Coupling time constants of striated and copper-plated coated conductors and the potential of striation to reduce shielding-current-induced fields in pancake coils

Abstract

The shielding-current-induced field is a serious concern for the applications of coated conductors to magnets. The striation of the coated conductor is one of the countermeasures, but it is effective only after the decay of the coupling current, which is characterised with the coupling time constant. In a non-twisted striated coated conductor, the coupling time constant is determined primarily by its length and the transverse resistance between superconductor filaments, because the coupling current could flow along its entire length. We measured and numerically calculated the frequency dependences of magnetisation losses in striated and copper-plated coated conductors with various lengths and their stacks at 77 K and determined their coupling time constants. Stacked conductors simulate the turns of a conductor wound into a pancake coil. Coupling time constants are proportional to the square of the conductor length. Stacking striated coated conductors increases the coupling time constants because the coupling currents in stacked conductors are coupled to one another magnetically to increase the mutual inductances for the coupling current paths. We carried out the numerical electromagnetic field analysis of conductors wound into pancake coils and determined their coupling time constants. They can be explained by the length dependence and mutual coupling effect observed in stacked straight conductors. Even in pancake coils with practical numbers of turns, i.e. conductor lengths, the striation is effective to reduce the shielding-current-induced fields for some dc applications.

Alternating Current Loss of Superconductors Applied to Superconducting Electrical Machines

“…Two typical electrical circuits of the pick-up coil method [132] and the lock-in amplifier technique [54] are presented in Figure 7. The pick-up coil method is often applied to measure the magnetization loss of superconducting samples [133][134][135][136]. The measurement system is usually composed of the AC power supply, AC electromagnet, isolation amplifier, pick-up coil, compensation circuit, compensation coil, cryostat, as well as data acquisition and processing parts, as shown in Figure 7a.…”

Round-Core-Radius-Dependent Electromagnetic Coupling of Multifilament Helical Superconducting Tapes in a Swept Magnetic Field

“…Here, we model a thermal stabilizer such as copper on top of the SC filaments by means of a finite-resistivity conductor as in Refs. [9]- [11]. Fig.…”

“…We assume that the coupling current between the SC filaments flows uniformly through the thermal stabilizing layer. The stabilizer information (e.g., conductivity, thickness, and effective length responsible for the transverse conductance) can be incorporated via the transverse conductivity (resistivity) σ n = 1/ρ n as discussed in Section III, thereby reducing a three-dimensional model to a two-dimensional model [9]. Here, we model a thermal stabilizer such as copper on top of the SC filaments by means of a finite-resistivity conductor as in Refs.…”

Dipole Magnets above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…As an example, the peak magnetization normalized to the wire volume of a CORC R wire at 4.2 K [175] is a factor of five higher than that of a Nb 3 Sn Rutherford cable measured at 1.9 K [176]. Tape striation is effective to reduce the magnetization in REBCO tapes [86,173,177,178], although it may increase the conductor cost/performance ratio. Compensation from the magnet aspect is another effective approach that should be explored [67,179,180].…”

“…An automated winding machine was developed to wind coils in complex 3D shapes using single tapes [81,82]. Detailed experimental and simulation studies have been performed to understand the current distribution and its impact on the spatial and temporal field quality critical for accelerator magnet applications [83][84][85][86].…”