Utilizing explosive forming to manufacture a resistive vacuum torus for the S-1 Spheromak flux core

The key component of the S-1 Speromak is the flux core, a torus of 2 meters major diameter and 38.8 centimeters minor diameter centrally located in the S-1 vacuum vessel. The flux core contains capacitor bank-powered poloidal field and toroidal field windings and a generator-powered equilibrium field winding, and is enclosed in a resistive torus to maintain high vacuum purity while permitting the magnetic fields to penetrate. The original torus was spun from 2.2 mm Inconel 601 and then chemically milled to reduce the wall thickness to nominally 0.25 mm everywhere but at the weld closure regions where the full thickness remained. Large variations in grain size and torus wall thickness due to the creation of ''spinning grooves'' made chemical milling difficult. Thus, numerous flaws induced in the torus wall during chemical milling and large grain size resulted in unacceptably short fatigue life. To mitigate problems created by the manufacturing method, an alternative method was sought to produce a more uniform and metallurgically optimum torus. The method selected was explosion forming. The factors that led to that choice and the details of manufacture are described in this paper