A stellarator is a device for thermonuclear fusion research that uses magnetic coils of various shapes to create a magnetic field that confines plasma. It looks like a large crooked metal donut with plenty of crooked rings on it. Its bizarre geometry is the result of a decade of supercomputer calculations. A precisely computed magnetic field enables it to confine 100 million Kelvin plasma and maintain it in a steady-state regime. The largest stellarator device in the world, Wendelstein 7-X, weighs about 750 tonnes, has an outer diameter of 16 m, a height of about 4.5 m, and a nearly circular cross section with a diameter of 4.5 m.

The vacuum vessel and superconducting magnetic coils are enclosed in a toroidal steel container called a cryostat. It is a large thermos with ultrahigh vacuum inside. The vacuum helps to minimize thermal transfer from the plasma inside the vacuum vessel (about 100 million Kelvin hot) to the superconducting coils placed very close to the vessel wall (4 Kelvin cold). Heat transfer is also minimized by thermal insulation. Up to 300 ports protrude through the vacuum vessel and cryostat.

The vacuum vessel has the same shape as a crooked donut and the plasma it surrounds. It is made from high-grade steel with water cooled graphite tiles on the inner wall. These tiles must withstand a high thermal load up to 10 MW/m². An ultrahigh vacuum around 10^-⁸ millibar is maintained inside the vessel. There are numerous ports for measuring, heating and maintenance in the wall of the vacuum vessel.

The stellarator is equipped with 70 superconducting coils of 7 different types. 50 non-planar coils of 5 different shapes closely encircle the vacuum vessel. Their bizarre shape is a result of a decade of supercomputer calculations. Their purpose is to create a helically twisted magnetic field capable of effectively confining hot plasma. As a magnetic cage is created by coils only, the stellarator can in principle work in a steady-state regime. In the largest to-date stellarator Wendelstein 7-X, pulses up to 30 minutes long are expected. Non-planar coils have a height of about 3.5 metres and weigh 6 tonnes. They are made from a niobium-titanium alloy embedded as thin strands in copper wires braided to form a cable. Liquid helium for cooling to 4 Kelvin flows between the individual wires through the cable cavities. The mechanical strength is provided by fibre-glass bands and synthetic resin. They are able to produce a magnetic field of 2.5 Tesla and by changing the direction and intensity of current flowing through the coils, operators are able to create over seven different types of magnetic configurations.

The stellarator is equipped with 70 superconducting coils of 7 different types. 20 planar coils create additional magnetic field that allows the modification of plasma confinement parameters and thus increase the experimental flexibility. In addition, it will allow the plasma to shift farther inward or outward to position it properly with respect to the plasma-facing components. The planar coils are nearly circular with a diameter of about 4 m and a weight of about 3 tonnes. They are made from a niobium-titanium alloy embedded as thin strands in copper wires braided to form a cable. Liquid helium for cooling to 4 Kelvin flows between the individual wires through the cable cavities. The mechanical strength is provided by fibre-glass bands and synthetic resin.

The divertor covers the areas of the vacuum vessel in which particles from the edge of the plasma are magnetically directed. Divertor plates run in ten double strips along the vacuum vessel walls following the curved contour of the plasma edge. Plates cover an area of 19 m² and are made of 18 000 carbon tiles mounted on water-cooled plates made of a copper-chromium-zirconium alloy. The heat load to which they will be subjected will be enormous, comparable to heat stress experienced by a space shuttle re-entering the Earth’s atmosphere. The surface of each tile had to be milled three-dimensionally into shape – with tolerances of sometimes only 0.1 millimetre to avoid any overheating of protruding edges. In the gaps between the plates, powerful cryogenic pumps are installed to remove impinging particles and impurities from the plasma. In this way, the divertor can be used to control the purity and density of the plasma.

A twisted ring of highly ionized gas reaches a temperature up to 100 million Kelvin and has a volume of 30 m³ . In a stellarator discharge, first the modular magnetic field is switched on and it creates a magnetic cage. Then gas (hydrogen or helium) is blown into the vacuum vessel and heated by high-frequency electromagnetic waves that pass energy to the electrons. The accelerating electrons transfer their energy to the ions through collisions thus ionizing the plasma completely. The plasma can be confined as long as heating is available. So, in principle, the stellarator could operate in a steady-state regime.

Plasma can be heated by electron cyclotron resonance heating, ion cyclotron resonance heating or by neutral beam injection. The first two systems pass energy of microwaves to electrons or ions, respectively. Neutral beam injection is in principle a particle accelerator that injects neutral particles with high velocities into plasma. The particles then transmit their energy to plasma particles via collisions.

Openings connect the plasma vessel to the outside world and serve for diagnostics, maintenance, and heating of the plasma. They are leak-tight and protected from plasma heat by water-cooled liners. There are up to 300 ports on the Wendelstein 7-X stellarator.

To control the plasma form, it is necessary that all the 20 planar and 50 non-planar coils should be positioned within a tolerance of 1.5 mm. To meet this requirement, a complex coil support structure was created consisting of the central support ring and the different inner coil supports.

Pipes leading liquid helium toward superconducting coils ensure their cooling to 4 Kelvin.