Kazutaka Seo

Asymmetrical normal-zone propagation observed in the aluminum-stabilized superconductor for the LHD helical coils

Transient normal-transitions have been observed in the superconducting helical coils of the Large Helical Device (LHD). Stability tests have been performed for an R&D coil as an upgrading program of LHD, and we observed asymmetrical propagation of an initiated normal-zone. In some conditions, a normal-zone propagates only in one direction along the conductor and it hence forms a traveling normal-zone. The Hall electric field generated in the longitudinal direction in the aluminum stabilizer is a plausible candidate to explain the observed asymmetrical normal-zone propagation.

Development of UPS-SMES as a protection from momentary voltage drop

We have been developing the UPS-SMES as a protection from momentary voltage drop and power failure. The superconducting system is suitable as electric power storage for large energy extraction in a short time. The most important feature of superconducting coil system for the UPS-SMES is easy handling and maintenance-free operation. We have selected low temperature superconducting (LTS) coils instead of high temperature superconducting (HTS) coils from the viewpoint of cost and performance. However, it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium. Thus, a conduction cooled LTS pulse coil has been designed as a key component of the UPS-SMES. The development program of 1 MW, 1 sec UPS-SMES is explained.

Results of stability test in subcooled helium for the R&D coil of the LHD helical coil

Helical coils of the Large Helical Device are pool-cooled superconducting magnets. The operating current is restricted below about 90% of the design current because a normal-zone has propagated dynamically at several times at almost the same current. In order to estimate the effect of lowering temperatures on the cryogenic stability, an R&D coil was made of the same conductor. The cryogenic stability of the R&D coil was examined in saturated and subcooled helium. A normal-zone was initiated by a heater inserted between the conductor and the layer to layer spacer. The propagation was detected by voltage taps. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to begin propagation is 10.7 to 10.8 kA. It becomes higher at the lower temperature, and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil.

Analyses for inter-strand coupling loss in multi-strand superconducting cable with distributed contact resistance between strands

Abstract Multi-strand superconducting cables are utilized to large superconducting machines and/or alternative or pulse current ones, such as fusion machines and superconducting magnetic energy storages (SMESs). We have been studying for electromagnetic phenomena in multi-strand superconducting cables by using both analytical and experimental methods aiming at such large-scale applications. When we are going to evaluate alternating current (AC) loss numerically, especially inter-strand coupling loss, it is important to establish the model of crossover contact resistance distribution between strands and/or sub-cables. For the cable with multiple cabling stages, the contact resistances between two strands (or two sub-cables) change drastically due to the traces of them. They contact well each other at the places where they are crossover. On the other hand, when they are separated, there is no direct contact between them. The relation between AC loss and DC contact resistance was evaluated for a short sample, which is a 3×3 cable with Cr plated Nb 3 Sn strands. The AC loss was measured by the calorie metric method at LHe temperature. The contact resistances between two strands were also measured for the same sample in the same condition. Adapting measured contact resistance distribution to the numerical model, the AC loss calculation was fulfilled by using the distributed constant electric circuit. Comparing the calculated loss with the measured one, the modeling method of the contact resistance distribution was modified suitably. Finally the relation between the inter-strand coupling loss and the contact resistance distribution were discussed from the viewpoint of construction of the cable.

Experimental evaluation of loss generation in HTS coils under various conditions

To develop the high performance HTS coil operated in persistent-current mode, loss mechanisms of the HTS coil have been studied. We consider that the loss generation is associated with the shielding current and its temporal variations. To investigate the shielding current characteristics, the decay of shielding currents were measured under various conditions, and we evaluated these effects in simple experiments.

Analysis of joint-resistance-induced, non-uniform current distribution

The observation of nonuniform current distribution (NUCD) in multi-strand cables, in particular cable-in-conduit conductors (CICCs), has been reported often. NUCD may have important effects on AC loss and stability margin in such conductors; therefore, this phenomenon must be better understood. In this study, we have used numerical simulations to study the magnetic flux changes caused by NUCDs that themselves derive from details of lap-joint construction between cables composed of hundreds of strands and twisted in specific patterns. In the steady state, a NUCD is governed by the distribution and quantitation of contact resistances between individual strands in one cable to individual strands in the other. In an otherwise well-made joint, contact resistances will be lowest between strands that approach each other across the lap-joint-interface, and a strand that has many close encounters with the interface can be expected to carry larger current in the steady-state condition than a strand that has fewer or none. The length of the joint and the cable pattern determine the number of close encounters to the lap-joint-interface by individual strands. We present the results of our simulations using different joint length, we suggest experiments to observe these effects, and we discuss their significance with regard to conductor and magnet performance.

Design parameters influencing non-uniform current distributions in superconducting multi-stage stranded cables

We have suggested that non-uniform currents occur due to certain design parameters, for example a combination of cabling pitches, in a previous study. In the present study, we evaluate the occurrences of non-uniform current distributions in terms of design parameters, i.e., the combination of cabling pitches, winding methods (solenoidal and pancake winding) and cross-sectional shape of the cable by numerical analyses. Finally, some basic design concepts to solve this problem are proposed.

Improvements of Current Decay Behavior of HTS Coils in Persistent Current Operations

In order to improve the current decay behavior of high-temperature superconducting (HTS) coils using silver-sheathed Bi-2223 tapes in persistent current operations, the effect of the shielding current on the current decay behavior has been studied for magnetically levitated superconducting coils in internal ring devices. Generally, the current decay behavior of HTS coils is evaluated by the inductance of the HTS coils, the joint resistance and electric field versus current density relationship of the HTS tapes. In this paper, the effect of shielding currents in HTS tapes is discussed. It is shown that improvements of their properties of HTS tapes, which are the n-value and the critical current, are not so effective to decrease the shielding current in HTS tapes. Numerical electromagnetic analysis using finite element method and experiments have been carried out. Furthermore, improvements of the current decay behavior using excitation techniques are discussed.

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.

Critical Current of Mechanically Loaded

We developed a novel critical current and stability experimental setup utilizing a closed electric circuit with a multi-stand superconducting cable. The feature of this setup is transverse mechanical loading implied to the multi-strand cable in the transverse direction. It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. Furthermore, these phenomena were only observed in the ITER full size conductors with large Lorentz forces under high magnetic field. The advantage of our setup is a critical current measurement with comparable mechanical stress under high magnetic field. Employing an inductive critical current measurement technique, we conducted the experiment with transport current of around 10 kA without any power supply nor current lead. As experimental results, we observed significant degradation due to compressive stress of around 30 MPa. This degradation was found irreversible, when it was unloaded.