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

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.

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

Test Results of Three Poloidal Field Superconducting Samples in SULTAN

Pierluigi Bruzzone, Boris Stepanov, Rainer Wesche

Superconductors for the ITER Poloidal Field Coils are large cable-in-conduit conductors (CICC) made of NbTi strands encased in a round-in-square stainless steel jacket. Three prototype conductor sections for poloidal field coils PF1/6, PF2/3/4 and PF5 have been fabricated in collaboration of the domestic RF, CN and EU agencies and tested in SULTAN Test Facility at the nominal operating conditions. The test aimed to characterize the DC and AC behavior of the conductors. The DC test was focused on the current sharing temperature (T-cs) at the nominal operating current and nominal operating background field. The take-off electric field at the nominal Helium mass flow rate was investigated versus the cable current density over a broad range of field and temperature. The AC loss measurement was performed before any electromagnetic loading and after a number of load cycles in order to define the impact of cyclic loads on the coupling currents constant of the cable. From the test results in SULTAN test facility, the margins in normal operation and the limits of the operation range of the ITER PF conductors are assessed.

IEEE-Inst Electrical Electronics Engineers Inc2012

Qualification tests for ITER TF conductors in SULTAN

Pierluigi Bruzzone, Boris Stepanov, Rainer Wesche

From February 2007 to May 2008, 18 short length conductor sections have been tested in SULTAN for design verification and manufacturer qualification of the ITER Toroidal Field (TF) conductor. The test program is focussed on the current sharing temperature, T-cs, at the nominal operating conditions, 68 kA current and 11.15T effective field, which can be fully reproduced in the SULTAN test facility. A broad range of results was observed, with over 2 K difference among the T-cs of the conductors. In average, the results are poorer compared to the potential performance estimated from the strand scaling law. The key parameters to mitigate the degradation are not yet clearly identified. The experimental challenges to test conductors with performance degradation are highlighted, including enhanced instrumentation sets, the application of gas flow calorimetry to sense the current sharing power and the post-processing of voltage data to cancel the transverse potential across the cable. The updated schedule of the tests in SULTAN is presented with the short-term action plan for conductor test. (C) 2008 Elsevier B.V. All rights reserved.

Elsevier2009

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

Ortensia Dicuonzo

EPFL2022