Thermal stability of superconducting magnets: static criteria

To analyse the stability of a superconductor, a static analysis is often employed. The static solution of the energy equation describing the superconductor forms the boundary between a stable and an unstable region of operation. We solved the static energy equation analytically for the constant property case, and numerically for the variable property case. The results are gathered in graphs, from which the critical energy as a function of j and αj² can be found. In the variable property case we introduced θ_s as an additional variable to characterize the conductor. In a second paper we will analyse the dynamic behaviour of a conductor and compare it with the criteria introduced in this paper.     

Disconnectable current leads for superconducting magnets

A pair of 100 A disconnectable current leads for superconducting magnets has been tested. This research note describes the device and gives the results of the tests. The liquid helium consumption due to the Joule dissipation of the gold contacts is small compared to the saving when the leads are disconnected. During an average duty cycle, the overall efficiency is, therefore, increased by a factor of two over that for permanent current leads.     

The role of quench back in quench protection of a superconducting solenoid

This report demonstrates the role of quench back in the quench protection of high current density superconducting solenoid magnets with well-coupled shorted secondary circuits. The phenomenon of ‘quench back’ can be used to greatly reduce the size of an external quench protection resistor or even to eliminate the need for an external quench protection system altogether. A comparison is made with conventional magnet quench protection systems with and without a closely coupled secondary circuit.     

电力推进船舶中央冷却系统动态特性建模及控制仿真

通过对电力推进船舶中央冷却系统的综合分析，建立电力推进船舶中冷系统的稳态换热模型和动态换热模型。对中冷系统的冷却水出口温度的控制方案，进行了传统的PID控制和基于功率的前馈、反馈控制研究，并在此基础上对控制方案计算机仿真，比较两种控制方案的区别。     

Induction-drive magnetohydrodynamic propulsion

The use of magnetohydrodynamic propulsion for marine applications is reviewed with emphasis on induction-drive systems such as the ripple motor. Comparisons are made with direct-drive MHD propulsion systems. Application to pumps for hazardous fluids and liquid-metal coolants are also discussed.     

Coarse balancing a superconducting gradiometer at room temperature

An electronic measuring system is described, by which capabilities of balancing superconducting gradiometers can be checked at room temperature before connecting them to SQUIDs.     

Influence of superconducting magnet winding structure on conductor cryostability

The influence of the volume of the superconducting winding interlayer space and the structure of the insulation coating upon the conductor recovery current value I_r has been investigated experimentally. An analysis of the optimum conductor insulation coating was performed, considering different forms of an organic fibre net. The optimum thickness and cell dimension of the net were chosen based on the I_r measurements. It has been shown that the optimal volume of interlayer cooling channels increases the conductor cryostability, and creation of additional gaps of 200 μm in the winding spacers is able to intensify the heat transfer.     

High-temperature superconducting dual-mode resonator on (100) MgO substrate

We report, for the first time to our knowledge, a clear resonant peak split in the range of 7.7–9.7 GHz in a perturbed dual-mode disk-type resonator (DMDR) made of YBa₂Cu₃O_7–x (YBCO) superconducting thin film on MgO substrate. Epitaxial YBCO superconducting thin films were grown on (100) MgO substrates by pulsed laser deposition technique. The critical temperature of superconducting thin film on MgO substrate was 85 K. Superconducting dualmode disk resonators were designed by microwave design software, EEsof, and patterned by photolithography and a wet-etch process. The unloaded quality factor (Q_UL) of the superconducting DMDR was found to be 1,312 at 77 K. We believe this type of DMDR can be utilized for dual-mode resonator-based filters for satellite communications.     

A novel high temperature superconducting magnetic flux pump for MRI magnets

This paper presents a kind of minitype magnetic flux pump made of high temperature superconductor. This kind of novel high temperature superconducting (HTS) flux pump has not any mechanical revolving parts or thermal switches. The excitation current of copper coils in magnetic pole system is controlled by a singlechip. The structure design and operational principle have been described. The operating performance of the new model magnetic flux pump has been preliminarily tested. The experiments show that the maximum pumping current is approximately 200 A for Bi2223 flux pump and 80 A for MgB₂ flux pump operating at 20 K. By comparison, it is discovered that the operating temperature range is wider, the ripple is smaller and the pumping frequency is higher in Bi2223 flux pump than those in MgB₂ flux pump. These results indicate that the newly developed Bi2223 magnetic flux pump may efficiently compensate the magnetic field decay in HTS magnet and make the magnet operate in persistent current mode, this point is significant to the magnetic resonance imaging (MRI) magnets. This new flux pump is under construction presently. It is expected that the Bi2223 flux pump would be applied to the superconducting MRI magnets by further optimizing structure and improving working process.     

Normal zone propagation within a superconducting magnetic system and cryostabilization of superconductivity

A theoretical study of normal zone propagation along composite conductors in contact with liquid helium which accounts for the resulting rate of heat release and non-uniformity of heat conduction coefficient is described. The Maddock-James-Norris theorem is shown to be true only for a constant heat conduction coefficient. The formulae are given for the speed of the normal zone propagation. Equations have been obtained which describe the collapse dynamics or normal zone growth within the superconducting magnetic systems; time of collapse and normal zone growth being evaluated. Non-linear heat waves of a new type are assumed to exist in a composite in addition to the solution with an interface of normal and superconducting regions.     

Integration of superconducting magnets with cryogen-free dilution refrigerator systems

There has been much recent research interest into “cryogen-free” dilution refrigerators. Cryogen-free systems have some advantages from a safety and convenience point of view as liquid cryogens are unnecessary. However, this also makes integrating the low-temperature system with a high magnetic field environment much more challenging. Here we shall describe recent successes of integrating superconducting magnets and dilution refrigerators into one system requiring a single pulse tube cooler. The resulting environment provides experimental temperatures between 7 mK and 30 K and magnetic fields up to 12 T. We shall describe the effects of AC loss heating in such systems on the pulse tube refrigerator when the field is ramped and also the effects of eddy current heating on the mixing chamber in sweeping fields.     

A Tl-based high-temperature superconducting X-band cavity

A nonplanar X-band cavity amenable to coating with high-temperature superconducting films by conventional physical vapor deposition processes has been designed, fabricated, and tested. The cavity geometry resembles a symmetric clamshell configuration. It consists of two truncated cones joined at their bases. The specific dimensions of the cavity were calculated using the SUPERFISH computer code. Cavities were constructed using a silver-based alloy, Consil 995. The separate cavity sections were coated with Tl-based high-temperature superconducting material using a two-step deposition and annealing process. The unloaded Q values for this coated clamshell cavity are 66,000 and 105,000 at 77 and 20 K, respectively.     

Acoustic emission during quench training of superconducting accelerator magnets

Acoustic emission (AE) sensing is a viable tool for superconducting magnet diagnostics. Using in-house developed cryogenic amplified piezoelectric sensors, we conducted AE studies during quench training of the US LARP’s high-field quadrupole HQ02 and the LBNL’s high-field dipole HD3. For both magnets, AE bursts were observed, with spike amplitude and frequency increasing toward the quench current during current up-ramps. In the HQ02, the AE onset upon current ramping is distinct and exhibits a clear memory of the previously-reached quench current (Kaiser effect). On the other hand, in the HD3 magnet the AE amplitude begins to increase well before the previously-reached quench current (felicity effect), suggesting an ongoing progressive mechanical motion in the coils. A clear difference in the AE signature exists between the untrained and trained mechanical states in HD3. Time intervals between the AE signals detected at the opposite ends of HD3 coils were processed using a combination of narrow-band pass filtering; threshold crossing and correlation algorithms, and the spatial distributions of AE sources and the mechanical energy release were calculated. Both distributions appear to be consistent with the quench location distribution. Energy statistics of the AE spikes exhibits a power-law scaling typical for the self-organized critical state.     

A model for calculating a.c. losses in multistage superconducting cables

Superconducting magnets in tokamaks for fusion experiments are subjected to fast variations in magnetic field. As the high current conductors used in these magnets are made of multistage cables, these variations induce interstrand coupling currents that create losses. These losses are usually characterized by the so-called time constant of the conductor. A model is given to calculate this time constant. Working formulas are also proposed to calculate the current induced in the different cabling stages. This model takes into account the strand characteristics and the detailed cabling pattern. Using it, a method is also given to deduce the time constant from resistive measurements. The influence of the resistive barrier (chrome plating, CuNi shell, outer bronze matrix) is pointed out. Finally, this model is applied to a conductor that is foreseen for the toroidal coils of the International Thermonuclear Experimental Reactor (ITER).     

一种可控激励的柔性基础-轴系推进实验系统

 为进行复杂海洋环境中因波浪载荷导致的船体柔性变形状态下的特种船舶-推进装置系统振动实验研究,提出了基于垫升船舶的激励可控的模拟实验系统的设计思想,设计了一种由柔性浮箱、弹性支承、变频电机、风机和空气螺旋桨负载、推进轴以及控制系统构成的船舶轴系推进装置的实验系统.为模拟波浪载荷对柔性船体变形的影响,研究了实验系统的激励控制元件、电路设计和激励控制策略.利用该实验系统,进行了10,20和30 mm等三种中拱变形状态下的基础变形控制测试.测试结果显示：该实验系统能对柔性基础的各弹性支承的高度进行组合调节,并具有良好的变形控制精度,能够模拟海洋环境激励对船体的影响.研究结果表明,该实验系统可为船体刚度较小的特种船舶进行推进装置振动特性研究提供有效的实验研究平台.     

In-situ deposition of superconducting YBCO thin films by magnetron sputtering from powder target

We have used stoichiometric Y₁Ba₂Cu₃O_7–x powder as magnetron sputtering target to deposit high-quality high-T _c superconducting thin films on MgO, SrTiO₃, and ZrO₂ substrates. The zero-resistance temperatures are 86–88 K, and the 77 K zero-field critical current density is 8 × 10⁵ A/cm². The films are highlyc-axis oriented. Films deposited on 10 × 10 × 1 mm³ ZrO₂ substrates have surface resistances below 25 m at 77 K and 94 GHz. Using powder targets instead of bulk targets has the following advantages: simple and low-cost target preparation, simple target replacement, and versatility for large-area deposition.     

Worldwide cryogenics — Kernforschungszentrum karlsruhe

The Institute for Technical Physics of the Nuclear Research Centre Karlsruhe has devoted its main activity to applications of superconducting magnets. In this article, results of its low temperature research and technical developments are presented.     

Experiences on high current leads for superconducting magnets; seven types from 1 kA to 30 kA

In order to extend the current lead techniques to higher currents, the operational minimum flow rates of leads from 1 to 30 kA were measured and compared with the design value.     

Exploration of new superconductors and functional materials,and fabrication of superconducting tapes and wires of iron pnictides

Abstract

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ～1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ～700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.