The cryogenic system for ITER CC superconducting conductor test facility

This paper describes the cryogenic system of the International Thermonuclear Experimental Reactor (ITER) Correction Coils (CC) test facility, which consists of a 500 W/4.5 K helium refrigerator, a 50 kA superconducting transformer cryostat (STC) and a background field magnet cryostat (BFMC). The 500 W/4.5 K helium refrigerator synchronously produces both the liquid helium (LHe) and supercritical helium (SHe). The background field magnet and the primary coil of the superconducting transformer (PCST) are cooled down by immersing into 4.2 K LHe. The secondary Cable-In-Conduit Conductor (CICC) coil of the superconducting transformer (SCST), superconducting joints and the testing sample of ITER CC are cooled down by forced-flow supercritical helium. During the commissioning experiment, all the superconducting coils were successfully translated into superconducting state. The background field magnet was fully cooled by immersing it into 4.2 K LHe and generated a maximal background magnetic field of 6.96 T; the temperature of transformer coils and current leads was reduced to 4.3 K; the inlet temperature of SHe loop was 5.6 K, which can meet the cooling requirements of CIC-Conductor and joint boxes. It is noted that a novel heat cut-off device for High Temperature Superconducting (HTS) binary current leads was introduced to reduce the heat losses of transformer cryostat.     

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。     

Modified Roebuck compression device for cryogenic refrigeration system of superconducting rotating machine

Cryogenic refrigeration system installed on superconducting rotor has the merit that it can eliminate cryogen transfer between rapidly revolving rotor and stationary part and, therefore, the loss generated by it. Nevertheless, such an on-board cryogenic refrigeration system has not been realized yet because of the absence of compression device that can work reliably on rapidly revolving rotor. This paper presents the idea of modified Roebuck compression device as a potential on-board compression device. Although it has the disadvantage that it requires large radial space and external cooling and heating, a modified Roebuck compression device of 14 stages with diameter of 0.80 m mounted on a rotor revolving at 3600 rpm can produce substantially high compression ratio. J-T neon refrigeration system equipped with such a compression device can produce low temperature of 30 K and be useful for cooling superconducting rotor windings made of high temperature superconductor.     

Sub-cooled nitrogen cryogenic cooling system for superconducting fault current limiter by using GM-cryocooler

The 21st Century Frontier R&D Program was planned to develop and commercialize the inductive Superconducting Fault Current Limiter (SFCL) in Korea until 2011. The 1.2 kV/80 A inductive SFCL was planned to develop at the first year in the first phase (2001-2002) and the 6.6 kV/200 A inductive SFCL for short run operation test was planned to develop at the second and third year in the first phase (2002-2004). The experimental characteristics of conduction-cooled cooling system developed in the first year was very weak from the sudden large thermal disturbance. Therefore, the conduction-cooled cooling system was concluded not appropriate for the cryogenic technology of the application of superconducting fault current limiter. In the third year research, the improved sub-cooled nitrogen cooling system was adopted and investigated.In this paper, the characteristics of each cooling type was compared and the basic deign of ameliorated cooling system was introduced and the total heat load of the cooling system was calculated and compared with the heat load of the cooling system developed at 2nd year research.     

Magnetic-field calculation for a high-temperature superconducting cryogenic current comparator

The problem considered in this paper arises in the design of a high-temperature superconducting cryogenic current comparator (CCC). The CCC consists of two currents flowing in opposite directions inside a toroidal superconducting shield. The shield has a radial cut, necessary for the measurement of the current ratio, but causing an error in the obtained ratio. The problem of interest is the dependence of the error on the geometric parameters of the device: the major and minor radii of the shield, the cut width, the material thickness, and the location of the currents. In the first part of the paper, a toroidal shield with an infinitesimal cut is considered and analytic expressions are derived for the magnetic field and the surface-current distribution. In the second part, a cut of finite width is introduced. Since all the perturbing currents are present in the narrow region around the cut, a shield of cylindrical shape is assumed. Expressions are derived for the flux through the cut and the magnetic field around the cut. Analytical results are in good agreement with the numerical results obtained by a finite-element method. In the final part, the expression for the ratio error is derived, which shows that in order to minimize the error, currents should be concentrated around the shield axis, the major radius of the shield should be maximized and the bore radius minimized. The error depends logarithmically on the cut width.     

A simple and effective load-pull system for RF power transistorlarge-signal measurements for wireless communication power amplifierdesign

Conventional load-pull methods for RF power transistor large-signal measurements usually need complicated RF measurement instrumentation. A simple load-pull measurement setup and a test procedure with IEEE-488 GPIB control and data acquisition capability has been developed. This is used to facilitate the design of an RF power amplifier (PA) for wireless communication applications. Compared with the conventional load-pull method, it is simpler and less costly, yet effective. Measured examples of Siemens GaAs FET's CLY-2 and CLY-10, which are used to design a RF PA module for ISM-band (902-928 MHz) wireless communications, are presented     

Absolute cryogenic radiometer for high-current photometric measurements

A cryogenic radiometer with electrical substitution has been developed for precise photometric measurements. The radiometer design and specifications are described. The radiometer has an accuracy of 0.02%. Its main use is to calibrate the spectral response of reference radiometers. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 22–25, November, 1996.     

ac loss measurements on a superconducting transformer for a 25 kA superconducting rectifier

ac loss measurements have been performed on a superconducting transformer. The transformer is a part of a 25 kA thermally switched superconducting rectifier operating at a frequency of 0.1 Hz. The loss measurements have been automatized by means of a microcomputer sampling four relevant signals and processing these data to reliable loss numbers. Results were obtained at amplitudes of the secondary current between 5 and 15 kA at various current rates between 5 and 30 kA s^?1.     

Design and test of a simplified and reliable cryogenic system for high speed superconducting generator applications

Under the contract with Air Force Research Lab (AFRL), General Electric has successfully tested a high speed, superconducting generator for a Multimegawatt Electric Power System (MEPS). As the first successful full-power test of a superconducting generator for the Air Force, the demonstration tested the generator’s load up to 1.3 MW and over 10,000 rpm. A key component of the generator system is a closed loop cryo-refrigeration system to cool the field excitation coil at liquid neon temperature. This paper reports the design and tests of the cryogenic system, including the liquid neon dewar, cryogenic cooling loop for the high temperature superconducting (HTS) field coil and the cryostat. Performance data during both short-term load run and long-term non-load run were presented. Also, some key issues to design a reliable cryogenic system for a superconducting generator were discussed.     

A new cryogenic memory system

The recently developed "current stretch" technique, a low temperature integration and amplification of sense signals, offers new freedom in the design of cryogenic memory cells. This scheme makes the sense voltage independent of cell switching speed and gives a good signal-to-noise ratio. A memory system using a double hole tin film storage cell driven by cryotron trees is described. Current stretch sensing is done through the trees so that no sense line is required. The cell operating range, which can be made arbitrarily large, has been set at 40 percent in the present study. Cells have been tested at densities of 10³bit/in²and switching speeds of 100 ns. Densities of 10⁴bit/in²appear feasible with existing fabrication techniques. In sizes of 10⁸cells or above, this memory system appears cheaper than any known room temperature technique of equal speed.     

Upper critical field measurements on A15 NbGe superconducting films

Measurements of the upper critical fieldB _c2 (T) have been made on a range of high-transition-temperature A15 niobium germanium samples prepared by dc sputtering with varying additions of oxygen and differing sputtering conditions. An overview is presented on the formation of Nb₃Ge with particular reference to lattice parameter, composition, and normal state residual resistivity. The problem of sample inhomogeneity and its effect onB _c2 (T) measurements is discussed. Correlations of are made withT _cand resistivity, and the results compared with previous studies of neutron-irradiated and coevaporated material. Existing theories of are summarized and the problem of comparison of theory and experiment is discussed. An analysis is presented that does not depend directly on assumptions about the Fermi surface structure. The data indicate thatB _c2 has a fundamental dependence onT _cand resistivity that is similar to that found in Nb₃Sn. There is evidence that the mean Fermi velocity for clean Nb₃Ge is about twice the value for Nb₃Sn; the mean Fermi surface area is also about twice the value for Nb₃Sn. This suggests that Nb₃Ge is more strongly coupled than Nb₃Sn.     

Non-resonant RF and microwave response: A novel technique for the characterization of superconducting materials

When examined using continuous wave electron paramagnetic resonance and nuclear magnetic resonance spectrometers, the highT _c superconductors give rise to intense, low field, ‘non-resonant’ absorption signals in the superconducting state. This phenomenon can be used as a highly sensitive, contactless technique for the detection and characterization of superconductivity even in samples containing only minute amounts of the superconducting phase. Further, it can also be applied to the determination of material parameters of interest such asJ _c andH _c2 in addition to being a powerful way of distinguishing between weak-link superconductivity and bulk superconductivity. The details of these aspects are discussed.     

Penetration depth measurements on type II superconducting films

A penetration depth (λ) measurement technique, which is based upon the λ-dependence of the propagation velocity of electromagnetic waves in superconducting transmission lines, is refined and used to accurately measure λ's in Pb films containing Au, In, or Bi additions. The results are applied to test two distinct predictions for λ's in London (or type II) superconductors. First, the measured dependence of λ upon low temperature electrical resistivity is compared with the theoretical prediction for a type II superconductor containing a dilute homogeneous impurity concentration. We find that the measured λ's follow this simple prediction reasonably well, irrespective of the species of impurity, with two anticipated exceptions. Namely, deviations occur for pure Pb, which is not a type II superconductor, and for the highest impurity concentration, in which case the Fermi surface may be appreciably distorted from that of pure Pb. Second, the London-theory prediction for the dependence of the effective penetration depth (λ^eff) upon film thickness (d), λ^eff= λ(T) coth d/λ(T), is tested, in a d/λ range in which coth d/λ, is not closely approximated by 1 or λ/d. Separate experiments in which either film thickness or temperature, T, are varied reveal excellent agreement with the above formula. To our knowledge this is the first detailed confirmation of this explicit hyperbolic cotangent dependency.