On 27-28 July 2026, the ITER organization lowered the sixth vacuum vessel sector module into the tokamak pit, a milestone on the road to first plasma. The ITER vacuum vessel — 840 m³ of toroidal stainless steel designed to hold pressures below 10-6 mbar while withstanding neutron irradiation — is the largest ultra-high vacuum system ever engineered. When completed, it will be the heart of a machine that aims to demonstrate net-positive fusion energy.
ITER matters to vacuum engineers far beyond fusion. Every challenge it faces — enormous pumping, extreme bakeout temperatures, thousands of leak-tight penetrations, and motion components that must work for decades without maintenance — is the same challenge scaled down in every semiconductor tool, particle accelerator and research chamber we build today.
What the Vacuum Vessel Actually Is
The ITER vacuum vessel is a double-walled torus: an inner shell that faces the plasma, an outer shell that provides structural strength, and a space between them filled with shielding water. It operates at roughly 10-6 mbar during plasma operation, is baked at 200 °C to drive out water and hydrocarbons, and must withstand the electromagnetic forces of plasma disruptions measured in tens of MN.
Vacuum Engineering Lessons from ITER
1. Sealing at scale is about planning, not heroics
ITER uses thousands of flanged, metal-sealed and welded connections. Each one is a potential leak path that would be extremely expensive to fix after assembly. The lesson: design every seal for testability — individual leak testing of sub-assemblies before final integration is mandatory, exactly as it should be for smaller systems.
2. Motion inside vacuum must be designed for the machine’s lifetime
Diagnostic ports, remote handling tools and vacuum pumping ducts all include bellows and flexible connections. When the machine is radioactive, nobody walks in to replace a failed bellows. Cycle life and material selection are safety-critical decisions, not cost optimizations.
3. Bakeout changes everything
Heating the entire vessel to 200 °C means every component must tolerate repeated thermal expansion cycles. Welded bellows absorb differential thermal movement between flanges and structures — a role they play in every baked vacuum system, from synchrotrons to semiconductor load locks.
4. Leak rates are specified in the 10-10 regime
For a vessel this large, even small leaks integrate into unacceptable impurity levels in the plasma. ITER components are qualified to helium leak rates that most industrial tools never measure — pushing the whole supply chain to better testing discipline.
What This Means for Your Vacuum System
You may never build a tokamak, but the same principles apply to any UHV system:
- Test every welded bellows and feedthrough individually before integration — Alpha Technology ships every part with a calibrated helium leak test report.
- Choose bellows materials by duty cycle, not price — see the material guide for AM350 vs 316L vs Inconel behavior.
- Design for bakeout and thermal cycling from day one; a design guide level analysis of stroke and spring rate prevents field failures.
- Document everything — traceability is what makes large systems maintainable.
Alpha Technology and Fusion-Grade Components
Alpha Technology manufactures edge welded bellows and vacuum hardware used in research accelerators, synchrotrons and fusion-adjacent programs, with helium leak testing to 1×10-9 mbar·L/s and full material traceability. If your project — research or industrial — needs components that behave under bakeout and high cycle counts, contact us with your specification.
FAQ
How big is the ITER vacuum vessel?
Approximately 840 m³ with an internal surface area of about 1,000 m² — large enough to fill with multiple rooms of air, held at UHV during operation.
Why are bellows used in fusion machines?
They absorb thermal expansion, allow remote-handling tools to move inside the vessel, and connect components that must flex while maintaining leak integrity.
What leak rate does ITER require?
Individual components are typically qualified to helium leak rates in the 10-10 mbar·L/s range before assembly.
Can standard industrial bellows be used in UHV research systems?
Only if they are UHV-cleaned, leak tested and manufactured from vacuum-compatible materials. Alpha Technology’s bellows are built for exactly this service.
Specifying vacuum components for a research or industrial system? Send us your requirements — including bakeout temperature and cycle life — and we’ll propose the right design.