Researchers at Oak Ridge National Laboratory (ORNL) have engineered a new grade of stainless steel to serve as the structural conduit, or “jacket,” for the niobium-tin (Nb3Sn) superconducting cables in the ITER central solenoid. This material must provide structural integrity against immense electromagnetic forces while enduring cryogenic operating conditions. The central solenoid, a stack of six modules, will stand 18 meters tall and 4.3 meters wide, forming the backbone of the tokamak. Its function is to induce a powerful current within the plasma, a critical step for plasma initiation, shaping, and confinement. The new steel alloy is a key enabling component for the magnet's demanding operational requirements. Source: Oak Ridge (ORNL)

The operational environment of the central solenoid imposes severe constraints on its materials. The magnet will operate at 4 Kelvin (-269 degrees Celsius) to maintain superconductivity, yet must withstand repeated stress cycles from a peak magnetic field of 13 Tesla. These conditions create a high-fatigue, high-stress environment where material brittleness is a primary failure concern. The ORNL-developed steel is designed to exhibit high toughness and fatigue resistance at cryogenic temperatures, preventing fracture initiation and propagation through the 5.7-centimeter-thick jacket. The alloy's development was part of the US ITER project, managed by ORNL for the U.S. Department of Energy Office of Science. Source: Oak Ridge (ORNL)

The operational environment of the central solenoid imposes severe constraints on its materials.

The material's composition and processing were meticulously controlled to achieve the required properties. The steel is produced in 9-ton ingots, which are then forged, rolled, and machined into the final structural components. The production process ensures a uniform microstructure, which is essential for predictable mechanical performance under the extreme Lorentz forces generated during plasma operations. The jacket material encases the superconducting cables, which are heat-treated at high temperatures before being insulated and wound into coils. This multi-stage manufacturing process, involving international collaboration, highlights the complexity of building core components for next-generation tokamak reactors. Source: Oak Ridge (ORNL)

This materials science advance directly supports the operational goals of the ITER experiment, which aims to demonstrate the scientific and technological feasibility of fusion power. The central solenoid is the most powerful of ITER's magnets, providing the primary means of driving and sustaining the plasma current. Its structural reliability is therefore paramount to achieving the project's mission of producing 500 MW of fusion power from 50 MW of heating power, a Q ratio of 10. The successful development of this specialized steel provides a critical pathfinder for materials selection in future fusion power plants, which will require components capable of long-term operation under even more demanding conditions, including high neutron fluence. Source: Oak Ridge (ORNL)