A REBCO Persistent-Current Switch, Immersed in Solid Nitrogen, Operating at Temperatures near 10 K.

We present design and test results for a thermally-activated persistent-current switch (PCS) applied to a double pancake (DP) coil (151 mm ID, 172 mm OD), wound, using the no-insulation (NI) technique, from a 120-m long, 76-µm thick, 6-mm wide REBCO tape. For the experiments reported in this paper, the NI DP assembly was immersed in a volume of solid nitrogen (SN2), cooled to a base temperature of 10 K by conduction to a two-stage cryocooler, and energized at up to 630 A. The DP assembly operated in quasi-persistent mode, with the conductor tails soldered together to form a close-out joint with resistance below 6 nΩ. The measurements confirm PCS activation at heating powers below our 1-W design target, and a field decay time constant in excess of 900 h (i.e 0.1% h-1 field decay rate), limited by the finite resistance of the close-out joint.

A REBCO Persistent-Current Switch (PCS): Test Results and Switch Heater Performance.

In this paper, we report preliminary results of our on-going effort to develop a superconducting persistent-current switch (PCS) for REBCO pancake coils that will be operated in liquid helium. In the first part of this paper, we briefly describe experimental results of our PCS operated in the temperature range 77-57 K, i.e., liquid-and solid-nitrogen environments. The rest we devote to a new PCS heater design in which we target a heating power of < 1 W in liquid helium.

A Theoretical Design Approach for Passive Shimming of a Magic-Angle-Spinning NMR Magnet.

This paper presents a passive shimming design approach for a magic-angle-spinning (MAS) NMR magnet. In order to achieve a 1.5-T magic-angle field in NMR samples, we created two independent orthogonal magnetic vector fields by two separate coils: the dipole and solenoid. These two coils create a combined 1.5-T magnetic field vector directed at the magic angle (54.74° from the spinning axis). Additionally, the stringent magnetic field homogeneity requirement of the MAS magnet is the same as that of a solenoidal NMR magnet. The challenge for the magic-angle passive shimming design is to correct both the dipole and solenoid magnetic field spherical harmonics with one set of iron pieces, the so-called ferromagnetic shimming. Furthermore, the magnetization of the iron pieces is produced by both the dipole and solenoid coils. In our design approach, a matrix of 2 mm by 5 mm iron pieces with different thicknesses was attached to a thin-walled tube, 90-mm diameter and 40-mm high. Two sets of spherical harmonic coefficients were calculated for both the dipole and solenoid coil windings. By using the multiple-objective linear programming optimization technique and coordinate transformations, we have designed a passive shimming set that can theoretically reduce 22 lower-order spherical harmonics and improve the homogeneity of our MAS NMR magnet.

High-Resolution Magnetic Field Mapping System With an NMR Probe.

This paper presents a high-resolution magnetic field mapping system in development that is capable of collecting spatial magnetic field data for NMR magnets. An NMR probe was designed and built with a resonant frequency of 5.73 MHz. The measured Q-factor of the NMR probe is ~191 with a half-power bandwidth in the range of 5.72-5.75 MHz. An RF continuous-wave technique with magnetic field modulation was utilized to detect the power dispersion of water molecules. The zero-crossing frequency of the NMR dispersion signal corresponds to the magnetic field at the center of the water sample. An embedded system was developed to sweep the frequency and record the reflected RF power simultaneously. A numerically controlled digital oscillator is able to provide a precise frequency step as small as 0.02 Hz, which is equivalent to 4.7 e-7 mT for hydrogen atoms. An RF preamplifier was built to supply up to 4 W of RF power to a bidirectional coupler. The coupler supplies RF power to the NMR probe and channels reflect the RF power back to the detection circuit, which detects the reflected RF power from the NMR probe during the frequency sweep. The homogeneity of an NMR magnet can be determined by magnetic field data.

Strain in YBCO Double-Pancake Coil With Stainless Steel Overband Under External Magnetic Field.

This paper deals with the mechanical strain issue in a high-temperature superconducting (HTS) insert for a GHz-class (> 23.5 T) LTS/HTS NMR magnet. We present results, experimental and analytical, of hoop strains in a double-pancake (DP) test coil, wound with 6-mm wide YBCO coated conductor (CC) and equipped with strain gauges at their innermost and outermost turns. To keep the YBCO CC to within a 95% Ic retention, the conductor tensile strain must be limited to 0.6%. To satisfy this strain limit in our test DP coil, we wrapped 0.08-mm thick, 6-mm wide stainless steel strip over its outermost turn of an 4.8-mm overband radial build deemed sufficient by our stress analysis based on force equilibrium and generalized Hooke's law with plane stress approximation. A control test DP coil, actually the same test DP coil, without overbanding, was run under the same experimental condition. In each case the test DP coil was energized up to 350 A at 4.2 K in a background field of 4 T. We report the experiment and analysis, with discussion on the merit of overbanding as a means to limit hoop strain in high-field HTS inserts.

Construction and Persistent-Mode Operation of MgB2 Coils in the Range 10-15 K for a 0.5-T/240-mm Cold Bore MRI Magnet.

This paper presents construction and persistent-mode operation results of MgB2 coils for a 0.5-T/240-mm cold bore MRI magnet, wind-and-react with monofilament MgB2 wire at the MIT Francis Bitter Magnet Laboratory. The magnet, of respective inner and outer diameters of 276 and 290 mm and a total height of 460 mm, has center field of 0.5 T and current density of 11 kA/cm2. To limit the continuous length of Hyper Tech supplied MgB2 monofilament wire to ≤300 m, the magnet was divided into eight series-connected coils, each equipped with a persistent current switch and a superconducting joint. We have manufactured three coil modules. Before being tested as an assembly, each coil was tested individually to ensure its capacity to carry 100-A superconducting current in the range of 10-15 K. The three coils were then assembled, connected in series, and operated as a 3-coil assembly in persistent mode at nearly 100 A in the range of 10-15 K. We present results that include: 1) construction details; 2) component performances; and 3) a 3-coil assembly performance.

A 1.5-T Magic-Angle Spinning NMR Magnet: 4.2-K Performance and Field Mapping Test Results.

We present results of full-current testing at 4.2 K of a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole coil that comprise a 1.5-T/75-mm room temperature bore magic-angle-spinning nuclear magnetic resonance magnet developed at the MIT Francis Bitter Magnet Laboratory. Also included in the paper are results of the magnet performance when the magnet assembly is immersed, to enhance its thermal mass, in solid nitrogen, and operated in the temperature range of 4.2-4.3 K.

Operation of a 130-MHz/9-mm Compact HTS Annulus Magnet With a Micro-NMR Probe.

In this paper, we report final operation results of our compact annulus NMR magnet, named YP2800, with a homemade micro-NMR probe in a bath of liquid helium at 4.2 K. YP2800 comprises of a stack of 2800 YBCO "plate annuli," 0.08 mm thick, either 46 mm or 40 mm square, each having a 26-mm hole machined at the center. By the field-cooling technique, YP2800 was energized at 130 MHz (3.05 T); an overall peak-to-peak homogeneity of 487 ppm within |z| < 5 mm was measured at a moment when a field drift of 11 ppm/h was reached in three days after field cooling. Due to a small (9.2 mm) bore size, no commercial probes could fit into the bore; an 8.5-mm micro-NMR probe was designed and constructed. Following a general description of YP2800 and design construction details of the micro probe, this paper presents NMR signals captured by the probe for a dimethyl sulfoxide sample of ϕ 4.4 and 5 mm long at a base frequency of 130 MHz with a half-peak width of 60 kHz; the corresponding frequency impurity of 461 ppm is chiefly due to a spatial field error, i.e., 487 ppm in the target space.

A High-Resolution 1.3-GHz/54-mm LTS/HTS NMR Magnet.

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently in the final stage at the Massachusetts Institute of Technology Francis Bitter Magnet Laboratory. Its key component is a three-coil (Coils 1-3) 800-MHz HTS insert comprising 96 no-insulation (NI) double-pancake coils, each wound with a 6-mm-wide GdBCO tape. In this paper, after describing the overall 1.3-G system, we present innovative design features incorporated in 1.3 G: 1) an NI winding technique applied to Coils 1-3 and its adverse effect in the form of charging time delay; 2) persistent-mode HTS shims; 3) a "shaking" magnet; and 4) preliminary results of Coil 1 operated at 4.2 K.

A 1.5-T/75-mm Magic-Angle-Spinning NMR Magnet.

We are currently working on a program to complete a 1.5-T/75-mm RT (room temperature) bore MAS (magic-angle-spinning) NMR (nuclear magnetic resonance) magnet. The MAS magnet comprises a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole coil. The combination of the fields creates a 1.5-T field pointed at 54.74 degrees (magic angle) from the rotation (z) axis. During the 2nd year of this 3-year Phase I program, both coils have been wound and testing has begun. Some preliminary field mapping has been performed, and the design of the MAS magnet assembly has been completed. During the final year, the magnet assembly will be integrated into the cryogenic structure and tested at ~5.5 K in a solid nitrogen environment. Each coil will be energized separately, and the magnetic field will be mapped accurately. We expect a bare magnet uniformity of 100 ppm over a 10-mm diameter, 20-mm-long cylindrical volume. Then, using the field data, the uniformity will be improved to < 0.1.ppm with a combination of ferroshims and cryoshims. Final field measurement will be performed as the cryostat-magnet system is spun manually at ~0.1 Hz.

Temporal Enhancement of Trapped Field in a Compact NMR Magnet Comprising YBCO Annuli.

Temporal "enhancement" of trapped fields was observed in the central region of a compact NMR magnet comprising a stack of 2800 YBCO "square" annuli (YP2800), field-cooled at 4.2 K. This paper presents an analytical model to simulate the trapped field enhancement in YP2800. First, based on an inverse calculation technique, the current distributions in the 560 5-plate modules of YP2800 were computed from the measured trapped field distribution. Then, YP2800 was modeled as a set of "three magnetically-coupled subcoils": the "bottom" coil (CB, 140 modules); the "middle" coil (CM, 280 modules); and the "top" coil (CT, 140 modules). With the index resistance of each coil included, the circuit model shows that the average current in CM "slowly" increases, induced by "fast" current decays in CB and CT. As a result, the center field in YP2800, dominated by the CM currents, increases in time. The simulation agrees reasonably well with the measurement, which validates the analytical model.

Development of a 0.5-T/240-mm MgB2 MRI Magnet: Assembly Design and Module Coils.

This paper presents the latest results from our continued development of a 0.5-T/240-mm MgB2 MRI magnet at the MIT Francis Bitter Magnet Laboratory. Because we have successfully developed our superconducting joint technique with a monofilament MgB2 wire, manufactured by Hyper Tech Research, Inc. (Columbus, OH), we have decided to use a monofilament wire to wind our MgB2 MRI magnet. The magnet, comprising eight module coils, has a winding inner diameter of 276 mm, an outer diameter of 290 mm, and a total height of 460 mm. Each coil has its own persistent-current switch (PCS) and a superconducting joint. In order to guard against a few bad coils forcing the entire magnet to be inoperative, each coil will be heat-treated and tested individually. After eight coils are successfully operated, they will be assembled into an MRI magnet and series-connected with soldering joints between adjacent coils. The PCS in each coil is designed in such way that it will also serve as a detect-and-heat protection absorber when the magnet quenches over a small "localized" region: The conductor volume in the eight switches is designed to absorb the entire magnet energy while still remaining below 200 K. This paper reports 1) the design of the whole magnet and 2) the fabrication and test results of the two real-size test coils, with their PCSs and superconducting joints. The tests were conducted in gas helium in the temperature range of 10-15 K and in the self-field of the coils.

Field performance of a prototype compact YBCO "annulus" magnet for micro-NMR spectroscopy.

A prototype compact annulus YBCO magnet (YP1070) for micro-NMR spectroscopy was constructed and tested at 77 K and 4.2 K. This paper, for the first time, presents comparison of the 77-K and 4.2-K test results of our annulus magnet. With a 26-mm cold bore, YP1070 was comprised of a stack of 1070 thin YBCO plates, 80-µm thick and either 40-mm or 46-mm square. After 1070 YBCO plates were stacked ''optimally'' in 214 groups of 5-plate modules, YP1070 was ''field-cooled'' at 77 K after being immersed in a bath of liquid nitrogen (LN2) with background fields of 0.3 and 1 T and also at 4.2 K in a bath of liquid helium (LHe) with background fields of 2.8 and 5 T. In each test, three key NMR magnet field-performance parameters-trapped field strength, spatial field homogeneity, and temporal stability-were measured. At 4.2 K, a maximum peak trapped field of 4.0 T, equivalent to 170 MHz 1H NMR frequency, was achieved with a field homogeneity, within a |z| < 2.5 mm axial space, of ~3000 ppm. YP1070 achieved its best field homogeneity of 182 ppm, though at a reduced trapped field of 2.75 T (117 MHz). The peak trapped fields at 4.2 K were generally ~10 times larger than those at 77 K, in direct proportion to ~10-fold enhancement in superconducting current-carrying capacity of YBCO from 77 to 4.2 K. Temporal stabilities of ~110 and ~17,500 ppm/h measured at 77 K, with trapped fields respectively of 0.3 and 1 T, show that temporal stability deteriorates with trapped field strength. Also, temporal enhancement of trapped fields at 4.2 K was observed and reported here for the first time.

Magic-Angle-Spinning NMR Magnet Development: Field Analysis and Prototypes.

We are currently working on a program to complete a 1.5 T/75 mm RT bore magic-angle-spinning nuclear magnetic resonance magnet. The magic-angle-spinning magnet comprises a z-axis 0.866-T solenoid and an x-axis 1.225-T dipole, each to be wound with NbTi wire and operated at 4.2 K in persistent mode. A combination of the fields creates a 1.5-T field pointed at 54.74 degrees (magic angle) from the rotation (z) axis. In the first year of this 3-year program, we have completed magnetic analysis and design of both coils. Also, using a winding machine of our own design and fabrication, we have wound several prototype dipole coils with NbTi wire. As part of this development, we have repeatedly made successful persistent NbTi-NbTi joints with this multifilamentary NbTi wire.

YBCO and Bi2223 Coils for High Field LTS/HTS NMR Magnets: HTS-HTS Joint Resistivity.

This paper presents our latest experimental results on high-temperature superconducting (HTS) splice joints for HTS insert coils made of YBCO and Bi2223, that comprise a 1.3 GHz low-temperature superconducting/HTS nuclear magnetic resonance magnet currently under development at Francis Bitter Magnet Laboratory. HTS splice joint resistivity at 77 K in these insert coils must be reproducible and < 100 nΩ cm2. Several YBCO tape to YBCO tape (YBCO-YBCO) splice·joint samples were fabricated, and their resistivity and I c were measured at 77 K. First, we describe the joint splicing setup and discuss the parameters that affect joint resistivity: pressure over joint surface, solder, and YBCO spool batch. Second, we report results on YBCO-YBCO joints at 77 K in zero field. Measurements have shown that spool batch and solder are primary sources of a wide range of variation in YBCO-YBCO joint resistivity. By controlling these parameters, we expect to reproducibly achieve HTS-HTS resistive joints of resistance < 100 nΩ cm2.

Monofilament MgB2 Wire for a Whole-Body MRI Magnet: Superconducting Joints and Test Coils.

This paper presents recent results from our continued development of a 0.5 T whole-body MRI magnet at the Francis Bitter Magnet Laboratory. HyperTech Research Corp. (Columbus, OH) manufactures the MgB2 conductor for this project. During the past year, we have found that our technique, originally developed successfully to splice unreacted multifilament MgB2 wires, works much better, i.e., of higher reliability, with unreacted monofilament MgB2 wires. This has led us to wind the entire coil components in our persistent-mode MRI magnet with unreacted monofilament MgB2 wire, having a MgB2 core of 0.4 mm in diameter, an overall diameter of 0.8 mm bare, 1 mm S-glass insulated. To verify that these coils would not suffer from flux jumping, as they would if wound with monofilament NbTi wire, magnetization studies were performed on monofilament wires of MgB2 and NbTi (as a reference) at 4.2 K. For the monofilament MgB2 wire, the results were affirmative. To further ensure the absence of flux jumping that may quench these current-carrying coils, two test coils were wound with unreacted monofilament MgB2 wire. One MgB2 coil was operated in driven mode, while the other MgB2 coil, equipped with a persistent current switch and terminated with a superconducting joint, was operated in persistent mode. The operating temperature range was 4.2-15 K for these MgB2 coils. The driven mode coil was operated in self-field. The persistent mode coil achieved a persistent current of 100 A, corresponding to a self-field of ~1 T in the winding, for 1 hour with no measurable decay. Both test coils were operated quench free.

Bulk and Plate Annulus Stacks for Compact NMR Magnets: Trapped Field Characteristics and Active Shimming Performance.

We have constructed two "annulus" magnets, YP2800 and YB10; each consists of 2800 YBCO thin square "plate annuli" (YP2800) and 10 YBCO thick "bulk annuli" (YB10). Their trapped field characteristics, spatial and temporal, were investigated and compared, experimentally and analytically. Two sets of field-cooling tests were performed at 77 K: (1) maximum trapped field tests, where a 2-T background field was applied to investigate the maximum trapped field capability of the two magnets; and (2) reduced trapped field tests, where spatial homogeneity improvement of the two magnets was investigated after field cooling with a reduced background field. Also, a Z1 copper shim coil was designed, constructed, and operated, alone and with YP2800 and YB10. When it was operated with the annulus magnets at 77 K, a significant attenuation of the shim coil strength was observed due to the screening currents induced within the annulus magnets.

No-Insulation Coil Under Time-Varying Condition: Magnetic Coupling With External Coil.

This paper presents experimental and analytical studies on the time-varying behavior of an NI (no-insulation) high-temperature superconductor pancake coil, alone or magnetically coupled to an external coil. An NI coil and another insulated coil (as an external), both of identical winding i.d. and number of turns, were fabricated. Another external coil used in this study was a 300-mm/5-T low-temperature superconductor magnet. An equivalent circuit model is proposed to simulate the NI coil, and the external coil, under time-varying conditions. Good agreement between experiment and simulation shows that the proposed equivalent circuit model is valid to characterize the time-varying electromagnetic behavior of an NI coil, alone or magnetically coupled to an external coil.

No-Insulation (NI) HTS Inserts for > 1 GHz LTS/HTS NMR Magnets.

A no-insulation (NI) technique has been applied to wind and test a NI HTS (YBCO) double-pancake coil at 4.2 K. Having little detrimental effect on field-current relationship, the absence of turn-to-turn insulation enabled the test coil to survive a quench at a coil current density of 1.58 kA/mm2. The NI HTS coil is compact and self-protecting, two features suitable for large high-field magnets. To investigate beneficial impacts of the NI technique on >1 GHz LTS/HTS NMR magnets, we have designed six new NI HTS inserts for our ongoing 1.3 GHz LTS/HTS NMR magnet, which require less costly LTS background magnets than the original insulated HTS insert. A net result will be a significant reduction in the overall cost of an LTS/HTS NMR magnet, at 1.3 GHz and above.

A Stack of YBCO Annuli, Thin Plate and Bulk, for Micro-NMR Spectroscopy.

A new annulus magnet, the latest in a series of compact magnets being developed for NMR spectroscopy applications at the MIT Francis Bitter Magnet Laboratory, was built and tested in a bath of liquid nitrogen at 77 K. The magnet, YP2800, a stack of 2800 thin YBCO plate annuli, each either 40 - or 46-mm square and 0.08-mm thick with a 26-mm bore, is an upgraded version of the two earlier plate annulus magnets, YP750 (750 plates) and YP1070 (1070). This paper presents construction details and test results of YP2800. Its spatial field homogeneity and temporal stability were measured, analyzed, and compared with those of YP750 and YP1070. Also, four YBCO bulk annuli, each 26-mm i.d., 46-mm o.d., and 5.2-mm thick, were added to YP2800 and their impacts on field strength and homogeneity were investigated experimentally.