The Effect of Field-Dependent $n$-Value on Screening Current, Voltage, and Magnetic Field of REBCO Coil

Here we report the effect of field-dependent index value
$n(B,\theta)$
on screening current and consequential NI REBCO coil behavior. Through the ‘Little Big Coil (LBC)’ framework, an unexpected voltage behavior has been observed at the top and bottom coils in the fourth mini magnet LBC4 during the self-field operation at 77 K. The measurement demonstrates that the top/bottom coil voltage is temporarily decreased and then increased. However, this anomaly cannot be reproduced if the conventional simulation model uses the power-law
$E\!-\!J$
model with a constant
$n$
-value. Hence, we have revisited a property of the REBCO-coated conductor and, as a result, found that the field dependency of
$n$
is necessary to elucidate the behavior. This paper will provide the 77 K self-field operation results, e.g., module coil voltages and the central magnetic field. A numerical simulation considering the field dependency investigates unexpected coil voltage behavior. The comparison between simulation results if the dependency is included and if not will be discussed.