LIGHTWEIGHT, DISTORTION-FREE ACCESS TO INTERIORS OF STRONG MAGNETIC-FIELD SOURCES

A permanent magnet structure has been designed to provide field distor tion-free access to large magnetic fields in cylindrical and spherical cavities without the unacceptably large mass increases usually incurr ed to provide it. For example, a spherical shell that produces a 1.2-T field in a spherical cavity of 2-cm radius has a mass of about 2.3 kg if material of 12.2-kOe coercivity and 12.2-kG remanence is used. If the sphere is compensated for distortion-free equatorial access, its m ass must rise to 23 kg to maintain the same field in the cavity. If th e geometry is altered by the new technique to maintain field integrity under access, the mass is only 1.7 kg. Some applications are discusse d that afford, up to this time, unviable devices.