Study of second-generation high-temperature superconducting magnets: the self-field screening effect

Abstract

Second-generation high-temperature superconductors (2G HTS) have high current density in very high magnetic fields. They are good candidates for high field magnets, especially when the magnetic field exceeds the critical fields of low-temperature superconductors. However, the thin and flat geometry of these conductors allows persistent screening currents (or shielding currents) to flow in the conductors. The screening currents caused by the ramping of applied current to the coil is identified as the self-field screening effect. The screening-current-induced magnetic field changes the magnetic field distribution of the magnet, and it also generates drift. This paper employs both experimental and numerical methods to study the mechanism of self-field screening currents for 2G HTS magnets. A 2G HTS magnet was constructed and tested, and a finite element model was built based on the magnet. The comparison between calculation and measurement is presented with detailed analysis. Current distributions inside the HTS magnet are calculated to illustrate the effects of screening. The screening-current-induced magnetic field is quantified by comparing the magnetic field distribution with a baseline copper model. The model is also used to explain the mechanism of the current sweep strategy, which can be used to effectively eliminate screening currents.

Investigation of the Relaxation of Persistent Current in Superconducting Closed Loops Made Out of YBCO Coated Conductors

“…Coated conductors exhibit enhanced rate of relaxation in comparison with the low-temperature superconductors [8], [9], [12], [13]. In part, of course, this is 1051-8223 © 2014 IEEE.…”

“…In this method, which is widely used to stabilize the magnetic field drift in conventional low-T c superconducting magnets [15], the current is ramped up above the operating level and then reduced back to the desired operating level. This method was also applied to coils made out of high temperature superconductors [13], [16], [17], but these coils did not operate in persistent mode.…”

Analysis of Current Distribution in Bi-2223/Ag Insert Pancake Coil

“…To simplify the calculation, we ignore the multi-filament structure of the Bi-2223 tape, and apply a rectangular bar approximation [1]- [3] to the tapes in the coil. The current distribution in HTS coil is different from that of the low temperature superconducting (LTS) coil: Firstly, a certain tape in the coil is exposed to a transverse magnetic field generated by the other turns after the coil is charged, the transport current distribution will be nonuniform and asymmetrical to the tape conductor's center due to the combine effects of transport current and transverse magnetic field [2], [10]. Secondly, a screening current will flow in the superconducting tape after the coil is discharged [5], [19] or put into a transverse background magnetic field.…”

“…Until now, many works have been done to simulate the transport or screening current distribution in tape conductor [11]- [15] or HTS coil [1]- [10], [16]- [21]. References an infinitely long stack model is presented to calculate the current distribution and ac loss in the coil [16]- [18].…”

“…However, there is not a real screening current in the conductor after it is charged, and the actual transport current distribution will be non-uniform and asymmetrical to the center of conductor. The finite element modeling (FEM) is put forward to calculate the actual current distributions [6], [10], [20], [21], but it is not for the coil with a large number of turns. To calculate the current distribution in the coil with large number of turns, Naoyuki et al [2], Enric [7], and Min et al [10] presented different numerical methods, such as FEM, and minimum magnetic energy variation (MMEV) method.…”