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

Abstract

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 field gradients, and (4) screening-current-induced field in the HTS inserts.

Progresses and challenges in the development of high-field solenoidal magnets based on RE123 coated conductors

“…The detrimental effects of SCF on field quality were observed in the first decade of this century, when Bi2223 tapes were used to build inserts for NMR magnets [152,153]. In [154] and [155] the SCF of the Bi2223 insert was found to be responsible for reduction in the total magnetic field, a large gradient in the harmonic components, and large temporal drift (field decay). Although Bi2223 tape, unlike CC tape, consists of many filaments, because the filaments are embedded in silver matrix and thus electromagnetically coupled, its SCF is large.…”

No-insulation multi-width winding technique for high temperature superconducting magnet

“…[9][10][11][12][13][14][15][16][17][18] Commercial 2G conductor comes as tape with width/ thickness ratio in a range of 5-40. 19,20 Unlike a conventional assembly of DP coils, in which the DP coils are wound with the same-width 2G tape, [21][22][23] in our MW technique, we place DP coils of the narrowest tape width in the magnet midplane section, placing DP coils of gradually wider tapes away from the midplane, with the widest-tape DP coils at the top and bottom sections, where the normal field that limits 2G tape performance is at its peak. This MW technique significantly enhances the overall current density of such a DP coil assembly at a given operating current.…”

Persistent-mode high-temperature superconductor shim coils: A design concept and experimental results of a prototype Z1 high-temperature superconductor shim

“…13,14 Slim ($1-mm radial build), it can be placed inside the magnet assembly, avoiding the disadvantages inherent in the conventional NbTi shim. We believe that the HTS shim described here will become a key design option for NMR magnets, including those operated LHe-free, even at > 4:2 K. Although not restricted to GHz-class NMR magnets, 1,[15][16][17] the HTS shim may find its dddddssdsdsd application in these NMR magnets, because each >1-GHz NMR magnet contains an insert of nested HTS coils, which may collectively be modeled as a thick-walled diamagnet.…”

“…1,[4][5][6][7][8][9][10] We may thus consider a superconducting coil as a Bean cylinder of a diamagnetic wall D thick. 4 Although D cannot be equated directly to the size of a superconductor strand, typically $50 lm for LTS and millimeters for HTS, LTS coils generally have much less attenuating and distorting effects of the diamagnetic wall than HTS coils.…”

“…The "raw" field of most superconducting magnets is not good enough for NMR spectroscopy and therefore it must be shimmed to a homogeneous field over the specimen volume. [1][2][3] Generally, the superconducting NMR magnet is equipped with its own set of superconducting shim coils ("shims"), currently all wound with NbTi, a low-temperature superconductor (LTS). Because of their field limitations (<12 T even at 1.8 K) and size (typically 15-30 mm radial build), NbTi shims have always been located outside the magnet assembly, i.e., radially furthest away from the magnet center where the sample is placed.…”