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Abstract

The transport AC losses (Q_tr) of YBCO pancake coils wound from n = 1, 2, 4 non-transposed parallel connected (n-tpc) tapes have been investigated experimentally and theoretically. It was found that the Q_tr AC losses of the coils with several n-tpc tapes consist of hysteresis AC losses (Q_h) and coupling AC losses (Q_cc). AC losses Q_h of these coils are nearly the same as the AC losses of single-tape coils (n = 1). In contrast, Q_cc are unique for coils with n > 1 n-tpc tapes. We found that the distribution of the coupling currents in the coil turns is defined by the magnetic flux penetrating between n-tpc tapes. This flux is related to the self-field component of the coil parallel to the tape surface, which is antisymmetrically distributed (an odd function) with respect to the winding center. This antisymmetrical distribution leads to a compensation effect of the coupling currents if n-tpc tapes are insulated along the turn length, in spite of low resistance between n-tpc tapes in the soldered coil ends. Therefore, AC losses Q_cc of such coils are negligibly small compared to AC losses Q_h. On the other hand, AC losses Q_cc per current cycle are frequency dependent and have a maximum defined by the time constant τ_eff if tapes are non-insulated along the turn length. In this paper we estimated τ_eff for several special cases. In particular, we found that Q_tr of YBCO pancake coils wound from several n-tpc tapes are comparable to Q_tr of single-tape coils if the operating frequency f is far from the characteristic frequency 1/τ_eff.