A customized furnace from SECO/VACUUM, a SECO/WARWICK Group company, has been used to heat treat the seventh and final module of the world’s largest superconducting magnet ever made. General Atomics finished the five-week heat-treating sequence on the last of the seven modules: six to be stacked together to form the most powerful magnet ever built and one on standby for repairs. In order to convert the 3.7-mile-long (6-km-long) stainless-steel-jacketed coil of niobium-tin conductors into superconductors, each of the 13-foot x 6.5-foot (4-meter x 2-meter) 110-ton solenoid sections was heat treated for five weeks, exceeding 1202°F (650°C) at its peak.
The heat treatment served to alloy the niobium and tin strands together into Nb3Sn, which becomes a superconductor when chilled with liquid helium to 4 Kelvin. No such heat-treating furnaces existed, so General Atomics turned to SECO/VACUUM to build a custom heat-treating furnace large enough to fit these solenoids.
The customized furnace had to perform the following tasks:
- Quality control vacuum and pressure tests of the coil and the furnace itself
- Purge with argon
- Slow but uniform temperature ramp-up with tolerances within +/-10°C throughout the furnace
- Bake off any last residual impurities left from coil fabrication
- Anneal internal stresses introduced at different stages of fabricating the 50 x 50 mm 316L stainless steel jacket
- Reaction heat treatment of a mixture of niobium and tin strands into the superconducting alloy Nb3Sn
- Slow and uniform temperature ramp-down to avoid additional stresses
The superconducting magnet will serve as the heart of ITER, which is an international nuclear fusion research and engineering project.