Hundred Metre Laser Welded Bars for Fusion Magnets
The conductors for a fusion magnet are jacketed in a steel profile that has to run for a kilometre without a transverse joint, which is a manufacturing question long before it is a physics one. Working with the Swiss Plasma Centre at EPFL, part of the EUROfusion consortium, Montanstahl set out to demonstrate that longitudinal laser welding is industrially feasible on such a profile, built from two stainless steel 316L shells welded simultaneously on both sides. Three bars of 100 metres were produced to test the structural integrity of the joint and the repeatability of the process.
A Feasibility Study for a Kilometre Long Profile
The project belongs to the research and development programme led by the Swiss Plasma Centre at EPFL as a member of the EUROfusion consortium, whose work is directed at developing the superconductors for the magnets of the future DEMO Tokamak fusion reactor. The specific objective was to demonstrate the technical and industrial feasibility of longitudinal laser welding on a profile one kilometre long, formed from two stainless steel 316L shells welded simultaneously along both sides. The reasoning behind the requirement is that a superconducting cable is pulled into its jacket over its full length, so any interruption in the profile becomes a discontinuity in the magnet. To validate the approach, Montanstahl welded three profiles of 100 metres each, concentrating on the structural integrity of the weld joint and on whether the process repeats, which is the question that decides whether a laboratory result can become a supply chain for energy and power generation.
Four Problems That Only Appear at This Length
Each difficulty in this project is a familiar one made unfamiliar by scale. The first was hot rolling a very precise U-shape in bars 100 metres long, to be laser fused together afterwards, with the drawing fixing the section within three tenths of a millimetre across a 63 mm height and a 28.8 mm width. The second was handling and transporting bars of that length safely and under control, since a 100 metre section deflects under its own weight and can be damaged simply by being moved. The third was achieving precise alignment of the two shells along the entire length, because a misalignment of a fraction of a millimetre at one point produces a defective seam locally even if everything else is correct. The fourth was guaranteeing a constant, full penetration weld of high quality from beginning to end, which is what underpins both the structural integrity of the jacket and the repeatability the programme needed to prove.
Application Context
Montanstahl started from the shells themselves, producing the bars by precision rolling that delivered the exact geometry and dimensional accuracy required without any need to cold draw the sections, which removes an operation that would have been impractical at this length. The profiles were then welded to the specification of the project, with custom guides holding the two shells in precise alignment as they entered the welding zone, so the position of the joint is controlled continuously rather than set once at the start. In-line ultrasonic testing monitored the weld seam throughout, making quality and repeatability measurable along the full 100 metres instead of inferred from samples, an approach consistent with Montanstahl quality management. The trial proved that stainless steel profiles of this length can be welded reliably, opening the way to industrial applications in fusion technology. Sections like these are developed as special profiles, with details in the technical Q&A.