Numerical Analysis on the He II Heat Transport in Channels with a Porous Spacer

Heat transport characteristics in a pressurized He II channel have been studied, using two-dimensional numerical code that is based on the two fluid model. In general, He II heat transport performance gets worse in a either long or narrow channel [1]. If a porous medium is used as a part of the channel to transfer heat to a next channel, it will be expected to improve the heat transport in the channel. In this study, numerical model was based on the channel formed by two FRP plates in parallel and a spacer was inserted in the middle of the channel to divide into two regions. A heater was placed in one of FRP plate to input heat to the channel. Two kinds of spacers were used in the analysis to compare heat transport characteristics; (l) porous spacer, (2) FRP spacer. The temperature distribution and flow velocity vectors of He II in the channel were calculated with a steady state heat input. According to the calculated results, the temperature increase of He II in the heated channel was suppressed in the case of a porous spacer. In addition, the mass flow induced by the thermo-mechanical effect of He II enhanced heat transport capability. The paper also discusses the He II flows within the channel

Measurements of self-field and voltage for the REBCO stacked tapes assembled in rigid structure (STARS) conductor at 77 K

The measurements of self-fields and voltages for the stacked tapes assembled in rigid structure (STARS) conductor were carried out when the conductor immersed in liquid nitrogen was energized. In the measurements, a 3 m straight line shaped STARS conductor, which is composed of 45 REBCO tapes (15 tapes × 3 rows) embedded in a copper stabilizer, was used as a conductor sample. The measurement results indicate that the ramp rate at the conductor excitation and the one-time thermal cycle of the conductor do not affect the self-fields but the voltages.The current center position in the conductor cross-section during the conductor excitation was analyzed from the measurement results of the self-fields. The current center position always maintains the same position in the middle of the cross-section during the excitation. The analysis indicates that the current distribution of the STARS conductor is stable during the excitation. In addition, two-dimensional magnetic field calculations were conducted using the various models for the cross-sectional configuration of the STARS conductor. As a result, the calculation results agree with the measurement results regarding the self-fields

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

Observation of a non-uniform current distribution in stacked high temperature superconducting tapes

High Temperature Superconductors (HTS) improve upon low temperature superconductors in many ways and the ability to cope with a non-uniform current distribution might be one of those improvements. To put this to the test, an experimental setup is designed to force a non-uniform current upon a stack of 5 HTS tapes, using a worst case current feeding method. The experiment can help determine the potential of this conductor design and is part of the ongoing effort to develop a non-transposed stacked HTS conductor for the nuclear fusion reactor FFHR. The results clearly show that the conductor sample is able to stably conduct a current equal to its critical current, although at an elevated electric field of roughly 5 mV/m. This means non-transposed stacked tape conductors remain stable, even if a worst case nonuniform current is constantly forced upon them. A hypothesis to explain this abnormally high electric field is formulated on the basis of the results, however additional research is needed to verify it. It states that the electric field is necessary for the tapes to share current and would mean that in a properly engineered application, these losses due to the electric field, would only occur during start-up. Overall it is clear that this experiment proves the excellent stability of non-transposed stacked HTS tapes and their ability to conduct a non-uniform current

Design Concept of Supercritical CO2 Gas Cooled Divertors in FFHR Series Fusion Reactors

In the FFHR power reactor equipped with a supercritical CO2 gas turbine power generation system, an divertor cooling system is connected to this power generation system [S. Ishiyama et al., Prog. Nucl. Energy 50, No.12-6, 325 (2008) [1]]. In this paper, for the purpose of developing a diverter by supercritical CO2 gas cooling that can cope with a neutron heavy irradiation environment with a heat load of 15 MW/m2 or more, CFD heat transfer flow analysis was carried out for performance evaluation and its design optimization by a structural analysis models of a supercritical CO2 gas cooled divertors. As a result, in the supercritical CO2 gas cooled tungsten mono-block divertors (50 × 50 mm × 5 channel × 5,000 mL) with a flow path length of 5 m or less, the engineering designable range of these advanced diverters having the same cooling performance as the water cooling divertor was clarified, and its practicality is extremely high from the feature that the structural model has an extremely low risk during operation as compared with the water cooled divertor

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

NITA Coil—Innovation for Enlarging the Blanket Space in the Helical Fusion Reactor

An innovative idea is proposed for enlarging the blanket space on the inboard side of the torus for the helical fusion reactor FFHR-d1. A set of sub-helical coils, named NITA coils, with opposite-directed current outside the main helical coils, effectively reduces the helical pitch parameter and enlarges the blanket space. Dependence of the blanket space and plasma volume on the effective helical pitch parameter is examined. The obtained magnetic surfaces and their properties are compared with that of the original configuration

Seismic Analysis of Magnet Systems in Helical Fusion Reactors Designed With Topology Optimization

Superconducting magnets in fusion reactors are subjected to a huge electromagnetic force of >100 MN/m. The magnets have to be sustained with a strong-body structure to avoid high stress and deformation. The total weight of the magnet system in the fusion reactor is estimated to be more than 20,000 tons. We applied topology optimization technique to the magnet support structure to reduce the weight of fusion reactors. Compared with the conventional design, we achieved a weight reduction of >25%. Static and seismic analyses were carried out to validate the soundness of the topology-optimized design. Consequently, the stress against the electromagnetic force in the structure was within the permissible range. It was discovered that using seismic isolation structure can adequately prevent the damage to the magnet system even when directly subjected to a massive earthquake