Superconducting solenoids

Figure Bl.12.4. Construction of a high-field superconducting solenoid magnet.

NMR instrumentation consists of three chief components a magnet, a spectrometer console, and a probe. While in the past much solid state NMR research was conducted on home-built equipment, the current trend is toward the acquisition of commercial systems. The magnets used for solid state NMR applications generally are superconducting solenoids with a cylindrical bore of 89-mm diameter. The most common field strengths available, 4.7, 7.0, 9.4, and 11.7 Tesla, correspond to proton resonance frequencies near 200, 300, 400, and 500 MHz, respectively. 

It now appears that such difficulties can largely be overcome by the application of proton-proton spin-decoupling and especially by means of the very high resolution now available in spectrometers of the superconducting solenoid type (see below). 

Cyclitol Spectra at 220 MHz with the Superconducting Solenoid. In 1964, Nelson and Weaver (34) at Varian Associates constructed a superconducting solenoid with which proton spectra can be observed at 51.7 kilogauss (220 MHz.) or even higher fields. Other nuclei have been observed at suitable field/frequency combinations. 

A second improvement was that we measured the cyclotron frequency in the precision trap simultaneously with the Larmor frequency. This reduces to a large extent possible errors induced by a temporal variation of the magnetic field which occurs in superconducting solenoids typically at a level of 10-8 per hour. In the final experiment we measure the rate of spin Hips at different ratios of the Larmor- and cyclotron field frequencies. An example is shown in Fig. 10. The linewidth is of the order of 10-8 and the g factor can be determined with a statistical uncertainty below 1 ppb [19]. 

An instrumental development of considerable importance to the p.m.r. spectroscopy of carbohydrates has been the introduction of high-resolution magnets based on superconducting solenoids.47 As already mentioned, the (homogeneous) magnetic field-intensity in conventional electromagnets is restricted by the properties of the iron core and by the fact that the addition of auxiliary coils that are... 

The magnetic field inside a superconducting solenoid is not particularly homogeneous, and compensation of the field gradients is quite a complex problem that has been solved49 by the provision of mathematically designed arrays of superconducting or normal, resistive shim-coils, or both. 

The high cost of commercial superconducting spectrometers and of the continuing supply of liquid helium needed has limited the number of installations of these instruments, and, in some countries, such high-frequency p.m.r. spectra are available only from a national service center. However, some savings are realized because of the fact that, once established in the persistent mode, superconducting solenoids require neither a power supply nor cooling water. 

Superconductivity is the absence of resistance to dc conduction this occurs only below a critical temperature Tc, a critical magnetic field (which is a function of T and current density j), and a critical current density j, which is a function of T and H. For alloys, does not exceed 23 -24 K (by contrast, some of the recently discovered ceramic high-Tc cuprate superconductors, such as HgBa2Ca2Cu30j , have Tc values as high as 140 K and can have comparable y c values), Designers of superconducting solenoid magnets... 

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