Transport measurement of MgB₂ wire under the sub-cooled water ice compared to other cooling conditions

Abstract

The standard DC transport measurement of MgB2 composite conductor was done in vacuum, low pressure helium gas, sub-cooled solid nitrogen and water ice. The effect of these coolants on electro-thermal behaviour was studied at self-field and temperature range from 32 K to 36.5 K. The obtained results show that even small volume of Helium gas enhances the sample thermal stability during the I-V measurement in comparison to those obtained in vacuum. The cooling by solid nitrogen improves the sample stability in contrast to helium gas. The measurement performed in water ice exhibits an excellent and stable behavior of superconducting wire carrying high transport current below and above the quench as well. Consequently, water ice shows simple, safe and promising He-free mode of cooling for future superconducting systems.

E–I characteristics and critical currents of small Bi-2223/Ag coil thermally stabilized by solid and liquid nitrogen compared to water ice

“…Figure 6 shows the coil's I c (T) characteristics measured in SN 2 and H 2 O with some deviation at temperatures lower than 30 K. This can be attributed to the weaker ability of SN 2 to homogenize the temperature in the coil due to the significantly lower thermal conductivity of SN 2 (0.25 W mK −1 ) [16], in comparison to H 2 O ice (∼23 W mK −1 ) at 30 K. A more detailed comparison of E-I characteristics measured in self-field and at an external magnetic field of 8 T is shown in figure 7.…”

“…Cooling of water from room temperature down to 12 K consumes some energy, but due to its favourable thermal properties, such as high sublimation pressure (∼10 −7 Pa @40 K) [8,13], relatively high thermal conductivity and capacity (k = 15 W m −1 K −1 and c p = 351 J Kg −1 K −1 @40 K) [14,15], the water ice still seems to be an inexpensive and safe cooling medium for superconducting windings and future He-free systems. Búran et al have performed transport measurements of MgB 2 wire under variable cooling conditions, which show that water ice allows stable operation in the normal state at currents above 200 A, which is not possible for any of the compared coolants [16]. However, there are still aspects of sub-cooled nitrogen and water ice, utilized for the efficient cooling of superconducting devices, that are not thoroughly understood, and thus require further detailed investigation.…”

Electrical and mechanical limits of ex situ MgB2 wires for cabling

“…The current leads were soldered at 5 mm from the end of each wire, and the voltage taps were located at the center of the sample, separated from each other by 5 mm. Critical currents at higher temperatures, 10-36 K, were measured for wire samples inside the sub-cooled water ice [15]. The I-V curves, far above the critical current criterion, were acquired at a constant current ramping of 0.43 As −1 up to the quenched current value (I q ), at which the current is fully expelled from MgB 2 filaments into the metallic sheath.…”