The design and construction of the outer solenoid of the ANU 30T electromagnet

The solenoid described here is the outer coil of a two coil system for generating a 30T magnetic field. Analysis showed that a homogeneous plane helix would be adequately strong. The Bitter principle of constructing a plane helix was chosen for practical reasons but difficulties arose due to loss of strength from the slits in the discs and the fact that circumferential strength had to be developed through friction between discs. The problems were overcome by adopting a design where each turn was made up of 16 discs interwound in a staggered pattern which dispersed the slits and provided adequate friction bonding. The effect of the slits is small reducing the strength to 15/16ths of an ideal continuous plane helix. It is shown that such a Bitter solenoid may fail either by expansion or by unwinding. The relevant factors involved in both modes are discussed and it is shown that unwinding is the most serious mode and cannot be remedied by increasing the number of discs per turn. A system of keying the end turns via torsion cylinders is described and illustrated. The solenoid is in fact a two-start helix with one turn each end four-start. Reasons for these features are described associated with current distribution and ease of insulation checking. The unusual construction technique is described and illustrated.