Axial and Radial Clearbores - American Magnetics

A split coil magnet with both axial and radial clearbores. Section view on right.

Axial ClearBore

A standard magnet will have an ‘axial clearbore’ that travels through the center of the coil windings. This access exists without any special consideration to the winding process since the magnet is wound on an axial bobbin.

Radial Clearbore

Radial access clearbore magnets allow perpendicular access to the magnetic field. These 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 midplane spacer may not be capable of supporting the forces between the two coil sections. Additional forces the windings may cause the epoxy to 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.

Industry Terminology:

ClearBore: The cylindrical access into the magnetic field is referred to as its clearbore. They are oversized to interface with equipment entering it. A sample insert with a 60mm outside diameter would require a corresponding 60mm “clearbore” in the magnet measuring approximately 60.6mm inside diameter. Clearbores are approximately 1% larger than customer equipment outside diameter

Midplane Spacer: A rigid spacer separating two coils in a split coil magnet. The midplane spacer maintains the distance between the coils while magnetic forces pull the two coils towards each other. A midplane spacer allows the addition of a radial access clearbore, but requires the coils to be larger and stronger for any required field.