Development of an explosion bonded toroidal field coil inner leg for CIT

The high currents and high magnetic fields present in the Compact Ignition Tokamak (CIT) toroidal field (TF) system will necessitate an exceptionally strong conductor for the inner leg of the coils. One solution is to bond UNS 7718 to the copper conductor for reinforcement. Explosion bonding is a method of joining dissimilar metals. The joining of UNS 7718 to copper over large areas is one of the more challenging combinations for explosion bonding. Four vendors were selected to perform a series of 15 shots, starting with small-scale setups to determine bonding parameters, and scaling up to a size comparable to the inner leg of a CIT TF coil. The results of the development program offer encouragements as to the use of an explosion-bonded laminate in the CIT TF coils, pending the resolution of several problems: trailing-end cracking must either be eliminated or shown to fail repeatedly in an area of clad which will be removed during taper grinding; the nose insert caused much trouble with the bonding process; and the additional material needed for bond startup must eventually be machined off, which is not a trivial matter given the physical properties of the 718. Production feasibility is discussed, and the next phase of the development program is described