MIT Francis Bitter Magnet Laboratory Research Articles

26 T 35 mm all-GdBa2Cu3O7–x multi-width no-insulation superconducting magnet

A 26 T 35 mm winding diameter all-GdBa2Cu3O (GdBCO) magnet was designed by the MIT Francis Bitter Magnet Laboratory, and constructed and tested by the SuNAM Co., Ltd. With the multi-width (MW) no-insulation (NI) high temperature superconductor (HTS) winding technique incorporated, the magnet is highly compact; its overall diameter and height are 172 and 327 mm, respectively. It consists of a stack of 26 NI double pancake coils wound with MW GdBCO tapes in five different widths ranged 4.1–8.1 mm. In a bath of liquid nitrogen at 77 K, the magnet had a charging time constant of 16 min due to the intrinsic NI characteristics. In liquid helium at 4.2 K, the magnet generated a 26.4 T field at the center, a record high in magnetic fields from all-HTS magnets. The results demonstrate a strong potential of MW-NI GdBCO magnets for direct current high-field applications.

An analytical approach towards passive ferromagnetic shimming design for a high-resolution NMR magnet

This paper presents a warm-bore ferromagnetic shimming design for a high-resolution NMR magnet. The design is based on spherical harmonic coefficient reduction techniques. We have applied the technique to a 700 MHz magnet completed at the MIT Francis Bitter Magnet Laboratory. The 700 MHz magnet was composed of a low-temperature superconducting (LTS) 600 MHz NMR magnet (L600), purchased from a magnet vendor, and a 100 MHz high-temperature superconducting (HTS) insert (H100), designed and built at FBML. Based on the measured magnetic field data, we derived a total of 22 low-order spherical harmonic coefficients. Another set of spherical harmonic coefficients was calculated for iron pieces attached to the surface of a 50 mm wide and 72 mm high tube. To improve the magnet homogeneity, we applied a multiple objective linear programming method to minimize unwanted spherical harmonic coefficients. A result is a ferromagnetic shimming set of 74 iron pieces. Analysis shows that with this ferromagnetic shimming set, the magnet field homogeneity may be significantly improved, here by a factor of eight.

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.

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.

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.

On the 600 MHz HTS Insert for a 1.3 GHz NMR Magnet.

We update here on the status of one of the main components of a 1.3 GHz NMR magnet currently being developed at the MIT Francis Bitter Magnet Laboratory (FBML): the 600 MHz HTS insert coil. Started in 2000 as a 3-phase program and currently in its final phase (Phase 3A), the HTS insert will first be installed in the bore of a 500 MHz all LTS magnet, generating a field of 25.8 T (1.1 GHz). In Phase 3B, we will place the HTS insert in the bore of a 700 MHz LTS magnet, achieving our ultimate goal of completing a high-resolution 1.3 GHz magnet. The HTS insert, built as a stack of double pancakes (DP), consists of two concentric stack of coils, the inner coil wound with YBCO tape and the outer coil with Bi2223/Ag tape. The series-connected coils, immersed in a bath of liquid helium at atmospheric pressure, will be operated in driven mode. The paper describes detailed aspects of: 1) design and fabrication of both coils, 2) testing of individual DPs at 77 K to characterize the current carrying capabilities and magnetic performance of each DP coil, 3) techniques and elements utilized in the DP-to-DP splice procedure and, 4) splice dissipation.

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.

High-Temperature Superconducting Magnets for NMR and MRI: R&D Activities at the MIT Francis Bitter Magnet Laboratory.

This paper describes the NMR/MRI magnets that are currently being developed at the MIT Francis Bitter Magnet Laboratory: 1) a 1.3 GHz NMR magnet; 2) a compact NMR magnet assembled from YBCO annuli; and 3) a persistent-mode, fully-protected MgB2 0.5-T/800-mm whole-body MRI magnet.

A Ferromagnetic Shimming Method for NMR Magnet Using Linear Programming

Abstract - Shimming is an important technique in development of nuclear magnetic resonance (NMR) magnets where image resolution is highly dependent on magnetic field homogeneity. Classically, shimming may be categorized into two types: 1) active shimming that incorporates with extra coils and precise tuning of their currents; and 2) passive shimming that incorporates with pieces of steel placed in a bore of a main magnet and their uniform magnetization under homogeneous external fields. Additional magnetic fields, produced by the coils and/or the steel sheets, compensate original magnetic field from the main magnet in such a way that the total field becomes more homogeneous. In this paper, we developed a passive shimming method based on linear programming optimization. Linear programming is well known to be highly efficient to find a global minimum in various linear problems. We firstly confirmed the linearity of magnetization of ferromagnetic pieces under a presence of external magnetic fields. Then, we adopted the linear programming to find optimized allocation of the steel pieces in the inner bore of a main magnet to improve field homogeneity. Key Words : - NMR magnet, Ferromagnetic shimming, Passive shimming, Linear programming †교신저자, 정회원 : 위덕대학교 에너지전기공학부 부교수E-mail : sjlee@uu.ac.kr*정회원:MIT Francis Bitter Magnet Laboratory Research Staff**정 회 원 : 한국전기연구원 초전도연구센터 선임연구원 접수일자 : 2010년 4월 20일 최종완료 : 2010년 5월 13일

A 0.6 T/650 mm RT Bore Solid Nitrogen Cooled MgB2 Demonstration Coil for MRI-a Status Report.

Aiming to demonstrate feasibility and practicality of a low cost superconducting MRI magnet system targeted for use in small hospitals, rural communities and underdeveloped countries, MIT-Francis Bitter Magnet Laboratory has developed a 0.6 T/650 mm room temperature bore demonstration coil wound with multifilament MgB2 conductor and cooled via an innovative cryogenic design/operation. The coil is to be maintained cold by solid nitrogen kept in the solid state by a cryocooler. In the event of a power failure the cryocooler is automatically thermally decoupled from the system. In this paper we present details of the MgB2 conductor, winding process, and preliminary theoretical analysis of the current-carrying performance of the conductively cooled coils in zero background field and over the 10-30 K temperature range.