AC Loss and Voltage Signal in a Pancake Coil Made of Coated Conductor With Ferromagnetic Substrate
The voltage signal and the AC loss in a pancake coil made of a coated conductor tape with magnetic substrate is measured. The experimental data have been analyzed with the help of numerical calculations. It is found that while for a single tape the AC loss is dominated by the substrate, for the coil it is dominated by the superconducting layer. Moreover, the substrate increases the AC loss generated in the superconducting material, making it similar to a slab. A compensation method for the voltage signal is also described, thanks to which the contribution from the substrate and the superconducting layer can be distinguished.
AC Loss in Pancake Coil Made From 12 mm Wide REBCO Tape
The design of a superconducting coil from high-performance REBCO coated conductors is often complicated because of complex anisotropy of the critical current density Jc . It is important to understand how much detail of this feature must be taken into consideration in the prediction of maximum achievable current and the expected ac loss. We present the results of investigation performed with a small (ten turns, 60 mm inner diameter) coil made from SuperPower tape of 12 mm width. The knowledge of Jc(B,θ) determined on short sample allowed prediction of the maximum achievable current of the coil and the ac loss behavior. We have also investigated the effect of the tape nonuniformity. Our results confirm that the lateral nonuniformity when Jc at tape edges is lower than in its center leads to significant increase of ac loss. A longitudinal nonuniformity, in particular a reduction of critical current in some portion along the tape length, is hardly observable in the ac loss result. On the other hand, using a piece of tape with lower Jc in the innermost coil turn would significantly reduce the maximum current. We also present calculations showing the change in current-voltage curve and redistribution of ac dissipation in the case of nonuniform tape quality.
Modeling Coherent Backscattering Errors in Fiber Optic Gyroscopes for Sources of Arbitrary Line Width
We present a new model of the noise and drift induced by coherent backscattering in a fiber optic gyroscope (FOG) interrogated with a light source of arbitrary temporal coherence. This study is critical to understand whether a FOG driven with a laser instead of a broadband source can attain high sensitivity and stability, which would have the overwhelming benefit of improving the FOG scale factor stability by at least ten-fold and would enable the use of FOGs for inertial navigation of aircrafts. Analytical and numerical solutions bring to light two significant new predictions. First, coherent-backscattering noise can be made negligibly small by utilizing a laser with a very narrow linewidth (less than ~ 20 kHz), although in this regime the drift is high. Second, by using a laser with a broad linewidth (greater than ~ 10 MHz), both the noise and the drift are low enough for aircraft inertial navigation. The dependencies of the noise and drift on fiber loss, loop coupling coefficient, and backscattering coefficient are also presented to define the optimum mode of operation of this new class of FOGs.