A simple method to eliminate shielding currents for magnetization perpendicular to superconducting tapes wound into coils

Abstract

Application of an external AC magnetic field parallel to superconducting tapes helps in eliminating the magnetization caused by the shielding current induced in the flat faces of the tapes. This method helps in realizing a magnet system with high-temperature superconducting tapes for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) applications. The effectiveness of the proposed method is validated by numerical calculations carried out using the finite-element method and experiments performed using a commercially available superconducting tape. The field uniformity expected for practical applications is estimated to be less than 1 ppm.Superconducting magnets used in magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) require a highly uniform magnetic field at their center.Hence, a multifilamentary superconducting wire with a circular or rectangular cross section composed of low-temperature superconductors such as NbTi or Nb 3 Sn is wound around the magnets. These materials are usually cooled by liquid helium, which has a boiling temperature of 4.2 K at atmospheric pressure. However, because of the rapidly increasing demand for liquid helium and its limited availability, the cost of this coolant has been increasing steadily. To realize a helium-free superconducting magnet system, 1)it would be necessary to use liquid hydrogen or nitrogen, whose boiling temperatures are about 20 K and 77 K, respectively, as a coolant, or carry out conduction cooling with the help of a cryocooler. Recently, a superconducting magnet with a nominal field of 0.5 T has been wound with a MgB 2 wire and installed in a private clinic for use in MRI. 2)Nevertheless, designing a very high normal field (several tesla or several tens of tesla) magnet using the MgB 2 wire for MRI and NMR applications is very difficult because the critical current density decreases drastically when the magnetic field is on the order of a few tesla.3) High-temperature superconductor (HTS) wires such as first-generation *

Progresses and challenges in the development of high-field solenoidal magnets based on RE123 coated conductors

“…Various techniques have been proposed to reduce the amplitude of the [161], though not really useful for the NMR magnet, reverses a current sweep, as small as 1% of the peak current, to impede SCF variation; apply a field parallel to the tape wide surface with copper coils that sandwich a CC coil [162].…”

Feasibility Study for Elimination of the Screening Current-Induced Fields in HTS Coil

“…3b, when the alternating current (I AC ) is supplied to the BG coils, the axial AC magnetic field (B AC ) becomes dominant due to cancellation of the radial B AC generated by reversely wound BG coils In the superconducting layer, circulation of the screening current is compelled to compensate for the axially oscillating B AC . Such a screening current causes the superconductor to become magnetized axially, leading to elimination of the SCIF generated by B DC because the axial magnetization hardly affects the center field [17,18]. This change of magnetization induced by the B DC reversing polarity into the axial direction perpendicular to the self-field of the coil is called the vortex shaking effect.…”

“…This technique can be applied to HTS to achieve reduced weight, size, and consumption of the conductor, especially for use in NMR spectroscopy as an NMR magnet generating an extremely high field. According to recent studies, the SCIF of the 2G HTS magnet, which is conventionally insulated every turn, can be effectively removed by exposing the magnet to an external AC magnetic field [17,18]. When the flat surface of the HTS CC lay in parallel to an external AC magnetic field, the direction of the magnetization changed from the radial direction to the axial direction, the so-called vortex shaking effect [19][20][21][22].…”

Reduction of Magnetization in Windings Composed of HTS Tapes

“…It has been well known that the combination of these magnetic fields yields the inhomogeneity of current across the tape width, and therefore the magnetic field in the central part of the magnet has very low uniformity [4]- [7]. In this study, simple and powerful methods to reduce the magnetization in the windings composed of HTS tapes are proposed comprehensively although a series of preliminary discussions has already been made by the present authors [8]. Next, one of the proposed methods is investigated in detail from experimental points of view by measuring central magnetic fields in a layer-wound coil fabricated with a coated conductor.…”