This paper reviews the fundamental thermodynamic behaviors of helium in superconducting rotating machine. Helium in a large diameter rotor of a superconducting generator would suffer the high centrifugal acceleration which ranges in several thousands fold normal gravity. In such a high centrifugal field, helium temperature near rotor periphery would rise due to the adiabatic compression. This temperature rise could seriously impair the current carrying capability of superconducting field winding especially in large diameter rotor. However, provided to arrange the helium coolant circuit appropriately in a rotor, one could realize the condition which would be favorable for the superconducting winding even in high speed rotor with large diameter. The thermosiphon effect and the self-pumping effect which are strongly related to the rotation would affect on the helium flow actively in a circuit and could offset the temperature rise resulting from compression of the helium in the rotor body. The several examples for the realization of these effects in helium management system inside rotors and the related problems are also described.
Ac losses of various kinds of pulsed conductors are measured. First, we found that the ac loss of the braid with Pb-40%Sn solder was extremely greater than that of the one without filler. Since the braid has the so-called three component strand, the intra-strand coupling loss will be quite small. Therefore, we concluded that the large increase of ac loss due to solder-filling was caused by the interstrand coupling through the outer copper sheath. To verify this, we measured the ac losses of the Cu-clad sample and the CuNi-clad one. Two samples have the same size and characteristics except that the one has copper outer sheath, the other does cupro-nickel one. With replacing Cu sheath by CuNi, the ac loss was decreased to the level of about one tenth of that of the Cu sheath strand. We also measured the ac losses of the compacted cables with and without Sn-5%Ag filler. Again, solder-filling caused a lot of increase in ac loss. Our conclusion is as follows: Even three component strands cause large interstrand coupling through the outer copper sheath if they are solder-filled. To avoid this, copper sheath should be replaced by cupro-nickel. When cables are not solder-filled, contact resistance among strands cuts the interstrand coupling effectively.