IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006 147 Transport AC Losses in YBCO Coated Conductors Luigia Gianni, Massimiliano Bindi, Franco Fontana, Stefano Ginocchio, Luciano Martini, Elena Perini, and Sergio Zannella Abstract—Modeling and design of superconducting power de- vices (e.g. fault current limiters), based on second generation HTS tapes, requires accurate evaluation and prediction of AC losses. Transport AC losses measurements have been performed on sam- ples of YBCO coated conductors at 77 K, as a function of current. The results have been compared with the classical analytical model for self field AC losses calculation, taking into account the role of the magnetic substrate. Index Terms—AC losses, magnetic substrate, transport current, YBCO coated conductors. I. INTRODUCTION D URING the last years, YBCO coated conductors (CCs) have gained a great deal of attention thanks to their high critical current density, above 1 at 77 K in self field, weakly dependent on magnetic field [1]. Many power applica- tions based on CCs have been envisioned, such as cables, fault current limiters and generators. The efficiencies and the techno- logical viability of these devices are strongly influenced by the energy dissipation in the conductor because of alternating fields experienced in operating conditions. Therefore, accurate mea- surements and prediction of CC’s AC losses are a critical issue for the development of HTS superconductor technologies. A number of potential routes are currently under develop- ment for the fabrication of YBCO CCs. In recent publication [2], [3], we demonstrated the suitability of CC’s production process based on the thermal co-evaporation technique and a simple ar- chitecture with a single buffer layer deposited onto cube textured Ni and Ni-5%at.W tapes. In this work, we have investigated the transport AC losses characteristic of YBCO CCs fabricated by this technique. AC losses measurements have been performed by a sensitive elec- trical method simulating the AC conditions typical of power devices at 77 K. The results have been compared with Norris for- mula for transport AC losses calculation. Respect to first gener- ation HTS tapes, transport self field losses in CCs exhibit a more complex behavior due to the magnetic substrate. To point out the substrate effect on overall transport losses, the hysteresis losses of Ni and Ni-5%at.W tapes have been evaluated by VSM mea- surements performed at 77 K. Moreover, the best fit between experimental AC losses results and calculations requires to take into account the internal field of the substrate, which is different from the self field of the superconductor film. Manuscript received September 20, 2005. This work was supported in part by Italian Project L95/95. L. Gianni, M. Bindi, F. Fontana, S. Ginocchio, E. Perini, and S. Zannella are with the Edison S.p.A., Milano I-20159, Italy (e-mail: luigia.gianni@edison.it). L. Martini is with CESI, Milano I-20134, Italy (e-mail: martini@cesi.it). Digital Object Identifier 10.1109/TASC.2006.870819 Fig. 1. Scheme of the AC losses measurements set-up. Lock-in amplifiers (LIA) are used to ensure accurate phase detection. II. EXPERIMENTAL A. YBCO CC’s Samples Details on the YBCO CC’s fabrication are reported in previous papers [2], [3]. The samples used in this work are Ni and Ni-5%at.W tapes buffered with a layer and cov- ered by a YBCO film. Superconductor sections are respectively and for the samples on Ni and Ni-5%at.W tapes. Zero-resistance critical temperatures are 87 K. End-to-end critical current densities , evaluated by transport measurements at 77 K self field, are 2 and 0.2 , respectively for the samples on Ni and Ni-5%at.W tapes. B. AC Losses Measurements of YBCO CCs Transport losses have been evaluated by a standard four probe measurements [4]. A scheme of the experimental set-up is shown in Fig. 1. A DAQ board drives an audio power amplifier (PA) which sup- plies, through transformers, AC current to the sample. The cur- rent amplitude is measured by the transducer TR, connected to a digital multimeter (DMM). The DAQ board also provides the reference signal to two lock-in amplifiers: LIA-1 is used to mea- sure voltage drop on the sample in phase with transport current, LIA-2 ensures accurate phase setting of current by the Rogowski coil RC. Variable inductor CC compensates the out of phase component of signals across the potential taps. Losses are evaluated by the following equation. (1) is the in-phase component of the voltage signals across the potential leads, is the current in the superconductor, f is the frequency and l is the distance between the potential contacts. 1051-8223/$20.00 © 2006 IEEE