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

Abstract

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 T_c 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.

Keywords: MRI magnet; MgB2 persistent joints; MgB2 persistent switch; MgB2 superconducting wire; MgB2 superconductor; active quench detection; mechanical stress in MRI magnets.

Figure 1

A cross section of a 1.5 mm × 1.0 mm rectangular MgB₂ wire for MRI applications. The example shown has 18 superconducting filaments with a Cu:SC ratio of 2.7:1.

Figure 2

(a) (left) I_c of a round, 0.83 mm diameter wire with 12% MgB₂, and (right) J_c (I_c normalized to MgB₂ cross sectional area) at various magnetic field strengths and temperatures, (b) index value, n, (slope of log–log plot near I_c) versus B and T for the same wire. (Wire HTR 1654 [27]).

Figure 3

(a) A comparison of critical current density, J_c, versus magnetic field strength at 4.2 K for 1st generation in situ MgB₂ wire (HTR 1654) and AIMI 2nd generation wire (HTR 2281) [28]. (b) Measurements of n-value for a 2nd generation HTR 2281 [28] wire versus magnetic field at various temperatures.

Figure 4

The critical current I_c in the wire at temperature T = 10 K as function magnet field of strength B for: (a) wire type #1027 used in the 1.5 T magnet design, and (b) wire types #1542 and #1524G used in the 3.0 T magnet designs [36]. These plots are for the wire at a temperature of 10 K. The maximum field on wire and the corresponding critical current are indicated in each plot.

Figure 5

1.5 T MgB₂ magnet design. (a) The field distribution on and around the bundles. (b) The hoop (tensile) stress distribution on bundles averaged over axial values and plotted at different radial positions of each bundles. (c) The DSV non-uniformity in ppm. (d) The 5 Gauss footprint.

Figure 6

3.0 T MgB₂ magnet design. (a) The field distribution on and around the bundles. (b) The hoop (tensile) stress distribution on bundles averaged over axial values and plotted at different radial positions of each bundles. (c) The DSV non-uniformity in ppm. (d) The 5 Gauss footprint.

Figure 7

Cross section of the conduction cooling layout for the 1.5 T magnet design. Individual coils of wire (red) are wound around a stainless steel former. Copper straps connect the coils to copper cooling rings that are then connected to a cryocooler (shown on the top of magnet). Layers of superinsulation (yellow) are placed between the magnet assembly and the wall of the vacuum vessel.

Figure 8

The temperature dependence of the thermal conductivity within the homogenized coil material. The thermal conductivity is given in the three directions.

Figure 9

Steady state temperature distribution of the 1.5 T magnet after cool down. The boundary conditions are described in the text.

Figure 10

Three stages of the manufacturing and operation of a superconducting magnet that create stress and strain in the superconducting wire: (1) winding and preparation of the coil, (2) cool-down of the magnet, and (3) electromagnetic charging.

Figure 11

Composite superconducting wire with 18 niobium wrapped MgB₂ filaments. These are imbedded in a copper matrix surrounded by a Monel or Glidcop sheath. The epoxy layer formed when winding the coils is also included in the hominization. The material directions referred to in table 6 are shown in the lower right side corner.

Figure 12

Three quarter view of the first principal strain in the coil bundles of the 1.5 T full body MRI magnet design using wire #1027. The indicated strain is that after the magnet has been wound, cooled, and energized (which is the maximum strain during the entire manufacturing and operating.) The maximum strain is well below the 0.2% safety factor limit criteria.

Figure 13

1st principal mechanical strain of the 3.0 T magnet design using the #1542 wire. Five of the coils are shown at the time of electromagnetic charging after wire winding and cooling down to operating temperature.

Figure 14

Material properties of the wire as a function of temperature. (a) volumetric heat capacity, (b) azimuthal thermal conductivity, (c) resistivity, and (d) thermal conductivity in the axial and radial directions.

Figure 15

Critical current of the used in the simulations of the 1.5 T magnet, wire as a function of magnetic field strength for various temperatures. Indicated are the operating current, I_c, the critical current at the locations of the maximum magnetic field, B_max, and the critical current at the location of the quench initiation.

Figure 16

Schematic of an active quench protection system.

Figure 17

The maximum temperature in each coil bundle as a function of time for the 1.5 T magnet design. The quench protection is triggered when the voltage on coil 1 reaches 100 mV.

Figure 18

The maximum temperature as a function of time for the 3.0 T magnet design with the Monel sheath. The quench protection is triggered when the voltage on coil #1 reached 100 mV.

Figure 19

The maximum temperature as a function of time for the 3.0 T magnet design with the Glidcop Sheath. The quench protection is triggered when the voltage on coil #1 reached 100 mV.

Figure 20

Representative volume element (RVE) as considered for quench stress and strain analysis.

Figure 21

Thermal strain and elastic modulus used in ANSYS simulations: (a) the accumulated thermal strain is referenced to T = 10 K; (b) elastic modulus.

Figure 22

Strains and stresses calculated in ANSYS during the quench simulation of the 1.5 T magnet. (a) Tensile strain in the MgB₂ superconductor; (b) maximum shear stress in the epoxy insulation.

Figure 23

MgB₂ persistent joint formed between two reacted (HT 3700) wires, (left) picture of the fixture and a persistent joint, (right) critical current of the joint as a function of B.

Figure 24

Loop fixture for direct decay measurements of persistent currents, shown with joint attached.

Figure 25

(Left) Decay of persistent current in MgB₂ W&R style joint based on expected MRI style MgB₂ conductor strand HTR 3700 at zero applied field (4.2 K), (right) initial persistent current as a function of field at 4.2 K.

Figure 26

Circuit diagram of a persistent current switch (PCS) used to charge a superconducting magnet. (a) The PCS circuit while charging the magnet. The heater keeps the persistent switch in a resistive state, thus the majority of current flows through the magnet. (b) The PCS circuit when heater is removed. The switch becomes a short circuit, allowing all of the current to flow through the superconducting loop, thus achieving persistent mode.

Figure 27

Bifilar wire built for a MRI persistent switch using MgB₂ superconductor. Note that two heater wires are used for redundancy. By twisting the three-wire bundle 180° at the mid-point fold, a close-packed build is achieved.

Figure 28

Persistent switch with bifilar MgB₂ wire and heater winding.

Figure 29

Plot of wire length versus temperature for the 1.5 T system using data in table 10.

Figure 30

Temperature of the persistent switch for the 1.5 and 3.0 T magnet designs during (a) heating of switch, and (b) cool-down of switch. The switch parameters are given in table 11.