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

Abstract

High-temperature superconductors (HTS) 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 the HTS conductors is focused on their protection in the case of a thermal runaway (quench). In spite of extremely high thermal stability of HTS materials, quench is still possible due to local defects along the conductor length or insufficient cooling. In such cases, the high stability results in slow propagation of a resistive zone. Thereby, risky hot-spot temperature (>200 K) can be reached, if applying conventional quench detection methods at the voltage threshold of 0.1 – 0.5 V, typical for fusion magnets. Aiming at experimental study of the phenomenon, a series of sub-scale 15 kA 3.6 m long conductors based on stack of tapes soldered in copper profiles are manufactured at Swiss Plasma Center, including twisted ReBCO and BiSCCO triplets, non-twisted and solder-filled ReBCO triplets, as well as indirectly cooled non-twisted ReBCO single strand. Applying either 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 quench is actually triggered, the quench propagation is studied by distributed voltage taps and temperature sensors able to monitor the external temperature of the jacket and internal one of the conductor (helium or copper). Thanks to the recent upgrade of the SULTAN test facility, quench propagation in the conductors is measured up to the total voltage of 2 V and peak temperature of 320 K. Furthermore, novel quench detection methods based on superconducting insulated wires and fiber optics are also instrumented and studied. Summary of the test samples, their instrumentation and corresponding test results is presented in this work.

Validation and Application of Hysteresis Loss Model for HTS Stacks and Conductors for Fusion Applications

“…EXPERIMENTAL TESTS AC loss tests were performed in SULTAN [8] in the framework of the Quench Experiment campaign launched by EUROfusion in 2020, see [7] and [13]. The tests were carried out applying a sinusoidal AC external field of ± 0.2 T with a frequency of 0.3 Hz up to 1.5 Hz, on top of a DC background field.…”

“…However, only limited number of experimental tests have been performed to quantify hysteresis losses in conditions relevant for fusion magnets. Recently, AC loss tests were performed on sub-sized HTS conductors [7], also measuring the losses in case of DC current and AC magnetic field [8].…”

A comprehensive machine learning-based investigation for the index-value prediction of 2G HTS coated conductor tapes

“…The n-value importance for practical aspects of superconductors' behavior was recently investigated, showing its role in stability by affecting the quench velocity propagation and temperature [6][7][8][9]. Moreover, lower n-values indicate a slow transition to the normal state, carrying significant implications for practical applications.…”

Performance test of REBCO CICC sub-cables with 10 kA current under 20 T background field

“…HTS materials are brittle and stress/strain sensitive [9][10][11][12], which means that the cables, conductors and magnets need to be designed and constructed in such a way that the REBCO tapes are not subjected to excessive stress/strain conditions during manufacture, cool down and operation. To verify the availability of applying the proposed design concepts and developed manufacturing technologies, cables [13][14][15][16], insert coils [17][18][19] and CICC specimens [12][13][14][15][16][17][18][19][20][21][22] were manufactured and tested at low temperature and in different background magnetic fields. Experiments show that REBCO cables are capable of maintaining high current-carrying capability under a high electromagnetic (EM) load [23].…”