Cryogen free cooling of ASTRO-H SXS Helium Dewar from 300 K to 4 K

Soft X-ray Spectrometer instrument (SXS) is one of the primary scientific instruments of ASTRO-H. SXS has a cold detector that is cooled to 50 mK by using a multi-stage Adiabatic Demagnetization Refrigerator (ADR). SXS Dewar containing ADR provides 1.3 K heat sink by using liquid helium in nominal operation. After liquid helium is dried up, 4 K heat sink is provided by using mechanical coolers. Both nominal operation and cryogen free operation were successfully demonstrated. This paper describes the test result of cryogen free operation and cool-down performance from room temperature by using only mechanical coolers without liquid helium. The coolers on the Dewar cooled down cold mass from around 300 K to 4 K with 260 W electric power in 40 days. Cold mass is 35 kg in 4 K area including the helium tank, ADR and detector assembly.     

Operational Characteristics of the 200-m Flexible Cryogenic Transfer System

A proper cryogenic environment is essential for the operation of superconducting devices. A test area for the superconducting radio-frequency modules (SRF) has been established in the RF laboratory at National Synchrotron Radiation Research Center in Taiwan; these modules require much liquid helium during conditioning and performance tests; a cooling capacity of 120 W is expected for the acceptance test of the SRF module. The cryogenic environment of the test area is completed on transferring the liquid helium over a remarkable length of 205 m from the two cryogenic plants at Taiwan Light Source, with a valve box located at each end to control and to measure the cryogenic flow. Flexible cryogenic transfer lines of concentric four-tube type are chosen for both the supply of liquid helium and the return of cold helium gas. Functional examination of this long transfer system was first achieved with a 500-L Dewar in the radio-frequency laboratory; an SRF module was then installed in the test area for practical operation. The primary concern about the cryogenic transfer system is the heat loss; a measurement technique based on the principle of thermodynamics is developed and proposed herein. With the available sensors inside the valve boxes and the heaters inside the 500-L Dewar and the test SRF module, this technique has proved promissing from the measured results.     

某火星探测器星外材料液氦温区贮存试验系统的研制与试验验证

为了考核某火星探测器星外材料在深冷宇宙空间中的性能,研制了一套用于液氦温区的试验装置,进行了环境模拟试验研究,结果表明该装置能满足试验要求.同时得到了液氦表面温度沿高度方向的变化规律.     

Thermal acoustic oscillations in triple point liquid hydrogen systems

Thermal acoustic oscillations are often observed in tubes which penetrate a cryogenic system and are closed at the warm end and open at the cold end. Such tubes are genrally used for filling or vetning the tank, providing relief pressure or inserting instruments taps. Large amounts of heat (of the order of ten to a thousand times more than by normal heat conduction) can be transferred into a cryogenic system when such thermaloscillations occur. A number of studies examining thermal acoustic oscillations in liquid helium systems have been performed by Rott et al. However, only minimal consideration has been given to such oscillationsin liquid and sluch hydrogen systems. This study extends Rott's theory to the stability aspects of thermal acoustic oscillations for a straight tube closed at the warm end and inserted into a Dewar flask filled with triple point liquid hydrogen when the cold open end is located above the liquid surface. These results can also be applied to a slush hydrogen when the pressure in the Dewar flask is reduced to the triple point pressure of hydrogen. Numerical results have been obtained in this study for developing stability curves, establishing oscillation frequency characteristics and identifying critical configurations for initiating such oscillation. The mechanisms associated with the two branches of the stability curves for thermal acoustic oscillations have also been investigated.     

Demonstration of Transfer of Liquid Helium Using a Pump System with MgB2 Wires

An electric pump composed of an MgB₂ motor is combined with a superconducting level sensor using a thin CuNi-sheathed MgB₂ wire to demonstrate the transfer of liquid helium. An impeller is attached to the lower end of a rotating shaft of the MgB₂ motor and covered with an outer casing to form a kind of centrifugal pump. After that, the MgB₂ motor with the impeller is located vertically inside a cryostat with an infill of the liquid helium. Another glass Dewar vessel is also prepared to receive the liquid helium transferred from the cryostat containing the MgB₂ motor. The MgB₂ sensor is used not only to detect the level of the liquid helium but also to control the electric pump on the basis of its pre-estimated calibration curve. By using the assembled pump system, the liquid helium is successfully transferred from the cryostat to the glass Dewar vessel via a transfer tube. The transfers of liquid hydrogen with the identical pump system will be carried out in the near future.     

超流氦低温系统发展及涡轮冷压缩机的应用

2 K下大型氦低温系统已采用离心式涡轮冷压缩机在低温低压下对饱和液氦槽减压操作,以获得超流氦或过冷氦.介绍了2K温度级超流氦制冷机发展情况和涡轮冷压缩机在氦制冷系统中的应用,以及中国科学院等离子体物理研究所EAST超导托卡马克氦低温制冷机中过冷氦的制取过程.     

Superconducting generator cooling system simulation

Rotating tests have been performed to evaluate continuous liquid helium transfer and the cryogenic environment established in a test rotor. The testing was related to high speed alternator development programmes and the rotating Dewar was designed to simulate a self-pumping liquid helium cooling system for superconducting rotors. Stable pumping to < 0.6 atm¹ and stable helium level regulation were achieved at several different levels. The established pool was, within the limitations of steady state heat transfer, isothermal and there was no evidence of a warm bore condition. Within the rotating Dewar, heat transfer devices, level gauges and other instrumentation have been tested using onboard microprocessors to log, digitize and then transmit the data from the rotor. All of the cooling system functions required by large synchronous alternators were observed to be present and stable.     

A self-circulation helium liquefaction system with five 4 K G-M cryocoolers

A self-circulation helium liquefaction system (SCHLS) with five 4 K G-M cryocoolers is developed to supply liquid helium (LHe) for SECRAL (a superconducting ECR ion source used in Lanzhou city, China). LHe is vaporized in SECRAL and warmed up to room temperature. SCHLS will re-liquefy the helium gas at a rate of 83.2 L/day under normal atmosphere pressure. With SCHLS, SECRAL system can run online without any interruption of refilling LHe.     

Development of a Superconducting Magnet System with Zero Liquid Helium Boil-off

A homogeneous magnetic field superconducting magnet with a cold bore of 250 mm and a central field of 4.3 T has been designed, manufactured, and tested with zero liquid helium boil-off. As a result of magnetic field homogeneity considerations, the magnet is composed of three coaxial coils: one main coil and two compensation coils. All coils are connected in series and can be charged with a single power supply. The magnetic field homogeneity is about ±3.0 % from ?200 mm to 200 mm in axial direction with 86 mm in diameter. The magnet can be operated in persistent mode with a superconducting switch. A two-stage GM cryocooler with a capacity of 1.5 W at 4.2 K was used to cool the superconducting magnet. The cryocooler prevents the liquid helium from boiling off and leads to zero helium loss during static operation. The magnet can be operated in liquid helium circumstance by cooling the gas helium with the cryocooler without additional supply of liquid helium. Under this condition, the magnet is successfully operated up to 4 T without quench. The magnet system can be generating 0.25 L/h liquid helium with the cryocooler by supplying the gas helium without loading the magnet. In this paper, the magnet design, manufacture, mechanical behavior analysis, and the performance test results of the magnet are presented.     

FRP Dewar for measurements in high pulsed magnetic fields

We describe a liquid helium glass-fibre reinforced plastic (G-FRP) Dewar which we designed, fabricated and tested for excitation spectrum measurements in high pulsed magnetic fields of up to 50 T. The sensitivity of high-resolution magnetic measurements carried out at low temperatures in such high fields is limited inevitably by magnetic and electric properties of the structural Dewar materials involved. Magnetic properties of various G-FRP Dewar-purpose materials are explored with a χ-meter furnished with a RF SQUID magnetometer. The Dewar materials and multilayer insulation effects contributing to the magnetic response signal are analyzed. It has also been discovered that field noise caused by the magnetization of the Dewar materials can be suppressed substantially by using special glass-epoxy technologies. The liquid helium evaporation rate is 3.8 l/day while the hold time is 27 h, the influencing factors are discussed.     

Electro-magnetic stress-induced degradation of insulation vacuum of a large cryo-magnetic system

In superconducting magnets, the cold mass is placed in a vacuum vessel to reduce heat load to the liquid helium system. Helium leaks into the vacuum vessel can degrade the insulation vacuum, which can, in turn, cause an increase in the heat load to the liquid helium system. These leaks are called cold leaks, as they show up when the coil is cooled with liquid helium. K500 superconducting cyclotron magnet at Variable Energy Cyclotron Centre, Kolkata has such cold leaks in the helium vessel that developed during cool down. The leak rate increases with the increase of current in the superconducting coils. This paper describes a series of experiments carried out on the superconducting cyclotron magnet to find the level of degradation of insulation vacuum and measure the increase in heat load with magnet current. The leak rate was also measured and the leak size was estimated analytically. Detail magneto-structural analysis was done using Finite Element Method (FEM) to identify highly stressed zones in the helium vessel and found out that highly stressed zones coincide with the weld zones. The magneto-structural stress was applied on an estimated size of single crack and found that crack tip stress could reach beyond elastic limit of the material. We can predict that the full design current may be unachievable in this situation. Mitigation of increased heat load was also done using an additional vacuum pump for the insulation vacuum space.     

基于液氮冷源超导磁体氦气循环预冷系统设计

为了提高超导磁体300-80 K预冷过程中的降温效率和安全性,开发了一种新的预冷方法.设计了一台以液氮为冷源、氦气为循环介质的可控温预冷装置,对其内部结构进行了优化设计,包括低温风机、板式换热器、气动调节阀、翅片换热器等主要组成部分,整个装置与磁体构成一个闭合循环系统.在预冷装置的作用下,该超导磁体从300 K到80 ...     

BEPC-Ⅱ低温系统的氦气净化技术

氦气净化技术是国家大科学工程北京正负电子对撞机重大改造(BEPC-Ⅱ)中低温系统的关键技术之一.在充分调研国内外大型低温系统氦气净化技术的基础上,结合自身情况,创造性提出氦气储罐内部处理及真空置换方案,一次性充入氦气,将储罐内氦气不纯度控制在40 vpm之内,同时辅之以80 K外置低温吸附器对氦气储罐内以及冷箱和超导设备端的氦气进行净化.高效而又经济的解决了BEPC-Ⅱ低温系统中的氦气纯度问题,成功地进行了制冷机的验收测试和超导设备的调试及运行.     

Escape rate of surface state electrons on liquid helium below 1 K

The escape rate of surface state electrons on liquid helium was measured in the temperature interval 0.56<T<1.0 K for electron areal density below 5×10⁷ cm^–2, where the rate is independent of density. It is found that the escape rate is dominated by the scattering from the helium gas atoms in the vapor phase above 0.8 K, while it is dominated by that from ripplons (surface wave quanta) below 0.8 K. Quantitative agreement is obtained between our experiment and the theoretical calculation by Nagano et al.Preliminary results were published inSurface Sci. 98, 17 (1980).     

Quantum evaporation from quantum liquids and solids

Quantum evaporation is the evaporation of atoms from the surface of a liquid or solid by single excitations, such as phonons. We show that this process can only occur in systems for which the deBoer quantum parameter is above a critical value. For superfluid helium we argue that, because the interface between liquid and gas is not sharp, it may be useful to consider quantum evaporation as a process in which excitations coming from the liquid are adiabatically deformed as they propagate through the interface and then emerge as gas atoms. Based on this picture, we make several predictions which can be tested experimentally. Finally, we discuss quantum evaporation in solid hydrogen.     

Development and Comparison of Two Types of Cryogen-Free Dilution Refrigerator

Dilution refrigerators are an important tool used in solid state and quantum fluid physics for cooling to temperatures below 0.3 K. Conventional dilution refrigerators consume a lot of liquid helium, which has to be recharged in a helium bath every few days. Cryogen-free dilution refrigerators, however, do not use liquid helium and then automatic operation by electricity can be possible from room temperature to the mK region. In near future, therefore, most conventional dilution refrigerators will be replaced by cryogen-free refrigerators because they are easy to operate, do not require maintenance and do not consume helium. We have developed two types of cryogen-free dilution refrigerator. One is directly cooled by a pulse tube refrigerator in the same cryostat using copper thin wires as a thermal link, and the other is cooled by a separate Gifford McMahon refrigerator using circulating helium gas through a flexible syphon tube. The latter has been developed as a vibration-free cryogen-free dilution refrigerator. These two types of cryogen-free dilution refrigerator are compared considering several key points: base temperature, precooling time, minimum temperature and vibration amplitude.     

模拟超临界氦迫流冷却系统

为ＣＩＣＣ子缆及子缆接头试验而设计的超临界氦迫流冷却系统由高压氦气钢瓶,汇流排,减压阀,流量控制阀,换热器等部件组成。     

Determination of evaporation loss from helium dewar vessels without nitrogen cooling

The author presents a method of calculation and nomograms for determining the evaporation loss of liquid helium from small Dewar vessels without nitrogen cooling.     

一种用于冲击压缩实验的液氦温度低温靶

介绍了研制的一种液氦温度低温靶靶体。该靶体具有独立真空夹层与气冷屏,采用连续流减压降温的液氦制冷方式,在约100 Pa真空环境下,获得了稳定的4.2 K制冷温度。它可以用于氦等气体样品的冲击压缩实验。     

An experimental study of heat exchange in nitrogen-free helium vessels with a multishield system of thermal insulation

We have investigated the exchange of heat in nitrogen-free helium Dewar vessels with a multishield insulation system, and we have derived the relationship between temperature and the coefficient of thermal conductivity for a multilayers insulation cooled by helium vapors in the temperature range 4,2– 293 K.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 2, pp. 216–220, August, 1990.