Magnetic field enhancement of high-T c superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.     

Counter flow cooling characteristics with liquid nitrogen for superconducting power cables

Counter flow cooling is one of effective cooling systems for superconducting power transmission cables to save space. In designing of HTc superconducting power cables, investigating cooling characteristics with liquid nitrogen is important. We have conducted counter flow cooling tests of a 100-m long duplex-counter-flow tube with liquid nitrogen and obtained basic data of initial cooling and temperature distributions along the tube. Measured temperature distributions are compared with those calculated that are based on our previous study. They agree with temperature distributions of inner tube but not for outer tube. During cooling tests, mass flow rate oscillations are observed. Relation between oscillation period and tube length is discussed. Reservoir pressure dependence of measured initial cooling time is also shown and compared with analytically estimated time in this paper.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

Capillary cooling of superconducting coils with two-phase hydrogen or nitrogen

Superconducting coils in AC application, such as in the stator of superconducting motors, have losses which warm up the coil and so limit the performance. Good thermal contact between the coil and the cooling agent is important. In this research we investigate the possibility of cooling the coils by a cryoliquid, such as liquid hydrogen or liquid nitrogen, flowing through capillaries in thermal contact with the coils.     

超导电缆波纹管内过冷液氮流动阻力特性研究

基于Fluent软件,采用30 m高温超导电缆模型,分析了波纹管几何结构对恒温器内液氮压力损失的影响,也与光滑管的流阻特性进行了对比分析.结果表明:压力损失与液氮流量的平方成正比,在相同体积流量下,过冷液氮在波纹管内的压力损失随着波纹管波距的增大而减小,随着波纹管波高的增大而增大.     

Magnetic field enhancement of high-Tc superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.     

Two-dimensional normal zone propagation in BSCCO-2223 pancake coils

We present the results of an experimental and analytical study of two-dimensional normal zone propagation in pancake test coils, wound with silver-sheathed BSCCO-2223 tapes. Two test coils were studied in detail, one having three and the other eight layers. Each test coil was housed in an adiabatic environment whose temperature (20-70 K) was controlled and maintained by a two-stage G-M cryocooler and placed in a background field (0-6 T) generated by a Bitter magnet. With a test coil carrying a transport current (0-200 A), a local heat disturbance was applied by a heater attached to the outermost layer of the coil. The resulting electrical and thermal responses of the coil were recorded with voltage taps and thermometers attached to the coil. A normal zone propagation code was developed to accurately simulate the voltage and temperature responses of each coil for both quenching and recovering events. The code solves the nonlinear transient heat diffusion equation in two-dimensional cylindrical coordinates with a finite difference method. As an application of this code, a two-coil system, with each coil comprised of one double pancake wound with silver-sheathed BSCCO tape, was studied for its quench behaviour as one of the coils was driven normal locally. The simulation results indicate that the value of a shunt resistor connected across the terminals of each coil has a profound effect on the level of hot-spot temperature reached in the quench initiation spot.     

Quenching of superconducting coils

Although the speed of propagation of the boundary between normal and superconducting regions in a wire falls with decreasing current, different, and even opposite behaviour can be observed in a coil during a quench. It is suggested that this is due to the changing field causing hysterestic heating of the conductor.     

Electrical characteristics of high-Tc superconducting mini-model cable under mechanical stresses in liquid nitrogen

To develop 22.9 kV class high-T_c superconducting (HTS) cable in Korea, we have been studying electrical insulation properties of dielectric paper, such as breakdown voltage, partial discharge, which is one of the HTS cable structure elements. However, the research on the mechanical stress of dielectric paper compared to breakdown properties of dielectric paper is insufficient. A cracking and variation of the electrical insulation due to mechanical stresses during cooling and bending of HTS cables in cryogenic temperature is a serious problem. Thus, we investigated tensile stress and breakdown stress of dielectric paper under mechanical stress. Moreover, we manufactured mini-model cables investigated breakdown stress under bending stress to design a cable drum for conveyance. In the AC, impulse and partial discharge properties, all test results showed a similar tendency, and the suitable bending radius ratio R/r was decided to be more than 25.     

A fast-acting superconducting switch for protection of superconducting coils in the persistent current mode

Superconducting switches which are normally used as shorting switches to operate superconducting coils in the persistent current mode are unsuitable for field energy removal from a coil in case of quenching by a parallel resistor because of the low resistance in the non-superconducting state. Moreover these switches have a long turn-off time compared with the characteristic transient time from the superconducting to normal state of high current density coils.The superconducting switch described in this paper permits switch off of current from 20–90 A within a few milliseconds with a normal resistance of about 200 Ω. The switch is activated inductively by a special winding configuration.     

The quenching of large superconducting coils

The growth of a normally conducting region in a superconducting coil is related to the propagation of such a region along a superconducting wire. The theory is compared with observations on two large coils which incorporated close coupled secondary windings, so that the voltage across the normal region could be observed directly. Reasonable agreement is found.     

超导电缆循环过冷液氮温度场模拟

建立了循环过冷液氮在恒温器内迫流冷却超导电缆的模型,利用Fluent软件模拟了30 m电缆系统内液氮温度场在不同流量、终端漏热、交流损耗等工况条件下的变化情况。仿真结果表明,在终端漏热和交流损耗一定时,液氮出口断面的最高温度随流量的增大而降低;在流量和交流损耗一定时,液氮出口断面的最高温度随终端漏热的增大而升高;在流量和终端漏热一定时,液氮出口断面的最高温度随交流损耗的变化不明显。     

Analysis of a flux-coupling type superconductor fault current limiter with pancake coils

The characteristics of a flux-coupling type superconductor fault current limiter (SFCL) with pancake coils are investigated in this paper. The conventional double-wound non-inductive pancake coil used in AC power systems has an inevitable defect in Voltage Sourced Converter Based High Voltage DC (VSC-HVDC) power systems. Due to its special structure, flashover would occur easily during the fault in high voltage environment. Considering the shortcomings of conventional resistive SFCLs with non-inductive coils, a novel flux-coupling type SFCL with pancake coils is carried out. The module connections of pancake coils are performed. The electromagnetic field and force analysis of the module are contrasted under different parameters. To ensure proper operation of the module, the impedance of the module under representative operating conditions is calculated. Finally, the feasibility of the flux-coupling type SFCL in VSC-HVDC power systems is discussed.     

Oval test coil for examining mechanical effects in superconducting coils

A method of investigating mechanical effects in superconducting coils has been demonstrated using an oval magnet constructed to produce high mechanical stress in a portion of a small test coil. The oval shape is maintained by an internal longitudinal strut which may be used to prestress the winding and control the longitudinal deformation. Lateral and longitudinal deflections during energization are monitored by linear variable differential transformers. Initial results show very complex deflection patterns, particularly at low field. There are indications of diamagnetic behaviour of the composite twisted multifilamentary conductor below Hc₁ and of possible conductor slippage but much remains unexplained.     

Tilted toroidal coils for superconducting magnetic energy storage systems

Superconducting magnetic energy storage (SMES) systems can be used to improve power supply quality and reliability. In addition, large amounts of power can be drawn from a small stored energy supply. Nevertheless, the strong electromagnetic force caused by high magnetic fields and coil current is a serious problem for SMES. To cope with this problem, we propose a new coil design, the tilted toroidal coil (TTC). The TTC, obtained from the toroidal field coil (TFC) system by varying two pitching angles, allows the balancing of the electromagnetic force in the major radius direction, maintaining the same manufacturing simplicity of the TFC system. After determining balanced configurations through computer simulation, we built an experimental device to confirm the feasibility of the balancing effect.     

Development of Bi-oxide superconducting tapes and coils

We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I_c of 310 A (criterion 10⁻¹³ Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T (I_c = 385 A (10⁻¹³ Ωm)) at 4.2 K and 1.53 T (I_c = 225 A (10⁻¹³Ωm)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu-x%M (x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J_c values of 5–7 × 10⁴ A cm⁻² at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J_c values.     

Protection of superconducting coils by induction coupling

A model that predicts temperature rise in perfectly coupled circuits is presented. The use of frozen cryogen in a secondary protection circuit coupled to a superconducting coil is discussed in this paper. Formulas for the required amount of the frozen cryogen and conducting metal in the protection circuit are derived as a function of the desired final temperature in the protected coil.     

液氮温区高温超导滤波器的漏热分析

为在液氮温区工作的高温超导滤波器的制冷机冷源选型,基于Sage 10软件对超导滤波器件的冷却装置进行了漏热分析,仿真计算了铜线和同轴线的几何参数对传导漏热量的影响,以及真空罩、冷盘的尺寸和发射率对辐射漏热量的影响,并综合上述分析计算了超导滤波系统的总漏热量。在仿真计算中发现,信号线的导热和真空罩中冷盘的辐射漏热在系统总漏热量中起主导作用。仿真计算结果表明,通过增大信号线长度、减小信号线直径的方式可将信号线漏热量降至0.72 W,约为初始导热量的1/3。与此同时,冷盘表面采用抛光镀金的方式减小表面发射率,使辐射漏热量降至原来的2/3。根据模拟计算的最优结果,选择制冷量为3 W@77 K的制冷机作为高温超导滤波器的冷源。     

Dc power system for mutual coupled large scale superconducting coils

The large helical device (LHD) is a fusion experimental facility which has a large scale mutually coupled superconducting coil system and it is now under operation at the National Institute for Fusion Science (NIFS). This paper shows the design and the operation results of the dc power systems to drive these coils. First, the outline of the LHD and the power supplies are described. Next, a control system and the current controller for the power supplies are introduced. Finally, some experimental results are introduced.