Anomalous behaviour of laser cooled fast ion beams

We have observed a sudden disappearance of intrabeam scattering in a laser cooled stored ⁹Be⁺ beam at the Heidelberg Test Storage Ring. The transition takes place at about 10⁶ ions corresponding to a mean ion distance of ≃50c μm. The disappearance of IBS is accompanied by a decrease of the longitudinal temperature by more than two orders of magnitude. At the same time, the transverse width of the ion beam shows an increase which is limited to the diameter of the laser beam. Experimental signatures of this anomalous beam behaviour are described, and possible interpretations are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anomalous positive ion mobility in liquid 3He

Measurements of the mobility μ₊ of positive ions in liquid ³He under a pressure of 1 bar in the temperature range 0.05 < T < 0.6K exhibit a sharp peak in μ₊(T) near 0.2K and irreproducible behaviour at lower temperatures.     

氦气冷却的MgB_2超导直流电缆本体热分析

清华大学、剑桥大学和麻省理工学院正在合作研制氦气制冷的MgB2超导直流电缆。针对此超导电缆本体结构,建立了简化热分析模型,求出了电缆的漏热量和氦气的轴向温度分布,并对氦气的进口温度、质量流率进行了优化分析;最后,利用FLUENT软件对整个电缆内流动和传热过程进行了2D建模和数值仿真;仿真结果和简化模型计算结果非常吻合,表明了简化分析模型可用于该类型超导电缆的热分析和热设计。     

Anomalous phonon dispersion in helium II

The present contribution reviews the experimental and theoretical developments to date in understanding the anomalous dispersion in the long-wavelength limit in superfluid ⁴He. Threshold energies for the occurrence of three-phonon processes are analysed in detail. Experimental studies are briefly outlined, their results compared and critically analysed. Besides giving details of phenomenological approaches, an attempt is made to sketch continuum models, some microscopic theories, studies based on sum rules and the polarization potential approach.     

X-rays from antiprotonic helium in helium gas

X-ray transitions to the 4F, 3D, and 2P atomic levels of p?He have been observed with antiprotons stopped in He gas at 4 and 1.1 atm NT. The population by radiative transitions of the 3D level in gas Y_M(4atm) = (28±14)% and Y_M(1.1 atm) = (43±22)% exceeds by more than one order of magnitude that measured in liquid He. The annihilation width of the 3D level Γ^a_3D = 2.8±1.0 × 10^?3eV is determined from the ratio between the numbers of X-rays feeding and depopulating the 3D level. The strong-interaction shift of the 2P level ε(2P) = ?14±6 eV is obtained by inputting the $\text{p}\text{He}$ experimental X-ray yields into a cascade calculation, the results of which are in good agreement with well-established data from muonic, pionic, and kaonic helium.     

线型离子阱中冷却离子所需高纯度氦气的渗透与实验

利用氦的渗透性质,向超高真空系统中充入高纯度氦缓冲气体,可以降低离子阱中囚禁离子的运动速度,减小二级多普勒频移,增加离子的存储时间,提高线型离子阱微波频标的稳定度.实验上得到了氦气渗透的实验参数、特性以及对系统真空度的影响、实现了用温度调节精确的控制充入氦气的分压强.     

Dependence of helium ion diffusion and drift characteristics in own gas on its temperature

The low-temperature gas discharge has a number of features which can appear in experimentswith dusty plasma. For example, at cryogenic temperatures of gas-discharge tube walls, the strong anisotropy of the velocity distribution function of ions takes place, which in turn can cause a significant change in dust structure properties. The dependences of the helium ion drift characteristics in a dc uniform electric field on the atomic temperature of own gas is analyzed.     

Anomalous mobility enhancement in Si doping superlattices

Enhanced Hall mobility has been measured in Si doping superlattices with pipi or nini doping profiles grown by molecular beam epitaxy. Although the carrier concentration is large, in the range of 10¹⁸cm⁻³, the mobility of holes in the pipi doping superlattice has the temperature dependence such as that expressed in the T^−2.2 law which is characteristic of high purity bulk crystals. The hole mobility is about twice that of high purity crystals, rising up to 40,000 cm²/V.s at about 30 K. In the case that p-type impurity concentration is comparable to n-type impurity concentration, no mobility enhancement is observed and the mobility is anomalously depressed. The mechanisms for drastical change in the mobility of doping superlattices have not yet been clarified.     

Dependence of characteristics of helium ion diffusion and drift in own gas on its temperature

The gas discharge at a lower atom temperature has a number of features which can appear in experiments with dusty plasma. At cryogenic temperatures of gas-discharge tube walls, strong anisotropy of the velocity distribution function of ions takes place, which, in turn, can cause a significant change in dust structure properties. In this study, the dependences of helium ion drift characteristics in a dc uniform electric field on the temperature of own gas atoms are analyzed.     

中国固态增殖剂试验包层系统的氦气冷却系统仿真计算

根据中国固态增殖剂试验包层氦气冷却系统的系统设计和布置情况,利用大型一维流体仿真软件 Flowmaster 建立了氦气冷却系统的仿真模型。利用该仿真模型,模拟了氦冷系统在产氚包层系统不同工况下氦气流动情况以及各种参数的分布情况,得到了热等待工况、热备用工况、正常运行工况和除氚工况下系统的主要工艺点的温度、压力和流速分布等相关参数,为系统的设计和设备选型提供参考。同时,该仿真模型和结果对中国聚变工程实验堆氦冷系统的仿真模拟具有一定的参考意义。     

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According to the system design and arrangement of helium cooling system of China helium cooled ceramic breeder (CHCCB), helium cooling system simulation model is built using large one-dimension fluid simulation software Flowmaster. For different operation conditions of tritium breeding blanket system, helium flow conditions and parameter distribution are simulated for helium cooling system, using such simulation model. Temperature, pressure and velocity distributions and other related parameters have been obtained on different scenarios for hot waiting state, hot standby state, normal operation state and tritium out-gassing state, which provide reference for the design of system and equipment selection. Meanwhile, the model and simulation results have some reference significances for the simulation of helium cooling system for CFETR.     

以液氮为冷源的氦气强制对流高低温箱实验研究

为模拟航天产品试验所需的极端温度环境和测试产品在较大升降温速率下的耐受能力,研制了充注氦气的高低温试验箱。通过强制对流及辐射换热相结合的方式提高升降温速率,显著区别于热真空罐。开展了该高低温箱的性能验证试验,包括设计温度的上限和下限,达到目标温度的升降温速率以及温场均匀性等,从而评估以液氮为冷源的氦气强制对流高低温箱的工作性能。     

Second sound in a laser cooled gas

We show that the second sound can exist in the ultra-cold gas of atoms confined and cooled in a magneto-optical trap. Our approach is based on a two-fluids model, similar to that used for superfluidity. Each of the fluids (the atomic gas and the phonon gas) is described by similar kinetic and fluid equation, which are coupled. The second sound modes are elementary excitations of these two coupled fluids. The ultra-cold gas could be used as an experimental test bed for detailed studies of the dispersion properties of the first and second sound.     

Suppression of intrabeam scattering in cooled heavy ion beams

The properties of electron cooled heavy ion beams in the ESR storage ring are dominated by heating due to intrabeam scattering. For low intensity ion beams a sudden reduction of the longitudinal ion beam temperature has been detected by Schottky noise analysis. This can be interpreted as the disappearance of intrabeam scattering heating which could allow a longitudinal ordering of the ion beam.     

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为进一步优化实验包层模块的热工水力性能,分别研究了实验包层模块的两种不同冷却方案在正常和极端两种不同工况下的温度分布。用流体数值分析软件FLUENT模拟了实验包层模块关键部件的实际温度分布状况。