IMPROVEMENTS AND VALIDATION OF THE NULL CALORIMETRIC METHOD FOR A.C. LOSS MEASUREMENTS IN SUPERCONDUCTORS

A new calorimetric method for a.c, loss measurements in superconductor s has been developed over the last few years, based on a null detectio n heating method. The measurements have shown general agreement with t heoretical predictions and with values obtained by other methods. More recently, a second version of the set-up has been built to solve some initial problems: mainly, the insufficient thermalization of the tape extremities and thermocouple reference junctions. These have been cor rected by the use of copper centred feed-throughs. It was then found t hat the new set-up yields correct results but with the use of an excha nge gas. The method has been validated through a comparison of the a.c , losses measured by the null calorimetric method for a copper tape wi th those deduced from standard electrical measurements. The new versio n of the null calorimetric method has been used to measure a,c. losses of silver and silver alloy sheathed Bi-2223 tapes. Dependencies on cu rrent and on frequency of the a.c. losses are correlated with the V-I curves, for a ''poor condition'' tape sample and a ''good condition'' tape sample, characterized by different n-values according to a fit V = k.l(n) . (C) 1998 Elsevier Science Ltd. All rights reserved.