A fast field-cycling NMR/NQR spectrometer

Abstract

The authors describe a fast field-cycling NMR/NQR spectrometer with a switching time of only 0.7 ms between zero field and maximum field (1 T) which uses a novel air-cored magnet design in which the magnet is cut out of a solid copper tube by means of electric spark erosion. The pitch and cross section of the windings vary and have been optimized for small inhomogeneity of the field (6 parts in 105) and the highest field for the available power (20 kW). The design of the power supply, which uses one capacitor as energy storage, and considerations concerning the safety of the instrument are also described as well as the probe and the more conventional parts of the NMR spectrometer. The authors conclude with examples of the performance and the main applications: field-dependent spin-lattice relaxation time measurement (T1 dispersion), cross relaxation and nuclear quadrupole double resonance (NQDR).

Fast-field-cycling NMR: Applications and instrumentation

“…The main problems are lethal transistor damages, a current offset in the magnet and undesirably long transients. The first problem is typical for systems with highly inductive coils where the switching time is shortened by using prestored capacitive energy [20,37,186,187]. In this case, the transistor bank must be protected against the high power peaks produced at the magnet coil during delivery of the capacitive energy.…”

“…An even number of cylinders was needed in order to compensate azimuthal and radial field components due to the helix configuration. This kind of magnets was successfully used by Noack's group, and later by the groups in Cordoba [150,186] and London [187] in different field cycling experiments. The idea was also successfully translated to an industrial environment by Stelar.…”

17O Quadrupole Dips in Ammonium Persulphate

“…1, was obtained using a mechanical field cycling instrum ent [11] with fixed residence times of 22 s in high field and 8 s in low field. This measure ment was carried out at room temperature, approxi mately 295 K. The measurement of 7\ (*H) dispersion was carried out using a fast field cycling spectrometer [12], in which the sample remains static whilst the field is switched electronically from about 1 T to low or zero field in 0.7 ms [12]. The dispersion was measured in low fields from the E arth's field (ca.…”

Magnet Design with High B0 Homogeneity for Fast-Field-Cycling NMR Applications

“…A coolant flow of 15 the coil at the inside of the inner layer from the top and from the bottom and pours through the windings to the outside. We consider that this has some advantages compared to a flow in the axial direction used by other authors (15,16):…”

“…The idea of these designs is to correct the inhomogeneity in the axial direction by varying the current density along the coil axis. This can be done by adding extra turns (1,20), by using a variable pitch angle (17), by varying the coil diameter (5), or by using a ribbon with continuously variable width (15,16). In principal the last design is the most favorable one, but so far it is only realized using an algorithm (15) which takes into account an axial distribution of ring currents instead of currents following the real helical form of the conductor.…”