Numerical study on the quench propagation in a 1.5 T MgB₂MRI magnet design with varied wire compositions

Abstract

To reduce the usage of liquid helium in MRI magnets, magnesium diboride (MgB2), a high temperature superconductor, has been considered for use in a design of conduction cooled MRI magnets. Compared to NbTi wires the normal zone propagation velocity (NZPV) in MgB2 is much slower leading to a higher temperature rise and the necessity of active quench protection. The temperature rise, resistive voltage, and NZPV during a quench in a 1.5 T main magnet design with MgB2 superconducting wire was calculated for a variety of wire compositions. The quench development was modeled using the Douglas–Gunn method to solve the 3D heat equation. It was determined that wires with higher bulk thermal conductivity and lower electrical resistivity reduced the hot-spot temperature rise near the beginning of a quench. These improvements can be accomplished by increasing the copper fraction inside the wire, using a sheath material (such as Glidcop) with a higher thermal conductivity and lower electrical resistivity, and by increasing the thermal conductivity of the wire’s insulation. The focus of this paper is on the initial stages of quench development, and does not consider the later stages of the quench or magnet protection.

Conceptual designs of conduction cooled MgB2 magnets for 1.5 and 3.0 T full body MRI systems

“…The amount of superconducting material can be reduced (compared to 1st generation wire), allowing more space to increase the percentage of copper in wire. A recent study has shown that increasing the copper fraction helps reducing the peak temperature of hot spot during quench [39]. The 2nd generation wire also allows for a higher operating temperature, which reduces the demands on the conduction cooling system [25].…”

“…Also, once a quench begins in NbTi magnets it spreads rapidly with the NZPV being roughly in the range of 10–50 m s −1 [1]. In contrast, the MQE for a MgB 2 superconducting magnet can be as large as several Joules, which is orders of magnitude larger than the MQE for NbTi magnets [39]. The NZPV can be in the range 5–80 cm s −1 which is orders of magnitude slower than in NbTi magnets [1].…”

“…Ideally, the material properties of the wire would be experimentally determined using an actual coil. With these measurements unavailable, the material properties of the composite are determined from a weighted average of the individual components of the wire as described in detail in [39, 59]. …”

“…The material properties of the composite wires used in the simulations (figure 14) are calculated using the material properties of the individual components taken from the published references tabulated in [39, 54, 59]. Compared to the wire with Monel (Wire #1542), the thermal conductivity of the wire with Glidcop (Wire #1542G) is significantly higher while the electrical resistivity of the Glidcop wire is significantly lower.…”

“…Changing the composition of the bulk wire has an effect on the MQE. The MQE can increase if the wire’s sheath material is replaced by a more thermally and electrical conductive material such as Glidcop [39, 63] and when the copper fraction of the wire or tape is increased [39, 61]. It has also been shown that the MQE for a one-dimensional length of wire is less than that of the wire when wound in a coil of either two or three-dimensions in both simulations [64–66] and experiments [61, 62].…”