LOSSES AND STABILITY IN SUPERCONDUCTING WIRE IN AC FIELDS WITH FREQUENCIES UP TO 2.5 KHZ

Results are presented of losses and stability measurements in a small superconducting (SC) coil with an alternating current at frequencies u p to 2.5 kHz. The mutual influence of losses and instability on the ma ximum attainable current is studied and discussed. It is shown that th ere are different modes of quench. SC wire can go to the normal state due to the development of instability with almost no overheating over the ambient temperature. In another mode, overheating over the current sharing temperature occurs due to losses under stable conditions. The experimental results agree with a recently developed stability theory for composite wires, taking into account the smoothness of the transi tion characteristic and its influence on stability. This theory explai ns why cryogenically unstable SC wires work at currents close to the c ritical current, why SC wires are more stable at lower rates of field change and current charge rate and why wires with a smoother transitio nal characteristic are generally more stable than SC wires with sharp transitions to the normal state. The theory thus allows us to approach the problem of the relationship between losses and stability.