2008 年 43 巻 10 号 p. 423-429
As a thermal property of MgB2 wires, we observed the propagation velocity of a normal zone for those immersed in liquid helium. The normal zone propagation was initiated by a resistive heater and the propagation velocity was estimated from the response of voltage difference between potential taps attached to the wire in a transverse magnetic field. The specimen was a mono-filamentary bare wire with a composite sheath of iron and copper. A part of the wire was cooled directly by liquid helium and the other was covered with a vacuum-grease layer for thermal insulation. The propagation velocity observed in each part was of a very low level in comparison with typical ones for usual NbTi wires, which may come from the high critical temperature and the low overall current density. We numerically calculate the propagation velocity with transient cooling models and discuss the normal zone propagation in relation to the cooling processes. We also evaluated the propagation of the normal zone in the MgB2 wires with advanced transport properties, which were attained in short specimens, under the condition of conduction cooling in a wide range of operation temperatures.