Statistics of Test Results for the ITER Nb3Sn and NbTi Conductors in the SULTAN Facility

The series production of Nb3Sn and NbTi cable-inconduit conductors (CICC) for the winding of various coils in the International Thermonuclear Experimental Reactor (ITER) magnet system is being completed in 2015. The tests of the ITER conductor samples are running in the SULTAN test facility in Villigen, Switzerland, in the scope of a framework contract, flanked by six bilateral agreements between IO and the Domestic Agencies (DAs) of China, EU, Korea, Japan, Russian Federation, and USA. The key acceptance parameter of the tests for the conductor samples made from the series production ITER CICCs is the current sharing temperature Tcs at the nominal operating field and current after applied electromagnetic cyclic loadings at the nominal field and sinusoidal swing of conductor current from zero to nominal one. The Tcs is defined as the maximum temperature at which the conductors operate (nominal current and field) before developing an electric field of 10 μV/m. This paper gathers the latest test results of the ITER series production CICC. The qualified conductor units will be used in the winding of the toroidal field and poloidal field coils, as well as the central solenoid. The test results for the series production conductors are compared with the results of the conductors tested at the early stages of their development. The performance variation among the suppliers and the performance evolution since the start of the conductor production are discussed as well.

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

DC behaviour of NbTi cable-in-conduit conductors for ITER-FEAT

Pierluigi Bruzzone, Boris Stepanov, Martin Vogel, Rainer Wesche

Using the SULTAN test facility the d.c. behaviour of three subsize NbTi cable-in-conduit (CIC) conductors has been studied. The d.c. performance of the cables is compared with the critical currents and the current sharing temperatures expected from the strand data. As a basis for this comparison the critical current of the NbTi strands has been measured in the temperature range of 4.2-6.5 K in magnetic fields of up to 7.5 T. The d.c. performance of the NbTi subsize conductors has been found to be not significantly affected by cyclic loading. Taking into consideration the self field of the conductor the cable critical current and the current sharing temperature are in agreement with the strand data. (C) 2003 Elsevier Science B.V. All rights reserved.

2003

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

Ortensia Dicuonzo

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.

EPFL2022

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

Ortensia Dicuonzo

EPFL2022