Electromechanical investigations and quench experiments on sub-size HTS cables for high field EU-DEMO Central Solenoid

The aim of this thesis is to investigate the mechanical and thermal behaviour of HTS coated conductor cables for EUROfusion DEMO magnets.One of the issue in large HTS magnets is the safety against a quench. A quench is a sudden thermal runway in which the energy stored in the magnet is turned into heat in a localized volume of the magnet. If no countermeasures are applied, the localized temperature increment might damage the magnet. A Quench Experiment has been carried out to provide experimental data on the full quench evolution in sub-size HTS cables for fusion application. The SULTAN facility has been upgraded. This upgrade allows us to provide large power to the conductor for indefinite time. Previously, the maximum power was limited to few W for few seconds. Four different sub-size conductors designed for the Central Solenoid magnet of EU DEMO have been fabricated with REBCO coated conductors. All the cables are based on a triplet of round strands, each strand is composed of two half-round copper profiles surrounding a stack of tapes. The copper profiles and the tapes are soldered together. The triplet is insert in steel jacket to obtain forced-flow conductors. The quench is triggered by sending hot helium in the conductor and the temperature evolutions of the conductor components (jacket, helium) have been measured up to temperatures of 150 K. Due to the gradients between cable, He and jacket, the maximum estimated temperature value on the cable is above 200 K, depending on the specific cable design. The experimental results of the different conductors have been interpreted with the support of a multi-physics code (THEA). It has been seen that decreasing the transverse thermal resistance between strand and jacket of one order of magnitude leads to a decrease in the temperature rise rate from tens of K/S to just few K/s: the heat produced in the cable can be better absorbed by the large thermal inertia of the jacket. The he multi-physics model indicates that, in case of the quench is caused by hot helium flow, the voltage rises much faster in case of homogenous magnetic field, leading to a lower hot spot temperature at the quench detection.The combination of large current and high magnetic field results in very large transverse stress on the cable. The superconducting ceramic material is brittle, and the mechanical limit for coated conductors depends on the direction of the force. When the limit is exceeded, the current carrying ability of the cable is compromised. We quantified the critical transverse pressure at which the performance of the cable is decreased. Various round strands were fabricated, exploring different reinforcement options. The critical transverse pressure is measured on a strand by testing it at liquid nitrogen temperature, applying the force thanks to an anvil. The measured critical transverse pressure values indicate which strand designs should be discarded, i.e. when the measured critical transverse pressure values are lower than the nominal ones foreseen in the full cable design. For one of the reinforcement options, a finite element mechanical model was developed to better understand the mechanics and to propose improvement to the cable design. It was found that the performance might be improved by wrapping the strand with a wire whose thermal expansion coefficient is higher than the of copper. The strand is precompressed during the cool down, decreasing the stress that causes the rupture of the tapes.

Development of HTS Conductors for Fusion Magnets

Pierluigi Bruzzone, Davide Uglietti, Rainer Wesche

In view of the development of cables for next-generation fusion reactors, research activities are carried out on all HTS materials available from industrial production. Preliminary design of a react and wind cable using Bi-2212 wires is carried out, inspired by the Nb3Sn cable for European DEMO. The design and construction of a 60-kA prototype cable made of coated conductors have been carried out. The strands in the cable are composed of a stack of coated conductor tapes (4 mm wide) embedded in a copper profile of 6.3 mm in diameter. Tapes and copper profiles are soldered together in order to obtain a mechanically solid strand and to keep the inter tape resistance at a minimum, so that current can be easily redistributed among the tapes in a strand. The strands are fabricated in pieces 2 m long, but scaling up to industrial production should not present any major problem. A flat cable is manufactured by winding 20 strands around a central copper former; the cable will be inserted in a steel jacket for force flow cooling. The critical current of each strand was measured at liquid nitrogen in self-field just after the manufacturing process (twisted and straight) and after cabling: No reduction of the critical current was observed. Two pieces of cable, each 2 m long, are going to be prepared and assembled to form a sample that can be tested in the EDIPO facility.

2015

Prospects for the use of high-T-c superconductors in fusion magnets and options for their test in SULTAN

Pierluigi Bruzzone, Stephen Alfred March, Boris Stepanov, Davide Uglietti, Rainer Wesche

In the last few years, the critical current densities of long commercially available REBa2Cu3O7-x (RE-123, where RE represents Y or a rare earth element) coated conductors have reached values of 250 A/cm-width at 77K and zero applied field. Even higher values of 600 A/cm-w (77 K, B = 0) have been demonstrated in shorter lengths. The attractive features of the use of these high-T-c superconductors (HTS) are operation temperatures above 20K and/or magnetic fields higher than those envisaged for the ITER TF coils. Possible operation conditions for HTS fusion magnets have been studied taking into consideration the possible further improvements of RE-123 coated conductors. Investigations of stability and quench behavior indicate that stability is not a problem, whereas quench detection and protection need attention. Because of the high currents necessary for fusion magnets, many tapes need to be assembled into a transposed conductor. The qualification of HTS conductors for fusion magnets would require their test at magnetic fields of 11 land currents well above 10 kA. The possibilities to test straight HIS conductor samples in SULTAN have been considered. For a test at 4.5 K, only the development of a low resistance joint between the HTS conductor under test and the NbTi transformer of SULTAN would be necessary. Tests up to 20K would require that the HTS sample is connected with the NbTi transformer by a conduction-cooled HTS bus bar of large thermal resistance similar to the HIS module of a current lead. HIS conductor tests at temperatures around 50K would be possible with modified cryogenics. (C) 2013 Elsevier B.V. All rights reserved.

Elsevier Science Sa2013

Critical current test results of 13 T-46 kA Nb3Al cable-in-conduit conductor

Pierluigi Bruzzone

In the framework of ITER-EDA, a 13 T-46 kA Nb3Al conductor with stainless steel jacket has been developed in order to demonstrate applicability of an Nb3Al conductor with react-and-wind technique to ITER-TF coils. Using a 3.5 m sample consisting of a pair of conductors with 0% and 0.4% bending strain, the critical current performances of the Nb3Al conductors were studied to verify that the conductor achieves the expected performance and the bending strain of 0.4% does not originate degradation. The critical currents were measured at background magnetic fields of 7, 9, 10 and 11 T at temperatures from 6 to 9 K. The expected critical currents were evaluated taking into account the variation of the strain in the cross-section due to the bending strain as well as self-field and non-uniform current distribution as results of an imbalance in the joint resistance and inductances. The calculation results indicate that the current distribution is almost uniform and the experimental results showed good agreement with the expected critical currents. Accordingly, we can conclude that the fabrication process of this conductor is appropriate and the react-and-wind technique using the Nb3Al conductor is applicable to ITER-TF coils. In addition, the critical current of the Nb3Al conductor is expected to be 108 kA at 13 T and 4.5 K, resulting in a sufficient margin against the nominal current of 46 kA. Furthermore, it was found that the decrease in the critical current by thermal strain can be made small by applying the bending strain to the conductor so as to reduce the compressive strain at higher fields, i.e. inner side of the coil, in the conductor cross-section. (C) 2003 Elsevier Science Ltd. All rights reserved.

2002