Towards beyond 1 GHz NMR: Mechanism of the long-term drift of screening current-induced magnetic field in a Bi-2223 coil

Abstract

The screening current-induced magnetic field in the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) insert coil proposed for a beyond 1GHz nuclear magnetic resonance (NMR) spectrometer may generate a long term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ~1000hr after coil excitation, the field-drift rate approaches the field decay rate of the persistent current of 10-8 /hr, suitable for a long-term NMR measurement in a beyond 1GHz NMR spectrometer. 4/32

Effect of coil current sweep cycle and temperature change cycle on the screening current-induced magnetic field for Ybco-coated conductor coils

“…We also reported that the temporal magnetic field drift due to screening current for a Bi-2223 coil is effectively suppressed by a current sweep reversal technique [6][7]. A similar effect was demonstrated for a 500 MHz (11.75 T) HTS/LTS NMR using a Bi-2223 innermost coil [8].…”

“…The reduction rate depends on the coil shape and is mostly remarkable in the case of a minimum volume design [4]. The second problem is a temporal positive drift in the central magnetic field; the screening current relaxes with time due to flux creep, resulting in long term drift in the central magnetic field [4][5][6]. These effects are undesirable for NMR magnets as NMR requires a homogenous and stable magnetic field.…”

Operation of a 500 MHz high temperature superconducting NMR: Towards an NMR spectrometer operating beyond 1 GHz

“…1 (black-solid line) shows the initial change in the magnetic field intensity with time measured by an NMR meter (Model PT 2025, Metrolab Instruments); note that the vertical axis is normalized to the NMR operation field. After attaining the operation current at t = 0 h, the magnetic field intensity increased with time at a rate of 1.9 Â 10 À5 (19 ppm)/h at first, and then tended to saturate at 5 Â 10 À8 /h (50 ppb/h) at t = 110 h. Screening current was induced in the Bi-2223 tape by the radial component of the NMR magnetic field, generating a negative central magnetic field at the coil center [15]. Relaxation of the screening current resulted in positive drift of the magnetic field intensity as seen in Fig.…”

“…Its relaxation results in the positive drift of the central magnetic field with time, after the magnet is charged to the operation current [15]. If the temporal change in the screening current-induced magnetic field exceeds 3 Â 10 À6 (±1.5 ppm), the field-frequency lock system loses operation and it is impossible to achieve a high resolution NMR spectrum.…”

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.…”