A platform for vibration damping, leveling, and height control of high-resolution superconducting magnets

Abstract

Three years ago, we designed and constructed a platform that provides good vibration isolation, level control, and height control for a high-resolution superconducting magnet. This platform was installed under the magnet of a 200 MHz (‘H frequency) spectrometer, as part of our efforts to develop ultrahigh resolution (I-12). Recently, we have observed that a perfectly vertical magnet significantly improves the resolution for a spinning sample in a 500 MHz spectrometer (13). Also, we have received several inquiries from users of 500 MHz spectrometers who are encountering problems caused by mechanical instabilities. Therefore, we have decided to share the design of our platform. It requires a constant source of compressed air, at a pressure dependent on the load (see below). It provides (i) a high degree of vibration isolation, (ii) the ability to adjust the level ofthe platform, and (iii) an invariant vertical position independent of load. The last feature may be important for maintaining homogeneity of magnets located above concrete floors which contain embedded steel rods. The “rubber tire” vibration isolators provided as extra-cost accessories by some instrument manufacturers obviously yield a gradual change in vertical position as the weight of liquid nitrogen ( 1.76 lb/liter) changes. Figure 1 shows the components of the system and how they are connected together with air hoses. Figure 2 is a top view of the aluminum platform; the dashed lines show the location of the components of Fig. 1, sandwiched between the aluminum platform (which holds the magnet) and two rectangular aluminum slabs on the floor. The platform is 1 inch thick and 40 X 40 inches in size. For a Bruker AM-500 system, it is probably necessary to increase the length to about 50 inches, and to drill holes for attaching an adapter for an automatic sample changer. The floor slabs are $ inch thick and 11 X 44 inches. Figure 3 shows a frontal view, and Fig. 4 gives a cross section of one of the four vibration-isolation components (labeled S in Figs. l-3). Details are presented below. The four items labeled S in Figs. l-3 are 1 S3-0 11 Super-Cushion Air Springs from Goodyear Tire & Rubber Co., Greensburg, Ohio, each designed for a load of 60370 lb. Our magnet weighs about 500 lb when full of liquid nitrogen. The required compressed air pressure for a 125 lb (per device) load is about 35 psi. The pressure gauge shown in Fig. 1 acts as a load monitor; it shows the effect of load change as a result of liquid nitrogen evaporation. The much larger weight of a magnet of a 500 MHz instrument (about 1200 lb fully loaded) will require a pressure of about 80 psi;