High-field quench behavior and dependence of hot spot temperature on quench detection voltage threshold in a Bi₂Sr₂CaCu₂O_xcoil

Abstract

Small insert solenoids have been built using a multifilamentary Ag/Bi 2 Sr 2 CaCu 2 O x round wire insulated with mullite sleeve (~100 µm in thickness), and characterized in background fields to explore the quench behaviors and limits of Bi 2 Sr 2 CaCu 2 O x superconducting magnets, with an emphasis on assessing the impact of slow normal zone propagation on quench detection. Using heaters of various lengths to initiate a small normal zone, a coil was quenched safely more than 70 times without degradation, with the maximum coil temperature reaching 280 K. Coils withstood a resistive voltage of tens of mV for seconds without quenching, showing the high stability of these coils and suggesting that the quench detection voltage shall be greater than 50 mV to not to falsely trigger protection. The hot spot temperature for the resistive voltage of the normal zone to reach 100 mV increases from ~40 K to ~80 K with increasing the operating wire current density J o from 89 A/mm 2 to 354 A/mm 2 whereas for the voltage to reach 1 V, it increases from ~60 K to ~140 K, showing the increasing negative impact of slow normal zone propagation on quench detection with increasing J o and the need to limit the quench detection voltage to < 1 V. These measurements, coupled with an analytical quench model, were used to access the impact of the maximum allowable detection voltage and temperature upon quench detection on the quench protection, assuming to limit the hot spot temperature to <300 K.

Feasible voltage-tap based quench detection in a Ag/Bi-2212 coil enabled by fast 3D normal zone propagation

“…data in [2] but considering that most of the data in [2] were obtained in liquid helium and our sample is a fully impregnated coil and therefore the helium cooling effects on slowing down normal zone propagation are depressed, this discrepancy is reasonable. Figure 6 shows the transverse normal zone propagation velocity in a background field of 7 T, 9 T, and 11 T, in comparison to that in a coil built with the same specifications but with wires insulated with a thick mullite sleeve insulation.…”

“…The importance of figure 7 and the impact of using thin insulation are better appreciated by comparing it to behaviors found in another coil we built recently to the same specifications but with the wires insulated with a thick mullite sleeve [2]. For the mullite coil, the hot spot temperature increased from ∼40 K to ∼140 K when the resistive voltage quench detection threshold was increased from 0.1 V to 1 V, and such dependence also quantitatively depends on the operating current density (figure 8).…”

“…After critical current measurement, the coil was subjected to a heater-induced quench experiment to determine the quench characteristics such as the minimum quench energy and normal zone propagation velocity. To simulate quenches, quenches were initiated at layer 6 (the outermost layer, turn 24) using an epoxy heater (figure 2) made from graphitebased electrically conductive epoxy ECOBOND 60L, previously used by Ghosh et al [7] and us [2]. To start a quench, rectangular current pulses of variable duration of 100-300 ms were supplied by a 200 W KEPCO power supply (KEPCO Bipolar 50-4D, ±50 V/±4 A).…”

“…We previously measured the hot spot temperature upon quench detection in a Bi-2212 coil fabricated from wires insulated with mullite sleeves and used it to measure the effectiveness of measuring the resistive normal zone voltage for quench detection [2]. The results showed that the effectiveness of quench detection using voltage taps is indeed compromised and the voltage development relies on the 1D growth of the normal zone along the wire.…”

Strain control of composite superconductors to prevent degradation of superconducting magnets due to a quench: I. Ag/Bi2Sr2CaCu2Oxmultifilament round wires

“…The simplification of quench degradation limit to T allowable is warranted by our earlier findings that the maximum hot spot temperature T max correlates well with J c degradation induced by a quench for a large pool of Bi-2212 wires and that the degradation is strain driven [11]. This experiment is representative of quenches in high-field magnets whereas prior quench experiments were performed either without electromagnetic stresses [11] or unknown (and well below 80 MPa) stresses in an epoxy impregnated coil [12,14]. Note that at 4.2 K, self field, when !…”