AC loss in ReBCO pancake coils and stacks of them: modelling and measurement

TL;DR

This study combines measurements and simulations to analyze AC loss in stacks of ReBCO pancake coils, revealing high losses in stacks and proposing design optimizations to reduce them.

Contribution

The paper introduces a validated simulation method for predicting AC loss in pancake coil stacks using measured critical current data.

Findings

AC loss in four-pancake stacks is two orders of magnitude higher than in single tapes.

A double pancake acts as a single wider pancake, but larger stacks behave differently.

Roebel cable reduces AC loss in pancake stacks, but not in single pancakes.

Abstract

Many applications of ReBCO coated conductors contain stacks of pancake coils. In order to reduce their high AC loss, it is necessary to understand the loss mechanisms. In this article, we measure and simulate the AC loss and the critical current, I_c, in stacks of pancake coils ("pancakes"). We construct stacks of up to 4 pancakes and we measure them by electrical means. We also obtain the anisotropic field dependence of J_c from I_c measurements of the tape. This J_c is the only input to the simulations, together with the coil dimensions. After validating our computations with the measurements, we simulate stacks of many pancakes, up to 32. We found that the AC loss in a stack of (four) pancakes is very high, two orders of magnitude larger than for a single tape. A double pancake behaves as a single one with double width but a stack of more pancakes is very different. Finally, we found that a 2-strand Roebel cable reduces the AC loss in a stack of pancakes but not in a single pancake. In conclusion, the AC loss in stacks of pancakes is too high. However, our simulations are useful to predict the AC loss and optimise the coil design, reducing the AC loss.