H.W. Weijers

High Field Magnets With HTS Conductors

Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.

Normal zone propagation experiments on HTS composite conductors

Abstract As high temperature superconductor magnet applications become a reality due to increases in conductor performance, it is important to understand the behavior of normal zones in conductors and magnets in order to design safe magnet systems. Here we study the effects of localized heat pulses in Ag-alloy sheathed Bi 2 Sr 2 Ca 2 Cu 3 O x powder-in-tube conductors and Ni-alloy substrated YBa 2 Cu 3 O x coated conductor tapes cooled by N 2 gas. A heater was used to initiate a normal zone. The experimental setup to measure the energy required such that the normal zone propagates and the normal zone propagation velocity in the longitudinal direction is described. Results for both conductor architectures are presented.

Design of a Superconducting 32 T Magnet With REBCO High Field Coils

The design and fabrication of a 32 T, 32 mm cold bore superconducting magnet with high field REBCO inner coils is underway at the NHMFL. In support of the design, conductor characterization measurements have been made including critical current as a function of field, field orientation, temperature, and strain on conductors and joints. Various conductor and turn insulation systems were examined. The selected coil fabrication method for the 32 T magnet is pancake wind, dry wind coils with sol-gel insulation on a stainless steel co-wind. Quench protection of the REBCO coils by distributed heaters is under development. Small REBCO coils have been made and tested in a 20 T background field to demonstrate performance of the technology. The design of the 32 T magnet is described, including coil configuration and conductor lengths, fraction of critical current, selection of conductor copper content for protection, and stress in the windings.

Progress in the Development of a Superconducting 32 T Magnet With REBCO High Field Coils

The design and development of a 32 T, 32 mm cold bore superconducting magnet with high field REBCO inner coils are underway at the NHMFL. The two nested REBCO coils that form the high field section are dry wound, with uninsulated conductor and insulated stainless steel cowind reinforcement. Active quench protection uses distributed protection heaters. As part of the development activity, prototype coils of the two REBCO coils with full scale radial dimensions and final design features, but with reduced axial length are being constructed. The first of these prototype coils was tested in a 15 T resistive background field magnet. The coil has inner and outer winding diameters of 40 mm and 140 mm, respectively, and consists of six double pancakes with a total conductor length of roughly 900 m. The construction of this prototype coil is described, including the protection heaters. Coil test results are reported including coil critical current, coil ramping characteristics, thermal stability, joint, and terminal resistance with field cycling. The corresponding operating stress in the windings is calculated. Importantly, the performance characteristics of the protection heaters will be measured including activation time.

Perspective on a Superconducting 30 T/1.3 GHz NMR Spectrometer Magnet

The COHMAG report of the National Academy of Sciences provides an assessment of high field magnet science and technology in the United States, and identifies possible new initiatives for future high field magnets. Regarding NMR, the report identifies high resolution NMR spectrometers in the range of 30 T/1.3 GHz as having great scientific interest. This paper discusses technology issues associated with 30 T superconducting magnets. The enabling technology is HTS conductors and coil technology. Significant issues remain for application of HTS conductors to large coils, including long length quality, heat treatment, anisotropy and strain dependence. The outer LTS coil sections are large, with high stress and stored energy. Structural issues, including high strength conductor and reinforcement, and quench protection issues are significant. The status of technology required for a 30 T NMR magnet is examined. A preliminary conceptual design of the 30 T magnet is presented

Development of 3 T class Bi-2212 insert coils for high field NMR

Based on a successful 1 T Class PIT insert coil, the authors are now pursuing a 3 T Class insert coil. This paper describes the design and the latest conductor and coil test results, as well as supporting experiments. The final product is envisioned to contain 3 concentric sections, requiring over one kilometer of conductor. This will be tested in a 20 T large bore resistive magnet at the NHMFL. Experimental work focuses on the use of conductor with a silver-alloy matrix in the outer sections, that are subject to the largest stresses when operated in a background field. Results from heat treatment optimization for wound coils, mechanical test of conductors and coil design studies are reported.

Critical current degradation in Nb/sub 3/Sn cables under transverse pressure

The critical current degradation of a few sample Rutherford-type Nb/sub 3/Sn cables is investigated as a function of transverse pressure. A comparison is made between Nb/sub 3/Sn strands produced by the powder-in-tube, bronze, and modified jelly roll processes. The (keystoned) Rutherford cables are charged at 11 T under transverse pressures up to 250 MPa. Large differences in critical current reduction are observed, ranging from 6 to about 60% at 200 MPa, depending on the type of Nb/sub 3/Sn. It appears that about 40% of the total reduction is irreversible. Moreover, the irreversible part shows relaxation, and a partial recovery is possible by thermal cycling.<<ETX>>

HTS insert coils for high field NMR spectroscopy

The availability of high field insert coils will extend the operating point of future NMR systems above the 1 GHz level. We report here on the continuing development of HTS insert coils fabricated with BSCCO-2212 surface coated (SC) superconductor in a tape form. In addition, we review a number of issues associated with the application of HTS superconductors to high resolution NMR. The latest conductor and coil test results to date are presented for a prototype layer wound magnet with a 52 mm bore. In order to accommodate the high stresses associated with high field operation, a nested set of coils is utilized. This coil set provided up to an additional 1.9 Tesla central field while operating at 4.2 K in a 19 Tesla background field.

Bi-based HTS insert coils at high stress levels

Bi-2212 and Bi-2223 conductors from commercial sources are wound into double pancakes using the react and wind approach. One of the Bi-2223 conductors as supplied is reinforced with stainless steel strips. I–V characterization at 4.2 K until failure of the pancakes, in a 19 T–0.17 m cold bore magnet assembly, is performed. This gives insight in the operational limits of these conductors in conditions representative of insert magnets. The results are compared to the linear stress–strain properties of the conductors.

Critical currents in Bi-Sr-Ca-Cu-O superconductors up to 33 T at 4.2 K

The I-V curves of Bi-Sr-Ca-Cu-O superconductors were measured in DC magnetic fields, oriented both parallel and perpendicular to the tape surface, up to 33 T, 4.2 K, in resistive magnets at the National High Magnetic Field Laboratory. This paper presents the experimental set-up, results of critical current measurements, and an analysis of the obtained I/sub c/(B) curves in terms of strongly and weakly linked current paths. A distinct double-step behavior is observed for field perpendicular to the tape surface. The scaling behavior of the applied field with its component perpendicular to the ab-plane, and the implied average grain-misalignment angle, is investigated.