What is a vacuum vessel in a fusion machine?
A vacuum vessel is the sealed steel chamber at the center of a fusion machine such as a tokamak. Air is pumped out so that the hot, charged gas called plasma can exist inside it. The fusion reactions take place there.
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What a vacuum vessel does
A fusion machine heats hydrogen fuel until it becomes plasma, a hot and electrically charged gas. The plasma needs an airless space to exist in. The vacuum vessel provides it.
Commonwealth Fusion Systems is building a tokamak called SPARC. It compares the inside of the vessel to a pocket of outer space. The company says even a wisp of air would put out the fusion reaction.
The vessel does not hold the plasma in place. Magnets around it do that. They keep the plasma away from the walls.
ITER lists several jobs for its own vessel:
- providing a high vacuum for the plasma
- acting as a first safety barrier and confining radioactivity
- improving radiation shielding and plasma stability
- supporting parts mounted inside it, such as the blanket and the divertor
How ITER's vessel is built
ITER is the international tokamak being assembled in southern France. Its vessel has double steel walls. Cooling water flows between them to remove heat. Forty-four openings, called ports, give access for heating, fueling, diagnostics and remote handling.
ITER gives these figures for the vessel:
- 19.4 meters across and 11.4 meters high
- about 5,200 metric tons, before inner parts are added
- an interior volume of 1,400 cubic meters
The vessel is made of nine sectors. Europe built five and Korea built four, ITER says. Each sector weighs 440 metric tons. ITER says a larger chamber makes it easier to confine the plasma and produce significant fusion power.
Where things stand in 2026
The ninth and last ITER sector was delivered on October 2, 2026. Delivery is not the end of the work. Each sector is first joined with thermal shield panels and two magnet coils. The result is called a sector module. It is then lowered into the pit where the machine is assembled.
ITER said on July 29, 2026 that six of the nine modules were in place. It expected to place the last by mid-2027. The nine sectors must then be joined into one vessel.
ITER is not the only tokamak receiving its vessel. Commonwealth Fusion Systems said in October 2025 that the first half of SPARC's vessel, a 48-ton steel piece, had arrived in Devens, Massachusetts. It said in May 2026 that the second half had arrived. The halves still had to be welded together.
Sources
- Vacuum Vessel, ITER Organization
- Half of SPARC’s heart, a 48-ton steel vessel, arrives at CFS, Commonwealth Fusion Systems
- CFS begins preparing a home for our 100 million degree fusion plasma, Commonwealth Fusion Systems
- In a Few Lines, ITER Organization
- Two-thirds of ITER tokamak core now in place, ITER Organization
- A remarkable chapter draws to a close, ITER Organization
