Quench, Normal Zone Propagation Velocity, and the Development of an Active Protection Scheme for a Conduction Cooled, R&W, MgB2 MRI Coil Segment

Finite element analysis of the temperature distribution within a Conduction-Cooled, MgB2-based MRI superconducting coil segment

Superconducting magnets used for Magnetic Resonance Imaging (MRI) scanners need to keep temperature gradients minimized in order to retain thermal and operating current margin. We have used 3D finite element analysis (FEA) simulation in COMSOL Multiphysics software that includes both conductive heat transfer and radiative heating to calculate the temperature distribution both along the winding direction and across the cross-section of an MRI segment coil at its equilibrium operating temperature. We have also modelled the evolution of the thermal properties during cool-down from ambient temperature. The heat capacity and thermal conductivity of the magnet winding were computed for use within this simulation. The heat capacity as a function of temperature was calculated using a rule of mixtures. This procedure was also used for the thermal conductivity along the direction of the wire. However, the thermal conductivity within the composite cross section (x- and y-directions) was computed using a 2D FEA model. Based on this, a time-dependent, 3D coil model was built to calculate the coil temperature throughout the winding during cool-down in our test cryostat system. The model included a heat leak component to the coil current contacts via conduction through the current leads as well as a radiative component from the surfaces of the cryostat. A key result was that a maximum coil ΔTmax = 5.1 K (=maximum temperature within the winding -minimum temperature in the winding) was seen and a coil Ic margin of 12.75 A was predicted at steady state, with our first current lead design. A second set of more optimized current leads significantly lowered the ΔTmax within the coil at the steady state. The coil Ic margin has been analyzed for different current lead designs.

Magnetic, Mechanical and Thermal Modeling of Superconducting, Whole-body, Actively Shielded, 3 T MRI Magnets Wound Using MgB2 Strands for Liquid Cryogen Free Operation

we present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB₂ strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB₂ short (1.37 m length) MRI magnet based on the performance parameters of existing MgB₂ wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

Enhancement of B c2 and B irr in bulk MgB2 superconductors with SnO2 Additions

Three (MgB2)1-x (SnO2) x samples with x ranging from 0 to 5 wt% were prepared by the in situ route to study the effect of tin dioxide additions on the superconducting properties of MgB2 bulk materials. All of the reacted samples were slightly Mg deficient although the starting Mg:B precursor powder ratio was 1:2. A heat treatment (HT) temperature of 700 °C with a dwell time of 30 min was used. XRD results showed evidence of peak shifts for MgB2 phases with SnO2 addition. The magnitude of the a-axis lattice constant change (0.361 ± 0.075 %) calculated for the 3 wt% doped samples is comparable in magnitude to that seen previously for the C-doped MgB2 bulks which exhibited enhanced B C2 . The upper critical fields (B C2 ) and the irreversibility fields (B irr ) were measured resistively in fields up to 14 T at 5 K to T c . The best B C2 value at 20 K (15.2 T based on extrapolation) was seen for sample IS3 (x = 3 wt%), and was comparable to the best B C2 values (≈ 15 T at 20 K) seen for C-doped MgB2 bulks. IS3 had a corresponding B irr = 10.8 T (20 K). The superconducting transition temperature (T c ) appeared to increase slightly with doping, although within the range of error bars (37.4 K to 37.6 K for 1.6 T B C2 increase at 20 K), in contrast to C doping which is accompanied by a significant decrease in T c (39 K to 36 K for 3.8 % C doped MgB2 bulk). We attribute the observed increase in both B C2 and B irr for SnO2-additions to lattice strain caused by the introduction of precipitates within the grains.