^4He超流转变温度复现研究

本介绍一种复现^4He超流转变点（2.1768K）的密封瓶器件，复现方法简单，易于实现，可以得长达数小时的^4He超流转变温坪。改变热流量，测出不同热流下对应的下压温坪值，外推到零热流，得到超流转变温度复现值。3只密封瓶多次复现的标准偏差(复现性）为0.04mK。     

~4He超流转变温度复现研究

本文介绍一种复现4He超流转变点 ( 2 1 768K)的密封瓶器件 ,复现方法简单 ,易于实现 ,可获得长达数小时的4He超流转变温坪。改变热流量 ,测出不同热流下对应的下压温坪值 ,外推到零热流 ,得到超流转变温度复现值。 3只密封瓶多次复现的标准偏差 (复现性 )为 0 0 4mK。     

EAST超导托卡马克热负荷波动对低温系统液氦槽的影响

EAST超导托卡马克的纵场和极向场磁体均采用NbTi超导材料,由3.8 K超临界氦冷却.在托卡马克实验运行时,极向场的放电脉冲和等离子体破裂产生的交流损耗带来的热负荷增加,经过超临界氦流带到低温系统控制阀箱内的液氦槽和过冷槽,造成槽内的液氦蒸发量增加.蒸发的氦回到制冷机中,从而影响制冷机的稳定运行.通过对实际超临界管道和液氦槽、过冷槽中换热过程建立换热模型,进行热工分析,分析液氦槽和过冷槽中的压力等参数的变化,指导低温系统的设计.     

超导材料的发展与应用

超导的产生和发展1911年荷兰物理学家昂尼斯用液氦冷却水银,当温度下降到4.2K时,发现水银的电阻完全消失,这种现象就称之为超导。后来还发现,在电阻消失的同时,还出现排斥磁通的性质,即抗磁性。因此零电阻和抗磁性就成了超导体的两个重要特征。超导现象一般要在很低温度下才能出现。能使导体电阻为零的温度,就叫超导转变温度,又称超     

50 kA超导变压器杜瓦及高温超导电流引线的设计与优化

CICC超导导体性能测试用50 kA超导变压器由初级线圈和次级线圈组成,初级线圈浸泡在4.2 K液氦低温杜瓦中,次级线圈为CICC导体采用4.2 K/354 637 Pa超临界氦迫流冷却,液氦和超临界氦均由500 W/4.5 K制冷机提供,变压器低温杜瓦的理论液氦蒸发率为1.52 L/h。为减少电流引线漏热,超导变压器采用B i-2223/AgAu高温超导(HTS)二元电流引线,并且在颈管中部设计了一个新型的直接用液氮冷却的热截流装置来截断电流引线高温端的热流;最后对铜电流引线部分进行了尺寸优化计算,得到最佳截面积和直径分别为28 mm2和6 mm。     

单回路液氦脉动热管气液两相流数值模拟

为了探究液氦脉动热管的传热机理,建立了单回路液氦脉动热管的二维数值模型。基于多相流VOF(Volume of Fluid)方法对该模型进行了数值求解,模拟了该液氦脉动热管的初始状态以及运行状态。其中初始状态为气液交替分布的饱和静止状态。运行状态为方向变化的脉动流动状态,流型为塞状流。分析了液氦脉动热管的流动与传热特性。     

1.8K常压超流氦低温系统的设计与分析

中国科学院等离子体物理研究所ITER CC导体测试装置背景超导磁体,由4.2 K液氦浸泡冷却,能够提供7 T背景场,为了满足超导导体测试需要更大背景场(10 T)的要求,将采用1.8 K超流氦浸泡冷却。针对该测试装置的低温系统设计了一种1.8 K常压超流氦低温系统,给出了该系统的关键组成部分并对获取1.8 K常压超流氦的流程进行了分析。针对预冷与节流相结合获取1.75 K超流氦方案进行了分析和计算,同时针对此方案给出了其物理过程的T-s图,计算了1.75 K超流氦液体得率。     

超导技术在强电领域的应用及市场分析

自1911年发现超导现象后,至今已整整100年。在这百年里,无数科技工作者一直为寻求高临界转变温度的超导材料和推进超导技术实用化而奋斗。如今,低温超导材料（以液氦为制冷剂）铌钛合金（NbTi）和铌三锡（Nb3Sn）已用于低温高场磁体,     

基于闭环制冷机的平衡氢三相点复现装置

为了解决平衡氢三相点复现装置复现难度大、耗时长、效率低的问题,研制了基于闭环GM制冷机的新型平衡氢三相点复现装置。采用准绝热方法复现平衡氢三相点;基于Labview程序,实现了复现过程的自动测量与控制。该套装置不仅提高了工作效率,而且提升了复现性。实验结果表明:该套系统平衡氢三相点的复现性优于0.05 mK,标准不确定度优于0.1 mK。     

超导HWR腔垂直测试低温系统试验研究

中国科学院先导科技专项ADS(Accelerator Driven Suberitical,ADS)嬗变系统中超导HWR(half-wave resonator,HWR)腔垂直测试需低温系统维持4.2 K(液氦)的低温环境,低温系统降温过程包括氮气置换、液氮预冷、氦气置换和液氦冷却。通过实验建立了低温系统降温4个阶段不同测点温度随时间的变化规律,在此基础上,计算了液氦的消耗速率和杜瓦的静态热负荷,分析了低温系统在稳定工作状态时最佳的液氦补液时间间隔。结果表明:该低温系统满足超导HWR腔垂直测试需求,消耗液氮约175 kg、液氦约2 048 L,低温系统稳定工作时液氦体积消耗速率为32 L/h,杜瓦静态热负荷为21.36 W,液氦合理补液时间间隔为4 h,为后续超导HWR腔垂直测试提供了保障。     

New Results for the Realization of the Superfluid Transition Temperature of Helium

A two-chamber sealed cell has been developed to realize the superfluid transition temperature of helium. A series of temperature plateaus are obtained while a series of small heat flows are applied to the capillary connecting the two chambers. The plateau temperatures are extrapolated to determine the transition temperature at zero heat flow. This paper reports the new results for the realization of the transition temperature of helium using seven cells. More than 30 measurements have been made in two laboratories since 2002, as five cells sealed in 2000 and two cells sealed in 2009 are used. The standard deviation of the measurements is ~0.02 mK.     

Study of a Helium Film in 10 μm Porous Glass by Torsional Oscillator in Free Decay Mode

Dissipation and superfluid density of a thin helium film (superfluid transition at temperature T_c=0.88 K), placed in 10 m porous glass are studied as a function of oscillation amplitude or AC amplitude at fixed temperatures near superfluid transition. The measurements are performed using a high-Q torsional oscillator in the free decay mode as well as constant drive mode. Results show decreasing superfluid density with increasing velocity and non-monotonic behavior of the dissipation in the film.     

Effect of channel restrictions on the axial heat transport in subcooled superfluid helium

The present study investigates the influence of partial restrictions on the axial heat transport and critical heat flux limits in subcooled superfluid helium (helium II) channels. Different size orifices are used to simulate partial plugging of superconducting magnets cooling channels by frozen oxygen, nitrogen, hydrogen, neon or moisture during the cool down process. Thin stainless steel sharp edged orifices of sizes 0.5, 1, 2 and 5 mm id are mounted between stainless steel flanges attached to 9 mm diameter (helium II) channel. The helium II channel is heated at one end with a copper block heater while the other end heat sinks to an atmospheric superfluid helium heat exchange. Temperature drop across the restriction is measured by two calibrated carbon resistors. Measurements are carried out at both temperatures ranging from 1.8 to 2.2 K.As the orifice/channel area ratio decreases, data show a considerable decrease in the axial heat transported by internal convection process resulting in lower critical heat flux at the phase transition from helium II to helium I by the destruction of superfluidity and inititation of boiling. A linear correlation between critical channel heat flux and orificeI channel area ratio gives a good fit to the experimental data. For heat fluxes higher than the critical heat flux, transient temperature measurements for a step heat input are correlated with the time required to reach the phase transition.     

Superfluid helium in porous media

The superfluid properties of ⁴He absorbed in porous media are discussed. Emphasis is given to the nature of the superfluid transition for helium contained in a variety of different porous structures. Recent heat capacity and superfluid measurements for helium in Vycor glass and other porous media are presented in some detail. The onset of superfluidity as a function of adsorbed helium coverage at zero temperature, or boson localization, is touched on briefly. The problem of dissipation of superflow in porous media, especially in the vicinity of the superfluid transition, is discussed.     

Ellipsometric Study of Superfluid Onset in Thin Liquid 4Helium Films

We have developed a modulated null ellipsometer capable of measuring single layers of adsorbed ⁴He films at 1.4 K. The small optical index of liquid helium, the extreme sensitivity to temperature gradients, and the requirement of sub-monolayer stability over many hours presents significant experimental challenges, which will be briefly discussed. The main goal of our experiments is to independently measure the superfluid and normal coverage in thin adsorbed ⁴He films. This is a particularly important issue for helium films on intermediate strength substrates such as rubidium and thin cesium, where previous measurements indicate that prewetting and the Kosterlitz-Thouless transition interact strongly, and the K-T transition appears to have nonuniversal features. Independent determination of the superfluid and normal fraction can be accomplished by using the ellipsometer in conjunction with a quartz crystal micro balance (QCM). QCM measurements rely on viscous coupling of the fluid layers, and therefore respond only to the normal component of a ⁴He film. In contrast, the ellipsometer is sensitive to the total thickness, independent of the state (superfluid or normal) of the film. By combining the QCM and ellipsometric measurements we can determine the total coverage, the normal fluid component and thus the superfluid fraction.     

A Study of the Superfluid Transition in ^4\hbox {He} Films Adsorbed to Rough \hbox {CaF}_2 Over a Large Temperature Range

Rough two-dimensional substrates, such as thermally deposited \(\hbox {CaF}_2\) , have been shown to modify the experimental signatures of the superfluid transition in adsorbed thin helium films. Previous experiments have investigated a series of increasingly rough surfaces over a limited temperature range and found that the features at the superfluid transition become less defined as substrate roughness is increased. In this work we study the superfluid transition in adsorbed helium films over a wide range of temperatures for a series of \(\hbox {CaF}_2\) substrates. Our results show that as the transition temperature increases the abrupt jump in superfluid density at the transition becomes less distinct. The changing characteristics of the transition on a single \(\hbox {CaF}_2\) substrate with temperature suggest that the reduced observability of the transition on rough substrates cannot be explained entirely by superfluid drag. We discuss several other possible scenarios which may be relevant to the helium films on \(\hbox {CaF}_2\) .     

The generation of particles to observe quantized vortex dynamics in superfluid helium

We describe a method to prepare a sample of superfluid helium-4 with hydrogen particles suspended within it. The method is to dilute hydrogen gas with helium at room temperature, and bubble the mixture through liquid helium at a temperature above the superfluid phase transition temperature, T_λ ≈ 2.17 K. The procedure yields a suspension of micron-sized particles whose total volume is about 10⁵ times smaller than the fluid volume. The fluid and suspension are then cooled to a temperature below T_λ. We show that the particles, so prepared in superfluid helium, are useful for studying superfluid flows and, in particular, the dynamics of quantized vortices. In addition, the particle-superfluid helium system is rich in not yet fully explained interactions. We review preliminary investigations that include observing the vortex lattice in rotating helium, vortex reconnection in quantized vortex turbulence, and vortex ring decay. These data illustrate the basic mechanisms of dissipation in superfluid turbulence.     

Possible criteria for the ac Josephson effect in superfluid helium

Based on the differences between superfluid helium and superconductors in the dissipation mechanism of a weakly coupled system, an attempt is made to explain the possibility of observing an ac Josephson effect in superfluid helium with an orifice much larger than the coherence length of the superfluid helium. The temperature criterion for the ac Josephson effect proposed by Biswas is also discussed. The temperature needed for observing an ac Josephson effect in superfluid helium may not have to be much below 1 K.     

Thermal convection and thermal conductivity of nondilute3He-4He mixtures

We present measurements of the critical temperature difference for the onset of thermal convection and the effective thermal conductivity in two³He-superfluid-⁴He mixtures. The mixtures were 6.8% and 9.8% by molar volume of³He in⁴He and the measurements were made from 0.65 K to just above the superfluid transition temperature for each mixture. The measurements were made as part of an effort to visualize convective flow patterns in helium mixtures using optical shadowgraph techniques. We discuss the implications of our results for this effort.