Cryogenic Stability of LTS/HTS Hybrid Conductors

Hybrid-type superconductors are proposed by utilizing a bundle of high-temperature superconducting (HTS) tapes as a stabilizer of low-temperature superconducting (LTS) cables in order to extend the basic research on the cryogenic stability of solid composite-type superconductors and to explore its potential. Since the effective resistivity of HTS is significantly lower than that of pure metals of equivalent cross-sectional area, a bundle of HTS tapes may work as a good stabilizer to achieve high current density. Short sample experiments have been carried out by modifying the aluminum-stabilized superconductor used for the LHD helical coils and the cryogenic stability was examined

Effective Resistance of the HTS Floating Coil of the Mini-RT Project

A magnetically levitated superconducting coil device, Mini-RT, has been constructed using high temperature superconductors for the purpose of examining a new magnetic confinement scheme of high-beta plasmas. The floating coil is wound with Bi-2223/Ag tapes, and it is operated in the temperature range of 20-40 K. The excitation tests of the coil were carried out and persistent current was sustained for magnetic levitation. The decay time constant of the persistent current was measured and the effective resistance of the coil cables was evaluated. The obtained resistance shows a considerable increase than that predicted by the n-value model. This might be caused by some electromagnetic effects such as the loss generation with long-lived shielding currents. This consideration was examined by measuring the magnetization of an HTS sample coil

Effects of spatially limited external magnetic fields on short sample tests of large-scale superconductors

For short sample tests of large-scale superconductor coil conductors, it is difficult to get sufficient spatial uniformity using external magnetic fields because of the size limitations of test facilities. The effects of spatially limited external magnetic fields on short sample tests are discussed by comparing the test results for narrow and broad external magnetic fields. The authors tested short samples of pool-cooled 10 kA class superconductors using two kinds of split coils which are different in bore size. The measured recovery currents for the narrow external field are more than twice those for the broad field. It shows that the insufficient spatial distribution of the external field biases the stability measurements of superconductor

Hysteresis Loss in Poloidal Coils of the Large Helical Device

Hysteresis loss in poloidal coils of the Large Helical Device (LHD) has been measured during single-pulse operation. The superconductors of the coils are Nb-Ti cable-in-conduit conductors (CICC) cooled by forced-flow supercritical helium. The loss was measured by monitoring the enthalpy increase of the helium coolant between the inlet and outlet. Although the hysteresis loss was extracted by extrapolating several data sets from pulse excitations with different sweep rates, the extrapolated loss was much larger than the estimation using the magnetic hysteresis of the conductor. The anomalous increase in the loss is likely due to inter-strand coupling loss with long time constants from the order of 10 to 1000 s. The calculations show that the additional coupling loss behaves like a hysteresis loss

Development of static magnetic refrigeration system using multiple high-temperature superconducting coils

It is expected to build a sustainable social system that uses “hydrogen” as a fuel to generate electricity without emitting CO2. To realize this, technology for storing a large amount of hydrogen is indispensable, and storage as liquid hydrogen is ideal. However, the efficiency of the cooling device in the temperature range around 20 K required for long-term storage with liquid hydrogen is low, and the equipment is huge and expensive, so it has not been established as a widely used technology. Magnetic refrigeration is expected to be a highly efficient refrigerator in the temperature range of around 20 K because it can realize an ideal refrigeration cycle. However, in magnetic refrigeration, it is necessary to give a magnetic field change to the magneto caloric material (MCM). Further, in order to perform cooling with a large capacity and extremely low temperature by magnetic refrigeration, the magnetic field strength of a permanent magnet is insufficient, and it is indispensable to use a superconducting coil capable of generating a strong magnetic field with low power consumption. This study aims to develop a static magnetic refrigeration system using multiple high-temperature superconducting coils. By utilizing the energy storage characteristics of the superconducting coil, we are considering a magnetic refrigeration system that can repeatedly generate magnetic field changes to save energy without the need for large amounts of energy to be taken in and out of the outside. We report on the technical feasibility of a static magnetic refrigeration system using HTS coils. The power consumption including the AC loss of two superconducting coils, which is the basic configuration of the static magnetic refrigeration system, is calculated, and the efficiency is estimated as a ratio to the assumed refrigeration capacity of the MCM

Operational status of the superconducting system for LHD

Large Helical Device (LHD) is a heliotron-type experimental fusion device which has the capability of confining current-less and steady-state plasma. The primary feature on the engineering aspect of LHD is using superconducting (SC) coils for magnetic confinement: two pool boiling helical coils (H1, H2) and three pairs of forced-flow poloidal coils (IV, IS, OV). These coils are connected to the power supplies by SC bus-lines. Five plasma experimental campaigns have been performed successfully in four years from 1998. The fifth operation cycle started in August 2001 and finished in March 2002. We have succeeded to obtain high plasma parameters such as 10 keV of electron temperature, 5 keV of ion temperature and beta value of 3.2%. The operational histories of the SC coils, the SC bus-lines and the cryogenic system have been demonstrating high reliability of the large scale SC system. The operational status and the results of device engineering experiments are summarized

Effects of Subcooling on Lengths of Propagating Normal Zones in the LHD Helical Coils

Propagation of a short normal zone was observed in a helical coil of the Large Helical Device, when the coil was cooled with subcooled helium, of which the inlet and outlet temperatures are 3.2 K and below 4.0 K, respectively. The normal zone was induced at the bottom position of the coil. It propagated to only the downstream side of the current with recovery from the opposite side, and stopped after passing the outer equator of the torus. The induced balance voltage is obviously lower and the propagating time is shorter than those of propagating normal zones observed in the helical coil cooled with saturated helium at 4.4 K. According to the simulation of the propagation of a normal zone, it is considered that such a short normal zone at the current close to the minimum propagating current propagates without full transition to film boiling

Feasibility Study of High-Efficiency Cooling of High-Temperature Superconducting Coils by Magnetic Refrigeration

As a high-efficiency cooling technology for high-temperature superconducting coils, we have begun research and development to examine the feasibility of a cooling assist technology that maintains a cryogenic state by combining the magnetic force generated by the superconducting coil with magnetic refrigeration technology. Magnetic refrigeration requires the magneto caloric material to change the magnetic field. In many cases, the magnetic field change is obtained by moving the magnetic source, but moving the superconducting coil is not a good idea. Although it is possible to generate a change in the magnetic field by turning on and off the power supply of the high-temperature superconducting coil, it is unlikely to be established as a system that assists cooling of the superconducting coil because the coil generates heat due to AC loss. Therefore, it was considered that the magnetic field change can be obtained if the magnetic force generated from the superconducting coil can be controlled by the magnetic shield. As a verification of the principle, it was clarified experimentally that a magnetic field change can be obtained by repeatedly inserting and removing the magnetic shield into and from the gap between the magnetic field generation source and the magneto caloric material, and the temperature change can be extracted by the magneto caloric effect. In the experiment, the temperature change obtained when a magnetic shield was inserted into the air gap was measured by a simple test device using an iron-based magneto caloric material having high magneto caloric effect performance at room temperature and a permanent magnet. The principle verification confirmation test was performed using several types of magnetic shielding materials such as Permalloy bulks and the electrical steel sheets. In addition, numerical analysis is performed on the magnetic shielding effect, and the shielding effect is improved by increasing the thickness of the shielding material and the possibility of using high temperature superconductors as magnetic shielding materials were also analyzed. In this study, we report the possibility of cooling the high-temperature superconducting coil with high efficiency by combining the magnetic field created by the superconducting coil and magnetic refrigeration

Engineering research and development of magnetically levitated high-temperature superconducting coil system for mini-RT project

A magnetically levitated superconducting coil system is being developed using high temperature superconductors for examining a new magnetic confinement of high-beta plasmas. A miniature double-pancake coil was fabricated with a Bi-2223 Ag-sheathed tape for the purpose of developing a floating control using laser displacement gauges. The coil was inductively excited with liquid nitrogen cooling and successfully levitated in the air. A persistent current switch is also being developed with a Bi-2223 Ag-0.3wt%Mn-sheathed tape, and a prototype model was successfully tested

Stable long-term operation of superconducting current-feeder system for the LHD

A superconducting (SC) current-feeder system is used as the current transmission lines for the experimental fusion device, LRD. It consists of nine flexible SC bus lines with total length of 497 m, and nine pairs of gas-cooled current leads. To avoid the propagation of the ice on the leads, the temperature of the terminals had been kept in the range between 5 and 20 degrees C by the heaters. The measured voltage drops of all leads were less than 20 mV. The liquid helium levels of the leads and the sub-cooler tank will equalize by the siphon method. The total time of the coil excitations exceeds 3000 hours. We have demonstrated successfully that the SC current-feeder system was stable and easy to handle, and is useful for the SC experimental fusion device