Tokamak Vacuum Vessel
A steel donut the size of a house, holding nothing but a whisper of gas hotter than the Sun.
The vacuum vessel is the primary container of a tokamak fusion reactor. It holds the plasma (the super-hot, electrically charged gas where fusion reactions happen) in a near-perfect vacuum, while magnetic coils wrapped around it prevent the plasma from touching the walls. It also acts as the first safety barrier against radioactive materials.
Inside the double-walled steel shell, engineers pump out almost all the air, creating a vacuum about a million times thinner than sea-level atmosphere. When the plasma is ignited, it floats inside that vacuum, guided by powerful magnetic fields into a donut shape (a torus). The double-wall construction allows cooling water to circulate between the layers, carrying away heat that leaks from the plasma through radiation. Ports in the wall give access for heating systems, diagnostics, and remote-handling robots.
In plain terms
Imagine a giant hollow donut made of thick steel — but instead of jam inside, there's a wisp of hydrogen gas ten times hotter than the center of the Sun. The walls never touch the gas; invisible magnetic fields act like a floating forcefield keeping the plasma suspended in the middle.
Also called: donut chamber, torus vessel, confinement vessel.
Why it matters
- Provides the near-perfect vacuum necessary for plasma to survive without hitting atoms of residual gas
- Acts as the first safety containment layer, stopping any radioactive tritium or activated particles from escaping
- Houses hundreds of diagnostic and heating ports that scientists use to monitor and control the plasma
- Its structural integrity defines the shape of the magnetic field and therefore the quality of plasma confinement
Where it lives
- ITER (International Thermonuclear Experimental Reactor) — 840 m³ plasma volume, Cadarache, France
- JET (Joint European Torus) — 100 m³ plasma volume, Culham, UK (world record 59 MJ fusion energy in 2022)
- EAST (Experimental Advanced Superconducting Tokamak) — held plasma at >100 million °C for 1,056 seconds (2021), ASIPP, China
Key numbers
- ITER outer diameter: 19.4 m (Taller than a six-story building)
- ITER plasma volume: 830 m³ (8× larger than any tokamak operating today)
- ITER vessel mass: ~5,200 tonnes (After installation of shielding blankets)
- Target plasma temperature: 150 million °C (10× hotter than the Sun's core)
Surprising facts
- The ITER vacuum vessel will be 19.4 m in outer diameter, 11.4 m tall, and weigh approximately 5,200 tonnes — yet must be assembled to millimeter tolerances
- The plasma inside weighs only about 0.5 grams (a paperclip's worth of hydrogen), yet carries enough energy to melt the entire steel wall if the magnetic confinement fails
- EAST held stable plasma at temperatures exceeding 70 million °C for 1,056 seconds in December 2021 — longer than any previous tokamak worldwide
- ITER's plasma volume of 830 m³ is more than eight times larger than JT-60SA, the next largest tokamak currently operating
Open questions
- Edge-localized modes (ELMs): Edge-localized modes (ELMs) — sudden bursts of energy that eject plasma from the edge of the confinement — release energy fluxes up to 20 MW/m² in milliseconds, far exceeding steady-state design limits; their origin in magnetohydrodynamic instabilities is well-documented but their exact trigger timing remains unpredictable in individual events
- Plasma disruptions can terminate fusion shots in less: Plasma disruptions can terminate fusion shots in less than 10 milliseconds, dumping the entire plasma energy into the vessel wall; researchers at JET have catalogued thousands of disruption types but cannot yet predict every individual event from real-time sensor data alone
A short history
The tokamak concept (the name is a Russian acronym for 'toroidal chamber with magnetic coils') was invented at the Kurchatov Institute in Moscow in the 1950s by Igor Tamm and Andrei Sakharov. The first operational tokamak, T-1, ran in 1958. The world's most influential early machine, T-3, demonstrated plasma temperatures of 10 million °C in 1968, surprising Western scientists and triggering a global race. JET, the largest tokamak operated in Europe, first plasmaed in 1983 and held the world record for fusion energy output (21.7 MJ in 1997, then broken by JET itself with 59 MJ in 2022). ITER, currently under construction in southern France, will be the first machine designed to produce more fusion energy than it consumes.
Sources
- ITER Organization — Vacuum Vessel page (https://www.iter.org/machine/vacuum-vessel)
- ITER Organization — Facts & Figures (https://www.iter.org/facts-figures)
- ITER Organization — EAST 1,056-second plasma record (https://www.iter.org/node/20687/east-demonstrates-1000-second-steady-state-plasma)
- EUROfusion — Plasma temperature FAQ (https://euro-fusion.org/faq/what-is-the-temperature-generated-in-a-tokamak/)