Superconductor Science & Technology最新文献

Brain imaging MRI comprises a significant proportion of MRI scans, but the requirement for including the shoulders in the magnet bore means there is not a significant size reduction in the magnet compared to whole-body magnets. Here we present a new design approach for brain imaging MRI magnets targeting ±20 kHz B ₀ variation over the imaging volume rather than the more usual ±200 Hz making use of novel high-bandwidth MRI pulse sequences and distortion correction. Using this design approach, we designed and manufactured a 1.5 T class ReBCO cryogen-free magnet. The magnet is dome-like in form, completely excludes the shoulders and is <400 mm long. The magnet was wound using no-insulation style coils with a conductive epoxy encapsulant where the contact resistance of the coils was controlled so the emergency shut-down time of the magnet was less than 30 s. Despite acceptable coil testing results ahead of manufacture, during testing of the magnet, several of the epoxy coils showed signs of damage limiting stable performance to <55 A compared to the designed 160 A. These coils were replaced with insulated paraffin encapsulated coils. Subsequently the magnet was re-ramped and was stable at 81 A, generating 0.71 T as several other coils had sustained damage not visible in the first magnet iteration. The magnet has been passive shimmed to ±20 kHz B ₀ variation over the imaging volume and integrated into an MRI scanner. The stability of the magnet has been evaluated and found to be acceptable for MRI.

In this paper, we apply a surface-shunting method to prevent quenches in no-insulation (NI) REBCO magnets triggered by external failures of magnet current leads or power suppliers (i.e., fault mode). In a high-field magnet system, an NI coil may still be at risk during the mentioned quench events even if the whole magnet is well-designed, non-defective, and properly operated. The mechanism of this fault-mode quench initiation and propagation still remains unclear, complicating the development of reliable quench protection. Here, we present this mechanism to demonstrate a corresponding practical quench-preventive approach named surface shunting, which utilizes a low-temperature solder attached to the top and bottom of pancake coils. We validate the effectiveness of this approach by comparing the electromagnetic, thermal, and mechanical behaviors in the fault mode with and without the shunt. We conclude that the surface shunt suppresses the fault-mode quench initiation and propagation by redirecting the original turn-to-turn current and induced overcurrent out of the NI winding. We anticipate this work can provide a solution to improve the operational safety of high-field HTS NI magnets against quench and potential damage during fault modes.

A compact benchtop high-field REBCO NMR is one of the most promising HTS applications. An all-REBCO, conduction-cooled magnet is a very attractive design option for demonstrating the unique potential of REBCO for forefront magnets. In this research, we have successfully constructed and tested a prototype all-REBCO, conduction-cooled, 23.5 T magnet operating at 10 K. We have applied the concept of an extreme No-Insulation (NI) winding technique, coupled with a solder-shunting procedure to improve magnet performance. We have also used a temperature-controlled charging sequence (TCCS) to reduce the screening current. The magnet was energized to 23.6 T at 14 K; it was further operated to 25 T at 10 K for nearly 60 hours.

Bulk samples of magnesium diboride (MgB₂) doped with 0.5 wt% of the rare earth oxides (REOs) Nd₂O₃ and Dy₂O₃ (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB₂ doped with 0.5 wt% Dy₂O₃ (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T _c), magnetic and resistive measurements of upper critical field (B _c2) and irreversibility field (B _irr), as well as magnetic and transport measurements of critical current densities versus applied field (J _cm(B) and J _c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB₂ lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F _p) versus reduced field (b = B/B _irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F _p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F _p(b) was still further increased by a Dy₂O₃ additive-induced increase in B _irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy₂O₃ increases B _irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB₂ conductors.

Overpressure (OP) processing of wind-and-react Bi₂Sr₂CaCu₂O_x (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4 % without change in wire length and substantially raising its J _c . However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT). We found that about 80 % of the total wire diameter shrinkage occurs during the 50 atm OP-PD before melting. We successfully wound such pre-densified 1.2 mm diameter wires onto coil mandrels as small as 10 mm diameter for Ag-Mg-sheathed wire and 5 mm for Ag-sheathed wire, even though such small diameters impose plastic strains up to 12% on the conductor. A further ~20% shrinkage occurred during a standard OP-HT. No 2212 leakage was observed for coil diameters as small as 20 mm for Ag-Mg-sheathed wire and 10 mm for Ag-sheathed wire, and no J _c degradation was observed on straight samples and 30 mm diameter coils.

The development of coils that can survive a quench is crucial for demonstrating the viability of MgB₂-based main magnet coils used in MRI systems. Here we have studied the performance and quench properties of a large (outer diameter: 901 mm; winding pack: 44 mm thick × 50.6 mm high) conduction-cooled, react-and-wind (R&W), MgB₂ superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents (I _op ), and distinguished from the minimum energy needed to generate a normal zone (MGE). During these measurements, normal zone propagation velocities (NZPV) were also determined using multiple voltage taps placed around the heater zone. The conduction cooled coil obtained a critical current (I _c ) of 186 A at 15 K. As the operating currents (I _op ) varied from 80 A to 175 A, MQE ranged from 152 J to 10 J, and NZPV increased from 1.3 to 5.5 cm/s. Two kinds of heater were involved in this study: (1) a localized heater ("test heater") used to initiate the quench, and (2) a larger "protection heater" used to protect the coil by distributing the normal zone after a quench was detected. The protection heater was placed on the outside surface of the coil winding. The test heater was also placed on the outside surface of the coil at a small opening made in the protection heater. As part of this work, we also developed and tested an active protection scheme for the coil. Such active protection schemes are of great interest for MgB₂-based MRIs because they permit exploitation of the relatively large MQE values of MgB₂ to enable the use of higher J _e values which in turn lead to competitive MgB₂ MRI designs. Finally, the ability to use a quench detection voltage to fire a protection heater as part of an active protection scheme was also demonstrated.

We present a kinetic-inductance-based superconducting memory element with non-destructive readout, femtojoule read and write energies, both read and write shunts, which is writeable with pulses shorter than 400 ps. The element utilizes both a high-kinetic-inductance layer made from tungsten silicide as well as a low-kinetic-inductance layer made from niobium. By using tungsten silicide-which has a long (20 ns) thermal time constant-and measuring bit error rates from 10 MHz to 1 GHz, we were able to verify that the thin-film elements could be operated at a data rate at least as fast as the material thermal time constant with a bit error ratio less than 10^-6. We also analyze the margins of the device, and outline the characteristics by which a more efficient device may be designed.

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.