Dynamic resistance of a high-T_ccoated conductor wire in a perpendicular magnetic field at 77 K

Abstract

Superconducting high-Tc coated conductor (CC) wires comprise a ceramic thin film with a large aspect ratio. This geometry can lead to significant dissipative losses when exposed to an alternating magnetic field. Here we report experimental measurements of the 'dynamic resistance' of commercially available SuperPower and Fujikura CC wires in an AC perpendicular field. The onset of dynamic resistance occurs at a threshold field amplitude, which is determined by the total DC transport current and the penetration field of the conductor. We show that the field-dependence of the normalised magnetisation loss provides an unambiguous value for this threshold field at zero transport current. From this insight we then obtain an expression for the dynamic resistance in perpendicular field. This approach implies a linear relationship between dynamic resistance and applied field amplitude, and also between threshold field and transport current and this is consistent with our experimental data. The analytical expression obtained yields values that closely agree with measurements obtained across a wide range of frequencies and transport currents, and for multiple CC wires produced by different wire manufacturers and with significantly differing dimensions and critical currents. We further show that at high transport currents, the measured DC resistance includes an additional non-linear term which is due to flux-flow resistance incurred by the DC transport current. This occurs once the field-dependent critical current of the wire falls below the DC transport current for part of each field cycle. Our results provide an effective and simple approach to calculating the dynamic resistance of a coated conductor wire, at current and field magnitudes consistent with those expected in superconducting machines.

Exploiting asymmetric wire critical current for the reduction of AC loss in HTS coil windings

“…The measured I c value of the no-flipped assembly is 16% higher than that of the both-flipped assembly at 77 K, and 7% higher at 65 K. The calculated I c,coil values can successfully predict the same tendency as the measurement even though the agreement between the measured and simulated results is not perfect. It is worth noting that the calculated E ave values contain a linear component (slope) which we attribute to dynamic resistance which occurs when superconductors carry DC current under changing magnetic field [33]. Similar simulation results were observed in previous research [10].…”

The dynamic resistance of YBCO coated conductor wire: effect of DC current magnitude and applied field orientation

“…Nonetheless, the agreement with the linear regime is still substantially improved over figure 3. It is interesting to note that at i = 0.9, the 2D-FEM model substantially over-estimates the 'lift-off' due to non-linear flux-flow resistance at high magnetic fields, and thus deviates substantially from the experimental data [16]. The magnitude of the flux-flow resistance is highly sensitive to the Jc(Ba,) behaviour of the wire, and one possible cause of the strong disagreement in this region is that the 2D-FEM calculation used here assumed a constant n value for all magnitudes of applied B.…”

“…Experimental reports to date have focused on dynamic resistance measurements in REBCO coated conductors under perpendicular AC magnetic fields [14][15][16][17]. Several analytical equations have been developed to predict dynamic resistance in this geometry [16,18].…”

“…Experimental reports to date have focused on dynamic resistance measurements in REBCO coated conductors under perpendicular AC magnetic fields [14][15][16][17]. Several analytical equations have been developed to predict dynamic resistance in this geometry [16,18]. If we define a thin superconductor as seen in figure 1, the dynamic resistance per unit length per cycle in a perpendicular AC magnetic field can be written as [16,18], (1) where, a is half-width of the conductor, Ic0 is the self-field critical current of the conductor, a,⊥ is the magnetic field amplitude.…”

“…Several analytical equations have been developed to predict dynamic resistance in this geometry [16,18]. If we define a thin superconductor as seen in figure 1, the dynamic resistance per unit length per cycle in a perpendicular AC magnetic field can be written as [16,18], (1) where, a is half-width of the conductor, Ic0 is the self-field critical current of the conductor, a,⊥ is the magnetic field amplitude. Recently [16,17] we have suggested that the threshold field, th,⊥ , can be expressed as (2) where, i is It/Ic0, Jc0 is critical current density and t is half-thickness of the conductor.…”