Liquid Neon system and characterization of ac loss and quench propagation for MgB2 conductors

Abstract

Magnesium diboride (MgB2) was found to have superconducting characteristics in January 2001 below 39K. The abundance and low cost of its components and the possibility to manufacture long length of wires and tapes using the powder in tube technique makes this material a good candidate for cheaper alternative to many low temperature superconductor application.
A closed system Neon liquefier and dedicated hybrid current leads have been designed to investigate properties of MgB2 based conductors in liquid Neon in the temperature range from 24.5K to 31K. The system is capable of liquefying 3 litres of Ne and recover all the boil off. The temperature of the liquid Ne can be adjusted by controlling the temperature of copper surface extending in the Ne vessel. 900A hybrid current lead was designed and built using BSCCO superconducting tapes at the lower end while the top end was constructed as a liquid nitrogen cooled copper lead.
In the Ne cryostat transport AC loss measurements of a non magnetic NiCr sheathed and a magnetic Ni sheathed Cu stabilized MgB2 tape have been conducted. Despite the NiCr alloy is non-magnetic, the high temperature treatment of the superconductor results in the formation of a magnetic layer at the interface with the superconducting core. For both samples the losses are found to be dominated by the hysteresis losses of the magnetic layer and sheath. The results are presented and analyzed.
The minimum propagation zone and the quench energy of a MgB2 superconducting wire has been measured in Ne vapour environment. Our results show a good agreement with published literature where the minimum propagation zone of MgB2 has been found to increase with the transport current.
AC losses in external applied magnetic field have been measured on MgB2 samples using a purposely built rig and sample holder employing sapphire fingers to effectively thermally link the sample to a cold source, consequently eliminating the eddy current losses in the sample holder. The losses of the MgB2 samples below Tc are dominated by the losses of the magnetic sheath and are higher than the losses above Tc. Our measurements suggest that the losses in the superconducting state are the result of an interaction between the ferromagnetic sheath and the diamagnetic superconductor, where as the sample is cooled below Tc the field in the outer magnetic sheath is increased.

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