Emanuel S. Bobrov

Experimental and theoretical investigation of mechanical disturbances in epoxy-impregnated superconducting coils. 2. Shear-stress-induced epoxy fracture as the principal source of premature quenches and training theoretical analysis

Abstract An epoxy-impregnated superconducting winding may be considered structurally as a unidirectional composite consisting of superconducting wires embedded in a matrix of epoxy resin. The epoxy, because of its low strength and brittleness at low temperatures, is susceptible to brittle fracture which occurs under stresses induced initially during the cooldown (by differential thermal contractions of epoxy and metal) and subsequently during the magnet charge-up (by the Lorentz forces). Various modes of matrix failure are discussed and analysed. For the composite winding represented by four principal characteristics - geometry; constituent material properties; winding boundary conditions; and microcracks which become stress concentration sites for the initiation of further cracking. It is demonstrated that the transverse shear stresses induced by Lorentz forces in windings with cylindrical symmetry are principally responsible for premature magnet quenches. It is further demonstrated that to minimize shear stresses and thus prevent epoxy fracture in the winding, the whole winding body must not be restrained by the coil form and must be free to take its natural shape as the magnet is energized. This unrestrained winding support design is called the floating coil concept. The conclusions of the analysis agree both qualitatively and quantitatively with experimental results reported in the next two parts of this work.

Operation and performance analyses of 350 and 700 MHz low-/high-temperature superconductor nuclear magnetic resonance magnets: A march toward operating frequencies above 1 GHz

Since 2000, a three-phase program with a final goal to complete a 1 GHz high-resolution low-/high-temperature superconductor (LTS/HTS) nuclear magnetic resonance (NMR) magnet has been conducted at the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology (MIT). In a LTS/HTS magnet assembly, a HTS insert is placed in the cold bore of a LTS background magnet. To date, two LTS/HTS magnets have been designed, constructed, and tested: a 350 MHz (LH350) in phase 1 and a 700 MHz (LH700) in phase 2. The program’s target has recently been upgraded from the original goal of 1 GHz to a new goal of 1.3 GHz. In this paper, we present extensive performance analyses of the two LTS/HTS NMR magnets. Spatial homogeneity and temporal stability of LH350 and LH700, examined with harmonic analysis, and four key issues that became evident in the operation of these two magnets are discussed: (1) field constant reduction, (2) “large” residual Z1 gradient and its temporal decay, (3) large one-periodic tesseral fi...

An LTS/HTS NMR Magnet Operated in the Range 600–700 MHz

As part of our 3-phase program to develop a high resolution 1 GHz LTS/HTS NMR magnet, a system composed of a 600 MHz LTS background NMR magnet and an HTS insert operating in the range 75 to 100 MHz has been built and tested. Operated at 4.2 K, immersed in a bath of liquid helium, the Phase 2 LTS/HTS magnet, the LTS magnet in persistent mode and the HTS insert powered by a stable supply, generated an NMR frequency of 692.2 MHz (16.26 T): the LTS magnet, with 234.4 A, at 588.7 MHz (13.83 T); and the HTS insert, with 115.95 A, at 103.5 MHz (2.43 T). This paper presents results of the magnet operation during test geared to the design of room temperature shim coils and ferromagnetic tiles. Magnetization effects of the HTS insert on field distribution are also reported.

A general method of design of axial and radial shim coils for NMR and MRI magnets

A novel, efficient method for the design of axial and radial shim coils to suppress undesired harmonic orders in regions of homogeneity is presented. It uses two or three positional degrees of freedom as in the conventional method, but it is not required that these positions be the zeros of any of the source functions. The use of the method is shown for first- through eighth-order axial gradient coils, radial gradient coils, X-gradient coils, and ZX-gradient coils. >

Magnet system of the 500 MHz NMR spectrometer at the Francis Bitter National Magnet Laboratory: I. Design and development of the magnet

A high field nuclear magnetic resonance spectrometer has been put into operation at the MIT, Francis Bitter National Magnet Laboratory. The spectrometer is built around a high homogeneity magnet, the design field of which was 11.74 T, and which in fact operates in persistent mode at a field of 11.71 T. The magnet is wound using niobium–titanium superconductor cooled to 2.3 K and is housed in a special dewar designed to permit continuous subatmospheric cryogenic operation and to minimize helium consumption. This and a companion paper describe the analysis, design, and construction of the magnet.

A low- and high-temperature superconducting NMR magnet: design and performance results

A nuclear magnetic resonance (NMR) magnet comprised of low- and high-temperature superconducting (LTS/HTS) coils has been built and operated at a frequency of 360 MHz. The magnet operates at 4.2 K with the LTS background coil operated in persistent mode, while the HTS insert is in driven mode. The HTS insert is an assembly of 50 double pancakes, each wound with Bi-2223/Ag tape. The paper describes design and performance results of the LTS/HTS magnet.

An HTS insert for phase 2 of a 3-phase 1-GHz LTS/HTS NMR magnet

As a component of Phase-2 LTS/HTS NMR magnet of a 3-phase program to complete a 1-GHz LTS/HTS NMR magnet by 2009, an HTS insert magnet has been assembled from 48 double-pancake (DP) coils, each wound of high-strength Bi-2223/Ag/stainless steel composite tape. The paper describes the design and actual parameters; and preliminary performance results of the insert. The results include: 1) voltage vs. current data of double pancakes; and 2) splice resistances between adjacent double-pancake coils.

68.4 T pulsed magnet fabricated with a wire-wound metal-matrix Cu/Nb microcomposite

A description is given of a high-strength metal-matrix microcomposite Cu/Nb conductor used in a multilayer wire-wound pulsed field magnet. The 0.5-in inner diameter magnet was precompressed in a hardened-steel structure and cooled to 77 K. A maximum field of 68.4 T was generated for a 5.6-ms half-period at the maximum energy available with the available capacitor bank (100 kJ at 4 kV). The Cu/Nb microcomposite has an ultimate tensile strength (UTS) of 180 ksi at 77 K. The tradeoffs of UTS, temperature, conductor resistance, pulse time, and field volume versus energy are discussed. The manufacturing process of the Cu/Nb metal-matrix microcomposite is outlined. Examples of some recent experiments using this magnet are presented. >

Experimental and theoretical investigation of mechanical disturbances in epoxy-impregnated superconducting coils. 3. Fracture-induced premature quenches

Abstract The theoretical correlation between shear stress and epoxy resin fracture developed in Part 2 was verified experimentally using a series of epoxy-impregnated, thin-walled superconducting test coils. A Lorentz shear stress field was created within the winding of each coil by placing it in a constant background magnetic field and energizing it with transport current, which was increased slowly at a constant rate. Because the boundary condition at each end of the test coil critically influenced the Lorentz shear stress field, which caused epoxy resin fracture, premature quench data were measured for test coils with different combinations of end boundary conditions. In test coils with both ends rigidly clamped, cracks occurred as transport current was increased; during a training sequence the test was terminated by a premature quench. Using acoustic emission and voltage signals, each premature quench was linked directly to a crack occurring near one of the ends. Test coils which had both ends unsupported, giving the winding freedom to expand radially, did not experience epoxy fracture and showed no premature quenches: these reached critical current at the first attempt.

A 700 MHz LTS/HTS NMR Magnet— A Status Report

Phase II of our three phase program to develop a 1 GHz High Resolution LTS/HTS NMR Magnet, entails the design, construction and testing of a 700 MHz LTS/HTS NMR magnet. Here we report on the design and construction of a 100 MHz HTS Insert magnet, its interface to a 600 MHz LTS magnet, and testing of the LTS/HTS assembly. Immersed in 4.2 K LHe at atmospheric pressure, the coils are independently powered with the HTS Insert operating at 116 A while the background LTS coil carries 235 A. Details of the mechanical, cryogenic and magnetic features of the system are presented