AC losses in a finiteZstack using an anisotropic homogeneous-medium approximation

Abstract

A finite stack of thin superconducting tapes, all carrying a fixed current I, can be approximated by an anisotropic superconducting bar with critical current density J c = I c /2aD, where I c is the critical current of each tape, 2a is the tape width, and D is the tape-to-tape periodicity. The current density J must obey the constraint Jdx = I/D, where the tapes lie parallel to the x axis and are stacked along the z axis. We suppose that J c is independent of field (Bean approximation) and look for a solution to the critical state for arbitrary height 2b of the stack. For c < |x| < a we have J = J c , and for |x| < c the critical state requires that B z = 0. We show that this implies ∂J/∂x = 0 in the central region. Setting c as a constant (independent of z) results in field profiles remarkably close to the desired one (B z = 0 for |x| < c) as long as the aspect ratio b/a is not too small. We evaluate various criteria for choosing c, and we show that the calculated hysteretic losses depend only weakly on how c is chosen. We argue that for small D/a the anisotropic homogeneous-medium approximation gives a reasonably accurate estimate of the ac losses in a finite Z stack. The results for a Z stack can be used to calculate the transport losses in a pancake coil wound with superconducting tape.

AC loss evaluation of an HTS insert for high field magnet cooled by cryocoolers

“…On the other hand, the magnetic interactions between turns [10][11][12][13][14] have to be taken into account for estimation of the perpendicular-field loss Q \ . It has been well known that there are three theoretical expressions to estimate AC losses in SC strips exposed to a cyclic external magnetic field perpendicular to the broad faces [18][19][20][21][22][23]. One is the theoretical expression of AC loss in a single strip [18][19][20].…”

AC Losses and Their Thermal Effect in High-Temperature Superconducting Machines

“…The homogenization technique introduced in [8]- [10] is used to compute AC losses. The advantage of this technique is that it disregards the inner details of an HTS winding, then the HTS winding can be studied as a homogenized bulk, giving rise to a faster computation without compromising the accuracy.…”

“…Moreover, since AC losses are directly linked to the cost of a cooling system and the overall efficiency, [7]. Among these, the homogenization technique has been proven to be effective and it shows great advantages in simplicity and computation speed [8]- [10]. Nevertheless, very few works addressing AC losses have taken into account real operating conditions and constraints of an HTS machine.…”

Potential and limits of numerical modelling for supporting the development of HTS devices

“…• Direct current homogenization in a single stack of tapes [14,15] and coils [16] in 2-D, or even to 3-D coils [12] (see figure 4);…”