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Experimental study of stability, quench propagation and detection methods on 15 kA sub-scale HTS fusion conductors in SULTAN

Pierluigi Bruzzone, Nikolay Bykovskiy, Ortensia Dicuonzo, Kamil Sedlák

High-temperature superconductors (HTSs) enable exclusive operating conditions for fusion magnets, boosting their performance up to 20 T generated magnetic fields in the temperature range from 4 K to 20 K. One of the main technological issues of HTS conductors is focused on their protection in the case of thermal runaway (quench). In spite of the extremely high thermal stability of HTS materials, quenching is still possible due to local defects along the conductor length or insufficient cooling. In such cases, the high stability results in the slow propagation of a resistive zone. Thereby, a risky hot-spot temperature (>200 K) can be reached if applying conventional quench detection methods at a voltage threshold of 0.1-0.5 V, typical for fusion magnets. Aiming at an experimental study of the phenomenon, a series of sub-scale 15 kA 3.6 m long conductors based on stacks of tapes soldered in copper profiles are manufactured at the Swiss Plasma Center, including twisted rare earth barium copper oxide (ReBCO) and bismuth strontium calcium copper oxide (BISCCO) triplets, non-twisted and solder-filled ReBCO triplets, as well as indirectly cooled non-twisted ReBCO single strands. Applying either an increasing helium inlet temperature, overcurrent operation or energy deposited by embedded cartridge heaters, critical values of the electric field and temperature are evaluated for a given operating current (up to 15 kA) and background magnetic field (up to 10.9 T). Once quenching is actually triggered, the quench propagation is studied using distributed voltage taps and temperature sensors able to monitor the external temperature of the jacket and the internal temperature of the conductor (helium or copper). Thanks to the recent upgrade of the Supraleiter Test Anlage (SULTAN) test facility, quench propagation in the conductors is measured up to a total voltage of 2 V and a peak temperature of 320 K. Furthermore, advanced quench detection methods based on superconducting insulated wires and fiber optics are also instrumented and studied. A summary of the test samples, their instrumentation and corresponding test results are presented in this work.

IOP Publishing Ltd2023

Progress in the design and manufacturing of the EDIPO 2 cryostat

Pierluigi Bruzzone, Roberto Guarino, Kamil Sedlák, Evgeny Solodko

A major upgrade of the European DIPOle (EDIPO) test facility, which will allow the testing of superconducting samples both for fusion and for high-energy physics applications, is taking place at the Swiss Plasma Center. With a target background field of 15 T, the new magnet design requires a large engineering current density and thus an accelerator-like magnet cooled by a saturated helium bath is foreseen. Therefore, a helium vessel has been designed and procured.In this work, we present recent advances in the design and manufacturing activities of the EDIPO 2 cryostat. First, we provide an analytical estimation of the thermal budget at the temperature levels of interest. Then, we describe the design and manufacturing of the helium vessel, according to relevant norms. Finite-element nu-merical simulations are employed both to refine the design and to provide a comparison with the experimental pressure tests.

ELSEVIER SCI LTD2023

Application of Copper Thermal Spraying for Electrical Joints between Superconducting Nb3Sn Cables

Pierluigi Bruzzone

This manuscript reports on the application of copper thermal spraying in the manufacturing process of an electrical connection between Nb3Sn cables for superconducting magnets of fusion reactors. The joint is realized through diffusion bonding of the sprayed coating of the two cables. The main requirement for such a connection is its electrical resistance, which must be below 1 n omega at B = 8 T, I = 63.3 kA and T = 4.5 K. Micrographs of the joint prototype were taken to relate the joint resistance with its microstructure and to provide feedback on the manufacturing process. Optical microscopy (OM) was used to evaluate the grain size of the coating, presence of oxide phases and to analyze the jointed surfaces. Scanning electron microscopy (SEM) and, in particular, energy-dispersive X-ray spectroscopy (EDX) were used to confirm the elemental composition of specimens extracted from the prototype. It is shown that the copper coating has an oxide concentration of 40%. Despite this, the resistance of the prototype is 0.48 n omega in operating conditions, as the oxides are in globular form. The contact ratio between the jointed surfaces is about 95%. In addition, residual resistivity ratio (RRR) measurements were carried out to quantify the electrical quality of the Cu coating.

MDPI2022

Experimental study of stability, quench propagation and detection methods on 15 kA sub-scale HTS fusion conductors in SULTAN

Pierluigi Bruzzone, Nikolay Bykovskiy, Ortensia Dicuonzo, Kamil Sedlák

IOP Publishing Ltd2023