AC Loss Characteristics of Stacks of YBCO Coated Conductors
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0946-HH10-06
AC Loss Characteristics of Stacks of YBCO Coated Conductors Francesco Grilli1, Stephen P. Ashworth1, and Svetlomir Stavrev2 1 Superconductivity Technology Center, Los Alamos National Laboratory, PO BOX 1663 - MS T004, Los Alamos, NM, 87545 2 Swiss Federal Institute of Technology - Lausanne, Lausanne, CH 1015, Switzerland
ABSTRACT Practical applications of YBCO coated conductors (CC) involving superconducting coils will utilize tapes packed together in an arrangement resembling a vertical stack. In such configuration there is an important electromagnetic interaction between the tapes, which strongly influences the loss characteristic of the device. In the presence of an external magnetic field, the losses are reduced compared to an isolated tape because of the reduced aspect ratio of the conductor and, at least for low fields, because of an effective screening of the central part of the stack. On the contrary, in the case of AC transport current, the losses tend to increase due to the enhancement of the local field caused by the interaction of the self-field produced by neighboring tapes. In practical situations the conductor is usually subjected to both transport current and magnetic field, so that there is a trade-off between the two effects. In this paper we investigate, both experimentally and by means of finite-element method calculations, the ac loss behavior of a stack composed by a finite number of tapes in different working conditions, and we compare the AC losses to the ones of non-interacting tapes in order to determine if the use of stacked tapes is advantageous from the point of view of power dissipation. INTRODUCTION In the next years HTS produced in the form of YBCO coated conductors will become prevalent, these present several advantages with respect to Bi2223/Ag tapes, such as lower production costs, better resistance to mechanical deformation and better performance in external magnetic field [1]. In AC applications, such as superconducting cables and coils, the HTS tapes are packed closely together, so that it is very important to be able to understand and quantitatively predict their mutual electromagnetic interaction and its influence on the value of the AC losses. Analytical models for thin tapes are based on the critical state model and consider only the case of a stack with an infinite number of tapes [2,3]. Finite-element method (FEM) calculations may take into account the smooth current-voltage characteristics of HTS superconductors and a finite number of interacting tapes, thus allowing a more realistic description of the problem [4,5]. In this paper we use a FEM model to evaluate the AC losses of vertical stacks composed of several tapes of YBCO coated conductors. The main purpose of this work is to quantify experimentally and by the FEM technique the influence of the tape interaction on AC losses, both in the case of externally applied AC magnetic field and AC transport current. The paper is
organized as follows: first we give a brief description of the experiments and the
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