Assembly of graphite tiles in the plasma vessel of Wendelstein 7-X.

Since stellarator plasma has the shape of a twisted ribbon with a changing bean-like cross-section, the vacuum vessel surrounding it must follow a similarly complex three-dimensional shape. It can be imagined as a hollow steel doughnut that has been twisted and locally deformed to match the plasma geometry.

Shape

The exact shape of the vacuum vessel depends on the magnetic configuration used by each device. Stellarators with continuous helical windings, such as heliotrons, generally have a more regular vessel geometry than highly optimised modular stellarators, but in both cases the vessel is more complex than a simple torus. The magnetic coils are positioned very close to the vessel, leaving only limited space for access.

Access is provided through numerous ports that pass between the coils and connect the vacuum vessel to external systems. They are used for plasma heating, diagnostics, maintenance, vacuum pumping, gas injection and other services.

Heat Load

The plasma inside a stellarator can reach temperatures of around 100 million kelvin, while magnetic confinement keeps most of the hot plasma away from the vessel wall. Nevertheless, plasma-facing components are exposed to substantial heat loads from particles and radiation and must be protected accordingly. In Wendelstein 7-X, the stainless-steel plasma vessel is fitted with graphite-covered wall protection components and divertor elements, many of which are actively water-cooled for long-pulse operation. Part of the vessel is equipped with divertor target plates, with pumping systems located behind them. The edge magnetic field guides escaping plasma particles, heat and impurities towards the divertor, where particles are neutralised and pumped away.

Cryostat

Segment of the plasma vessel for the Wendelstein 7-X stellarator.

While the plasma can reach around 100 million kelvin, the superconducting magnet coils must be kept at cryogenic temperatures of about 4 K. To minimise heat transfer to the magnets, they are surrounded by thermal insulation and cooled shields, and the space around them is evacuated to suppress heat transfer by gas conduction and convection. In Wendelstein 7-X, thermal shields cooled to around 80 K provide an additional barrier between room-temperature structures and the superconducting coils. This thermal insulation is provided by the cryostat, a large vacuum-insulated enclosure surrounding the superconducting coils. In Wendelstein 7-X, the plasma vessel forms the inner boundary of the cryostat and the outer vessel forms its external wall, with the magnet system located in the evacuated space between them.

Outer vessel (cryostat) of the Wendelstein 7-X stellarator.

Future Shielding

No stellarator has yet operated with deuterium–tritium fusion fuel. A future D–T stellarator power plant would therefore require a lithium-containing breeding blanket and substantial neutron shielding between the plasma and the superconducting magnets. The blanket would absorb much of the neutron energy and breed tritium for the fuel cycle, while the shielding would protect the magnets and other components from neutron damage. The required thickness would depend on the specific reactor design. The breeding blanket and neutron shielding would increase the distance between the plasma and the magnetic coils. Maintaining the required magnetic field at the plasma would therefore require stronger magnets, a larger device, or a combination of both.