研究的低温地面支持和冷却系统,该系统要完成将AMS-02超导磁体从环境温度到1.8K的冷却过程,超流氦(HeII)的获得是其重要环节。文中针对抽真空获取HeII的方案进行了比较详细的分析和计算,同时针对此方案给出了其物理过程的T-s图、流程简图和实现该过程的主要设备。最后针对此方案,对真空泵抽速进行了预测,这将对后续阶段真空泵的选型具有指导意义。     

Trace detection of metastable helium molecules in superfluid helium by laser-induced fluorescence

We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable He2 molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid 4He.     

Hydrodynamics of superfluid helium in a single nanohole

The flow of liquid helium through a single nanohole with radius smaller than 25 nm was studied. Mass flow was induced by applying a pressure difference of up to 1.4 bar across a 50 nm thick Si(3)N(4) membrane and was measured directly by means of mass spectrometry. In liquid He I, we experimentally show that the fluid is not clamped by the short pipe with diameter-to-length ratio D/L?1, despite the small diameter of the nanohole. This viscous flow is quantitatively understood by making use of a model of flow in short pipes. In liquid He II, a two-fluid model for mass flow is used to extract the superfluid velocity in the nanohole for different pressure heads at temperatures close to the superfluid transition. These velocities compare well to existing data for the critical superflow of liquid helium in other confined systems.     

Optical studies of atoms and ions in superfluid helium using a ccd-camera system

The spectroscopic study ions and atoms immersed into liquid helium can contribute to the understanding of the structure of pointlike defects in helium and their interaction with the superfluid phase as well. Ions and atoms serve as microprobes in the form of so calledbubble orsnowball type defects in the quantum fluid. The optical emission of these structures is recorded. From the optical spectra of previous experiments the influence of the surrounding helium on the electronic configuration of the impurity atoms or ions was examined. In this experiment the light emitted from the defect atoms is observed by a camera. The pictures obtained yield information about the distribution and the motion of the defect particles in the superfluid. As an example the fluorescence light resulting from the recombination of magnesium, barium and thallium ions with excess electrons in superfluid helium was recorded.     

Laser spectroscopy and optical pumping of alkali atoms in superfluid helium

In this paper, our recent works on the alkali atoms in superfluid helim (HeII) are reported. At first we mentions the laser-sputtering method for implantation, which is simple but is very efficient to produce various kinds of neutral atoms and molecules in HeII. Secondly, we report on the laser spectroscopy of alkali atoms in HeII. Optical excitation and emission spectra are found to be roughly explained by a spherical atomic bubble model, but the spectra corresponding to the D₂ lines indicate the quadrupole oscillation of the bubble shape. Optical pumping by a circularly polarized laser beam is found to produce perfect polarization, for both electron and nuclear spins. Using the rf-optical double resonance techniques, the magnetic and hyperfine resonances are observed. It is discussed also about the phenomena which have observed in the experiments done so far but have not been fully explained.     

Observation of a superfluid phase in solid helium

Evidence of a superfluid liquid phase present in polycrystalline helium at a temperature of 0.2 K and a pressure of 51 bar has been obtained by means of inelastic neutron scattering. The superfluid component is absent at a temperature of 0.6 K and the same pressure. Thus, a “solid helium-superfluid helium” phase transition has been discovered. The sample of solid helium in a porous medium (silica aerogel) has been prepared with the use of a capillary blocking technique. The shape of the structure factor of the superfluid phase indicates the presence of clusters or the effects of a restricted geometry. The results may be used to explain the nonclassical rotational inertia phenomenon in solid helium (often referred to as supersolidity, Nature, 2004).     

The Fulde-Ferrell state in superfluid helium

The Fulde-Ferrell state in a superfluid³He-⁴HeII solution is investigated. Starting from its broken symmetry, a relative translation-gauge symmetry, the linearized hydrodynamic equations to lowest order in wave number are presented. To this order, the superfluid current perpendicular to the preferred direction vanishes identically; at the same time, supercurrents generated by a uniform change in temperature or density become possible. The Goldstone mode of the Fulde-Ferrell state is shown to be a propagating spin-temperature-shear wave.     

Adsorption and superfluid onset of He films on intermediate strength substrates

On strong binding substrates, such as graphite or mylar that are wetted by ⁴He at all temperatures, an adsorbed ⁴He film consists of 2 atomic layers of “inert” helium covered by a liquid layer that becomes superfluid via a Kosterlitz–Thouless (KT) transition. On weak substrates, for example cesium, superfluid onset above the wetting temperature also conforms to the KT picture. In contrast, superfluid onset on intermediate strength substrates, specifically heavier alkali metals and monolyer films of cesium on gold, deviates strongly from KT behavior. Here we describe superfluid onset of ⁴He on intermediate strength substrates and discuss the contributions of weak bindings and disorder to the non-KT behavior.