Visualization of Coupling Current Paths in Striated Ybco-coated Conductors at Frequencies up to 400 Hz (postprint)

The magnetic flux density component perpendicular to the broad tape face was mapped by miniature Hall probes in the vicinity of a striated YBCO-coated tape at frequencies of external magnetic field from 21 to 400 Hz, applied perpendicularly to the tape surface. For reasons of modelling the coupling current behaviour in tapes with a conductive substrate, we amplified the coupling current–amplitude by soldering 25 μm thick copper foil on the top of the filaments. The aim of this procedure was to decrease the transverse resistivity of the tape. The longitudinal components of the total currents flowing in the tape were calculated by an inverse method from the field map corresponding to the zero phase of the applied field. The diffusion lengths, characterizing the flux penetration into the tape, were determined for the respective frequencies. The experimentally determined diffusion length is in good accordance with theoretical models. While at 21 Hz both weak coupling currents and distinctive hysteretic currents of individual filaments are observed, at 400 Hz the coupling currents are predominant in this YBCO tape. (Some figures in this article are in colour only in the electronic version) A significant amount of development has occurred for hightemperature superconducting (HTS) wires and more recently especially for the second-generation (2G) YBa2Cu3O7−x (YBCO)-coated conductor. In order to make these HTS tapes suitable for AC applications, a part of this developmental effort has gone toward studies leading to a more AC-tolerant architecture for the conductor [1]. As part of this, a variety of techniques are widely used for characterization of the electromagnetic properties of the HTS tapes, such as AC loss, current density versus electric field (E–J ), resistivity versus temperature (R–T ) measurements, etc. However, these techniques are typically integral methods describing the tapes as a whole on the scale of several millimetres or more. Much less is known about the behaviour of these tapes on a submillimetre scale when exposed to an external AC magnetic 3 Author to whom any correspondence should be addressed. field, where local variations of the critical current or the magnetic self-field may occur. As already determined, the sample length plays an important role in AC loss measurements. While the total hysteresis power loss is proportional to the sample length, the total coupling loss in a twisted multifilament conductor increases as the square of the twist pitch length l2 t [2]. A linear, not twisted multifilament tape with the length of L behaves like a twisted one with twist pitch lt = 2L . To reduce the coupling losses in an application, the multifilament HTS tape must be either physically twisted or have a ‘twist’ of the filaments incorporated into the conductor’s architecture [3, 4]. In a rectangular non-twisted multifilament sample, the coupling currents at the end of the tape will flow in a direction perpendicular to the filaments. In the central section of the tape, however, the coupling currents flow in the direction 0953-2048/07/010087+05$30.00 © 2007 IOP Publishing Ltd Printed in the UK 87