Transport AC Losses of YBCO Coated Conductor Coils
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1001-M11-04
Transport AC Losses of YBCO Coated Conductor Coils Francesco Grilli, and Stephen P. Ashworth Los Alamos National Laboratory, Los Alamos, NM, 87545
ABSTRACT Certain practical applications of YBCO coated conductors (CC) involve superconducting tapes wound in coils. In such a configuration the superconducting tape is arranged as closely packed turns, leading to an increase of the magnetic field generated by the current in the tapes and, consequently, a significant increase in the AC losses, with respect to an ëisolatedí tape. In order to predict and reduce the refrigeration requirements of applications, it is therefore very important to be able to quantify the magnitude of such AC losses, both experimentally and by means of numerical calculations. INTRODUCTION Several applications of YBCO coated conductors (CC) use tapes wound in a pancake coil: in such configuration tapes are closely packed and interact strongly, similar to a z-stack. Whereas the methods for measuring transport AC losses in a single tape are well established, in the case of interacting tapes and coils in particular, fewer results are reported in the literature [1, 2]. The interaction of the tapes makes it difficult to calculate the losses from any ëloss voltageí measured by electrical means. Analytical models for computing the losses exist, but are limited to an infinite numbers of tapes, which in most cases constitutes too coarse an approximation. In this paper we present the results of transport AC loss measurements on a pancake coil composed of 25 turns of superconducting tape. We also compare the experimental results to the predictions of a recently developed finite-element model. EXPERIMENTAL SETUP The coil is composed of a YBCO tape 4.8 m long and 12 mm wide manufactured by SuperPower, Inc. [3] using the IBAD method, with a self-field critical current (measured end to end before winding into a coil) of about 330 A and a power index n=35 at T=75 K. The coil was wound around a G-10 cylindrical support of sufficiently large diameter (5 cm), such that the transport capacity of the tape is not degraded. This results in a coil composed of 25 turns. The turns are electrically insulated with a layer of 25 µm kapton tape on each side, so that the superconducting strips are separated by a spacing (substrate, buffer, silver coating and kapton) of about 100 µm. A piece of the tape 10 cm long was removed for comparison of individual, isolated tape losses.
We obtained the AC losses of the coil using a phase sensitive voltmeter to determine a voltage in-phase with the transport current. The ëlossí voltage was obtained by placing voltage taps at the ends of the coil and by canceling the large induced voltage by means of a compensation coil. Since the induced voltage can be orders of magnitude higher than the loss voltage, the compensation process represents a challenge. For the numerical computation of the losses we have used a recently developed finiteelement model based on the use of edge elements [4]. In order to simulate the axisymmetric str
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