Dynamic resistance, which occurs when a HTS coated conductor carries a DC current under an AC magnetic field, can have critical implications for the design of HTS machines. Here, we report measurements of dynamic resistance in a commercially available SuperPower 4 mm-wide YBCO coated conductor, carrying a DC current under an applied AC magnetic field of arbitrary orientation. The reduced DC current, I _t / I _c0 , ranged from 0.01 to 0.9, where I _t is the DC current level and I _c0 is the self-field critical current of the conductor. The field angle (the angle between the magnetic field and the normal vector of the conductor wide-face) was varied between 0° and 90° at intervals of 10°. We show that the effective width of the conductor under study is ∼12% less than the physical wire width, and we attribute this difference to edge damage of the wire during or after manufacture. We then examine the measured dynamic resistance of this wire under perpendicular applied fields at very low DC current levels. In this regime we find that the threshold field, B _th , of the conductor is well described by the nonlinear equation of Mikitik and Brandt. However, this model consistently underestimates the threshold field at higher current levels. As such, the dynamic resistance in a coated conductor under perpendicular magnetic fields is best described using two different equations for each of the low and high DC current regimes, respectively. At low DC currents where I _t / I _c0 ≤ 0.1, the nonlinear relationship of Mikitik and Brandt provides the closest agreement with experimental data. However, in the higher current regime where I _t / I _c0 ≥ 0.2, closer agreement is obtained using a simple linear expression which assumes a current-independent penetration field. We further show that for the conductor studied here, the measured dynamic resistance at different field angles is dominated by the perpendicular magnetic field component, with negligible contribution from the parallel component. Our findings now enable the dynamic resistance of a single conductor to be analytically determined for a very wide range of DC currents and at all applied field angles.

Dynamic resistance occurs when HTS (high-temperature superconductor) coated conductors carry dc current under ac magnetic field. This dissipative effect can play a critical role in many HTS applications. Here, we report on dynamic resistance measurements of a four-tape YBCO stack comprising 4-mm-wide coated conductors, which experience an applied ac perpendicular magnetic field with an amplitude of up to 100 mT. Each tape within the stack carries the same dc current. The magnetic field amplitude, the frequency of the magnetic field, and the dc current magnitude are varied to investigate the influence of these parameters on the dynamic resistance. We find that the threshold field of the stack is significantly larger than that of a single tape when dc current is small, which we attribute to coherent shielding effects from circulating currents present in each wire in the stack.

Fiber Bragg grating (FBG) temperature sensors are now re-garded as a relatively mature sensing technology for tempera-ture measurement in ambient conditions, and may offer poten-tial as a cryogenic temperature sensor for quench detection. In this work, the temperature response of three different coated FBG sensors from 77 K to 293 K was studied; bare (silica) FBG, acrylic coated, and polyimide coated FBG sensors. For compar-ison measurements were repeated with sensors from two FBG suppliers. The measured temperature response was then com-pared and contrasted with the predicted sensor response. In or-der to assess the sensor sensitivity, repeatability and any hyste-resis, these sensors were then subjected to repeated cool-down / warm-up cycles (77 $\times$ 293 K). Finally, the measurement of quench induced temperature change in an epoxy potted HTS coil was simulated through fabrication of bare FBG sensor embed-ded in a 10 mmμ2 mmν4 mm epoxy block. The effect of epoxy resin impregnation on the temperature response characteristics was analyzed by comparing the sensor response characteristics before and after immersion. From these results, we propose the most cost-effective and practical FBG type for HTS coil quench detection.

This paper presents the power dissipation analysis on high-temperature superconducting (HTS) coated conductor tapes and coils under the action of various oscillating currents and fields. The loss calculation on HTS coil was carried out by the H-formulation model implemented in the finite-element method (FEM) software package COMSOL Multiphysics. There were four different oscillating (ac) signals in this paper: sine, triangle, sawtooth, and square. Two scenarios causing loss in HTS tapes and coils which have been analyzed: Scenario 1 ac transport current; Scenario 2 ac magnetic field. The simulation results were compared to the experiment results with the typical sinusoidal ac signal.

Eddy current loss in metal stabilized layers of a HTS (high temperature superconducting) coated conductor is affected by the superconducting layer. We present AC loss measurement results on a 10 mm-wide non-stabilized Fujikura tape, on 10 mm-wide 0.1 mm-thick copper strips, and on the Fujikura tape stacked vertically with the copper strips in a perpendicular applied field at 77 K. The applied magnetic field amplitude was varied up to 100 mT at 67.9 Hz, 87.6, and 107 Hz. The eddy current loss in the copper strips was estimated by taking the difference between the loss in the stack and the isolated Fujikura tape. The results show approximately complete shielding of perpendicular magnetic fields in samples at low magnetic field amplitude, and there is little dependence of the eddy current loss on the copper strip thickness up to 300 $\mu$m. We also present the shielding of perpendicular magnetic fields by copper strips stacked with a 30 cm-long 15/4 (fifteen 4-mm-wide strands) Roebel cable. The shielding of perpendicular magnetic fields in the Roebel samples was observed even at high field amplitudes.

Assembled coated conductors are essential in many high current high temperature superconducting (HTS) applications. A promising option is to use Roebel cable, as it offers both low AC loss and mechanical flexibility. In this work, we measured magneti-zation loss at 77 K in REBCO Roebel cables with varying strand numbers from six to fourteen to investigate the strand number de-pendence of AC loss characteristics of the Roebel cables. The 4 mm-wide Roebel strands were punched from non-stabilized 10 mm-wide Fujikura REBCO tapes. The applied field amplitude, frequency and the field angle (the angle between the magnetic field and the tape wide surface) were varied. Three 10 mm-wide vertical stacks with conductor number of three, five, and seven were prepared as refer-ence stacks using the same source Fujikura tapes. The measured magnetization loss values of the Roebel cables were compared with those of the equivalent stacks as well as with the results of COMSOL modelling of the stacks using the H-formulation. At magnetic field amplitudes greater than the effective penetration field of the Roebel cables the cables have lower AC loss than equivalent stacks.