Local calorimetry to measure ac losses in HTS conductors

We present an overview of the development and use of local calorimetry to determine ac losses in individual HTS conductors, in coils and high amperage multi-tape conductors. The practical aspects of this technique are fully described, alone, with suggestions for achieving the desired resolution in loss measurement. Our recently developed local calorimetry method has been shown to allow the straightforward determination of losses of an individual conductor in almost any combination of field and current and is not limited to simple sinusoidal waveforms. We have extended this technique to structures of conductors e.g. coils and tape arrays where the ac magnetic fields generated by the structure as a whole influence the losses of the individual conductors. In these structures the individual HTS tapes see a local 'applied' magnetic field generated by the structure and a 'self'-field due to the current flowing in the tape. The interaction of the various magnetic fields makes it extremely difficult to measure the losses in these structures using for example pick-up coils and a lock-in amplifier, but using our calorimetric technique we are able to measure the losses at specific locations within the coil or array. In this paper we present examples of loss measurements made, for example, at various positions on a simple solenoidal coil and the changes in losses as a multi-tape, high amperage conductor is assembled. The data of local losses obtained on small-scale models may allow the estimation and minimisation of losses in full-scale designs or may be used in the verification of numerical models. (C) 2001 Elsevier Science Ltd. All rights reserved.