An Active Quench Protection System for MRI Magnets

et al. 2016

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.

Section: Temperature Rise and Voltage Increasementioning

confidence: 99%

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

Poole

et al. 2016

“…Since it is difficult to predict a priori the noise levels on the voltage taps [50], the sensitivity of the peak temperature to the quench trigger voltage, V , th is investigated. With a coil made using wire #1, the temperature was calculated for two different detection voltages.…”

Section: Resultsmentioning

confidence: 99%

“…A trade-off also occurs between a more robust trigger at a higher voltage but lengthier delay to trigger the quench heaters, and a lower voltage threshold more susceptible to false triggers. For most of these simulations, the quench heaters were triggered when the voltage across coil i becomes greater than V th =100 mV [50].…”

Section: Numerical Integration and Modeling The Protection Heatersmentioning

confidence: 99%

Numerical simulation of quench protection for a 1.5 T persistent mode MgB₂ conduction-cooled MRI magnet

Poole

et al. 2016

The active quench protection of a 1.5 T MgB2 conduction-cooled MRI magnet operating in persistent current mode is considered. An active quench protection system relies on the detection of the resistive voltage developed in the magnet, which is used to trigger the external energizing of quench heaters located on the surfaces of all ten coil bundles. A numerical integration of the heat equation is used to determine the development of the temperature profile and the maximum temperature in the coil at the origin, or ‘hot spot’, of the quench. Both n-value of the superconductor and magnetoresistance of the wire are included in the simulations. An MgB2 wire manufactured by Hyper Tech Research, Inc. was used as the basis to model the wire for the simulations. With the proposed active quench protection system, the maximum temperature was limited to 200 K or less, which is considered low enough to prevent damage to the magnet. By substituting Glidcop for the Monel in the wire sheath or by increasing the thermal conductivity of the insulation, the margin for safe operation was further increased, the maximum temperature decreasing by more than 40 K. The strain on the MgB2 filaments is calculated using ANSYS, verifying that the stress and strain limits in the MgB2 superconductor and epoxy insulation are not exceeded.

“…A trade-off exists between a more robust trigger at a higher voltage but lengthier delay to trigger the quench heaters, and a lower voltage threshold more susceptible to false triggers. For these simulations, the quench protection heaters were triggered when the voltage across the quenched coil becomes greater than 100 mV [74]. A simulation of the quench propagation with this quench protection for the 1.5 T magnet gives the maximum temperature in each coil (figure 17) and leads to the conclusion that the peak temperature can be kept below 200 K.…”

Section: Active Quench Protection Methodsmentioning

confidence: 99%

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

Amin

et al. 2017

Conceptual designs of 1.5 and 3.0 T full-body magnetic resonance imaging (MRI) magnets using conduction cooled MgB2 superconductor are presented. The sizes, locations, and number of turns in the eight coil bundles are determined using optimization methods that minimize the amount of superconducting wire and produce magnetic fields with an inhomogeneity of less than 10 ppm over a 45 cm diameter spherical volume. MgB2 superconducting wire is assessed in terms of the transport, thermal, and mechanical properties for these magnet designs. Careful calculations of the normal zone propagation velocity and minimum quench energies provide support for the necessity of active quench protection instead of passive protection for medium temperature superconductors such as MgB2. A new ‘active’ protection scheme for medium Tc based MRI magnets is presented and simulations demonstrate that the magnet can be protected. Recent progress on persistent joints for multifilamentary MgB2 wire is presented. Finite difference calculations of the quench propagation and temperature rise during a quench conclude that active intervention is needed to reduce the temperature rise in the coil bundles and prevent damage to the superconductor. Comprehensive multiphysics and multiscale analytical and finite element analysis of the mechanical stress and strain in the MgB2 wire and epoxy for these designs are presented for the first time. From mechanical and thermal analysis of our designs we conclude there would be no damage to such a magnet during the manufacturing or operating stages, and that the magnet would survive various quench scenarios. This comprehensive set of magnet design considerations and analyses demonstrate the overall viability of 1.5 and 3.0 T MgB2 magnet designs.