Radial Access Port Magnets

Cold Bore

Radial access port magnets are made with two coil sections separated by a midplane spacer. Radial access to the field is usually provided through four holes drilled radially through the spacer. Occasionally, the radial holes are required to be square or rectangular. In neutron scattering experiments, the radial holes can take the form of sectors of a circle or "pie slice" shapes. All these latter forms of radial access are more difficult to construct than a simple circular hole. When the dimensions of the radial ports become large, the bobbin may not be capable of supporting the forces between the two coil sections and the epoxy in the winding may fracture.

The size of the radial ports in the magnet should be carefully considered and made as small as possible consistent with the application of the magnet. As the gap is increased, the separate sections of the magnet must be enlarged in order to generate the same field at the center of the magnet and consequently the difficulty of construction and the cost increase rapidly.

The achievable field in magnets of this type is lower than that of comparably sized solenoids wound with the same material since the field at the windings is considerably higher than that at the center of the magnets. Typically, a radial access port magnet having a central field of 7 Tesla will have a peak field on the windings of 8-8.5 Tesla. In addition, high homogeneities are more difficult to attain in these magnets than in solenoids.

These magnets are usually fabricated in the form of Type CH (Cold bore, Horizontal access), but can be fabricated as Type CV (Cold bore, Vertical access) if necessary. Type CH magnets can be mounted with the magnetic axis either horizontal or vertical.

Many of these magnets are used for measuring the short sample critical currents in superconductors in transverse fields, in transverse field vibrating sample magnetometers, in some Mössbauer Effect experiments, and in some laser optical experiments in which the beam is not absorbed by the liquid helium.

Vacuum Bore

The considerations relating to the radial access and homogeneity discussed for cold bore magnets also apply to vacuum bore magnets.

Many experiments require that the radial and axial bore of the magnet be evacuated to avoid absorption of optical or nuclear beams. These magnets are made by assembling the coil sections on a helium leak-tight vacuum bobbin that is sized to fit a specific dewar design. Such magnets are supplied only as a part of an integrated system, including the dewar, since the vacuum bobbin becomes an integral part of the helium reservoir. Type VH (Vacuum bore, Horizontal access) magnets are most frequently requested, although Type VV (Vacuum bore, Vertical access) can be supplied.

Vacuum bore magnet systems are most often used in low temperature optical experiments; similar systems are used for neutron beam experiments.