Repair of poloidal field magnets on Alcator C-Mod

Abstract

While bringing Alcator C-MOD on line, failure of a solder joint caused an open circuit in one of the PF coils located within the toroidal field magnet. A design review was conducted to analyze the failure and propose possible solutions. The exact reason for the failure was not determined, but the joint may have been weakened by high temperatures during bakeout of the vacuum vessel. Peeling forces also may have been induced by unforeseen temperature gradients and/or magnetic loads. Significant design changes, which are limited to highly stressed PF coils located within the toroidal field magnet, involved repositioning the joint away from the coaxial termination and eliminating the use of solder as a structural element. PF coils external to the toroidal field magnet are not so highly stressed and brazing is acceptable. The redesign easily accommodates repositioning the joint, but finding a substitute for solder, which was originally selected to avoid annealing the cold worked copper conductor, proved difficult. Localized annealing which occurs in welding and brazing processes eliminated the two most common methods of terminating copper coils. There is not enough space available in the vacuum vessel coil pockets to accommodate mechanical clamping devices. The use of fasteners such as screws and rivets was prohibited due to adverse effects on fatigue life. Electroforming, a process by which complex parts are formed by electroplating materials such as copper onto an electrically conductive mandrel, was selected to replace soldering the joint. Electroformed copper sheet exhibited superior material properties to those of the C-10700 coil conductor, which has yield strength of 290 MPa. Changes, development of an electroformed electromechanical joint, and coil manufacturing will be further described.