Transport AC losses of YBCO pancake coils wound from parallel connected tapes

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 tau(eff) if tapes are non-insulated along the turn length. In this paper we estimated tau(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/tau(eff).