Thibault Lecrevisse

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.

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 Ω·cm2. Several YBCO tape to YBCO tape (YBCO-YBCO) splice joint samples were fabricated, and their resistivity and Ic 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.

An 800-MHz all-REBCO Insert for the 1.3-GHz LTS/HTS NMR Magnet Program—A Progress Report

A critical component of the 1.3-GHz nuclear magnetic resonance magnet (1.3 G) program, currently ongoing at the Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Massachusetts Institute of Technology, and now approaching its final stage, is the all high-temperature superconductor 800-MHz insert (H800). The insert consists of three nested double-pancake (DP) coils fabricated with 6-mm-wide REBCO conductor. Coil 1, the innermost coil of H800, has already been fabricated and tested at 77 and 4.2 K. In addition, one third of the DPs for Coil 2 have been wound and each DP individually fully tested. Work described here includes details of Coil 1 fabrication: DP winding, DP testing, assembling, joint performance, overbanding, and coil testing; winding details of DPs for Coil 2 and their testing are also included.

Designs and Tests of Shaking Coils to Reduce Screening Currents Induced in HTS Insert Coils for NMR Magnet

Two types of shaking coils are focused on reducing screening currents induced in solenoid coils wound with high-temperature superconducting (HTS) tapes. One is a pair of copper shaking coils coaxially located inside and outside the HTS coil to apply an ac magnetic field in the axial direction. The other is an HTS shaking coil with notch located only outside the HTS coil to minimize the radial components of local ac fields applied to windings of the HTS coil as small as possible. It is found that the copper shaking coils yield the allowable amount of power dissipation in liquid helium. The effectiveness of the HTS shaking coil to reduce screening-current-induced fields generated by another magnetized HTS coil is also experimentally validated in liquid nitrogen using a commercially available coated conductor with narrow width.

Construction and Test of 7-T/68-mm Cold-Bore Multiwidth No-Insulation GdBCO Magnet

As a sequel to our previous report on the key concept of the multiwidth (MW) no-insulation (NI) winding technique that applies the NI technique to a magnet assembly of double-pancake (DP) coils wound with varying tape widths (thus MW), this paper presents construction and test results of a 7-T/68-mm cold-bore MW-NI GdBCO magnet. A total of 13 DP coils were fabricated: five central DP coils were wound with 4.1-mm wide tape, while each of the remaining four pairs of DP coils, axially symmetric to the midplane, was wound, from center to end, with 5.1-, 6.1-, 7.1-, and 8.1-mm wide tapes. Each DP coil was tested in a bath of liquid nitrogen (LN2) at 77 K, to obtain its critical current and characteristic resistance. After stacking, the 13-DP magnet was tested in LN2 at 77 K. The key focuses of this paper are: 1) winding and testing of MW-NI DP coils; 2) DP-DP joints; 3) charging response of the magnet; 4) spatial field distribution; and 5) self-protecting feature in overcurrent operation.

Tape-to-Tape Joint Resistances of a Magnet Assembled From (RE)BCO Double-Pancake Coils

We present the results of tape-to-tape joint resistances in a 1.3-GHz/54-mm nuclear-magnetic-resonance magnet comprising a 500-MHz low-temperature-superconducting magnet and an 800-MHz high-temperature-superconducting insert (H800), which is currently in the fabrication stage at the Massachusetts Institute of Technology Francis Bitter Magnet Laboratory. The H800, which is a three-coil assembly of double-pancake coils and wound with a 6-mm-wide (RE)BCO tape (rare earth element), requires a total of 94 tape-to-tape joints. Specific results obtained are: 1) an extrapolation technique to predict the resistances of the tape-to-tape curved bridge joints from those of the curved lap joint samples with the same 6-mm-wide tape batch and joint area; 2) increased resistance owing to a small gap (<; 0.4 mm) in the outer radii of the two adjacent pancakes, which is mainly caused by the variation in the (RE) BCO tape thickness (on the order of several micrometers); 3) a method to minimize solder-heating influences on the pancake current-carrying capacity; 4) the dependence of resistance on solder materials; and 5) the experience with the 25 joints on Coil 1, which is the first of three coils for our H800 insert (H800 Coil 1).

A flux pumping method applied to the magnetization of YBCO superconducting coils: Frequency, amplitude and waveform characteristics

This letter presents a flux pumping method and the results gained when it was used to magnetize a range of different YBCO coils. The pumping device consists of an iron magnetic circuit with eight copper coils which apply a traveling magnetic field to the superconductor. The copper poles are arranged vertically with an air gap length of 1 mm and the iron cores are made of laminated electric steel plates to minimize eddy-current losses. We have used this arrangement to investigate the best possible pumping result when parameters such as frequency, amplitude and waveform are varied. We have successfully pumped current into the superconducting coil up to a value of 90% of I c and achieved a resultant magnetic field of 1.5 T.

Construction and Test Results of Coil 2 of a Three-Coil 800-MHz REBCO Insert for the 1.3-GHz High-Resolution NMR Magnet

This paper focuses on the construction and test results of Coil 2 that is a part of a trio of nested coils composing the REBCO 800-MHz insert. Upon its completion, the REBCO 800-MHz insert will be placed in the bore of a 500-MHz low-temperature superconducting nuclear magnetic resonance NMR magnet (L500) to form the MIT 1.3-GHz high-resolution NMR magnet. Coil 2 is a stack of 32 double-pancake (DP) coils wound with 6-mm-wide REBCO tape using the no-insulation technique. Each pancake is wound on a stainless steel inner supporting ring to prevent the collapsing of its crossover due to the external pressure exerted by the winding pack. Coil 2 will be constructed in the following sequence: 1) after winding, each DP will be individually tested in a bath of liquid nitrogen at atmospheric pressure to determine its current carrying capabilities; 2) DPs will be then assembled as a stack with interconnecting joints, and 3) as in Coil 1, each pancake will be overbanded with a stainless steel tape, this time to a thickness of 5 mm, thickness determined by a stress analysis previously performed. Finally, the fully assembled Coil 2 will be tested in liquid nitrogen at 77 K and then in liquid helium at 4.2 K. We present here details of the stress analysis leading to the sizing of the DP inner supporting stainless steel ring and of the overbanding thickness required. Test results include coil index, critical current, and charging time constant.

Test of an 8.66-T REBCO Insert Coil With Overbanding Radial Build for a 1.3-GHz LTS/HTS NMR Magnet

A 1.3-GHz/54-mm LTS/HTS NMR magnet, assembled with a three-coil (Coils 1-3) 800-MHz HTS insert in a 500-MHz LTS NMR magnet, is under construction. The innermost HTS insert Coil 1 has a stack of 26 no-insulation (NI) double pancake (DP) coils wound of 6-mm-wide and 75-μm-thick REBCO tapes. In order to keep the hoop strains on REBCO tape <;0.6% at an operating current Iop of 250 A and in a field of 30.5 T, we overbanded each pancake in Coil 1 with a 6-mm-wide, 76-μm-thick 304 stainless steel strip: 7-mm-thick radial build for the central 18 pancakes, while 6-mm-thick for the outer 2 × 17 pancakes. In this paper, Coil 1 was successfully tested at 77 K and 4.2 K. In the 77-K test, the measured critical current was 35.7 A, determined by an E-field criterion of 0.1 μV/cm. The center field magnet constant decreased from 34.2 to 29.3 mT/A, when Iop increased from 5 to 40 A. The field distribution at different Iop along the z-axis was measured. The residual field distributions discharged from 10 and 20 A were recorded. In the 4.2-K test, Coil 1 successfully generated a central field of 8.78 T at 255 A. The magnet constant is 34.4 mT/A, which is same as our designed value. The field homogeneity at the coil center within a ±15-mm region is around 1700 ppm. This large error field must be reduced before field shimming is applied.