The Stellarator

Eight Minutes of Plasma

A stellarator holds a very hot hydrogen plasma in a twisted magnetic field made entirely by the coils around it. Wendelstein 7-X (W7-X) in Greifswald, Germany, is the largest stellarator in the world: 16 m across, 725 tonnes, first switched on on 10 December 2015.

On 15 February 2023 it held a plasma for eight minutes (480 s), with 1.3 GJ of energy put in and taken out. On 22 May 2025 it held a high value of the fusion triple product for 43 seconds: for discharges longer than 30 seconds, on a par with the best tokamaks (shared with JET). The same campaign reached 1.8 GJ of energy turnover. It runs on hydrogen, uses no tritium and makes no fusion energy.

480 s · 1.3 GJ (2023) · triple product held 43 s (2025) · 1.8 GJ

One Part in 100,000

A stellarator’s field comes entirely from its coils, so it can be checked before there is any plasma. In 2015 the team sent a beam of electrons along a field line in the evacuated vessel and swept a thin rod coated with a fluorescent powder through the cross-section: the electrons went round the same field line many times, leaving a bright spot each time they crossed the rod, and the spots drew nested magnetic surfaces.

The result, published in 2016: the field differs from the calculated one by less than one part in 100,000; a field line followed for 100 m stays within a millimetre. It needed coils built to tolerances of about ±1 mm. The small error measured came mainly from the planar coils’ placement and shape, and five external trim coils can cancel it.

Error < 1 in 100,000 · 1 mm in 100 m · coil tolerance about ±1 mm

A Twist Made by Coils

Charged particles circle along magnetic field lines. Bend the lines into a plain ring and the field is weaker on the outside: ions drift up, electrons down, the separated charges set up an electric field, and that pushes both out to the wall. The cure is to make each line wind round the cross-section as it goes round the ring: a particle drifts away on the upper half and back on the lower, and on average stays on its magnetic surface.

How many times a line goes round the cross-section for each time round the ring is the rotational transform, ι. A tokamak makes it with a large current in the plasma, driven by a transformer, so it runs in pulses, and current-driven instabilities can end in disruptions. A stellarator makes it with the shape of its coils alone: no large plasma current, no transformer, and it can run continuously. In W7-X’s standard configuration ι rises from about 0.85 at the centre to about 0.98 at the edge.

ι = turns round the cross-section per turn round the ring · about 0.85 → 0.98

From Bean to Triangle

Seen from above, the plasma ring of W7-X is a rounded pentagon: the machine is made of five nearly identical modules, and its field repeats every 72°. The magnetic axis is no flat circle either, but a spiral that rises and falls.

The cross-section changes all the way round: at a corner of the pentagon (φ = 0°) it is an upright bean, half a period on (φ = 36°) a flat triangle, and half a period later a bean again. Its long axis turns half a turn every period, and that, with the twist of the axis, gives the rotational transform.

5 periods · bean (0°) → triangle (36°) → bean (72°)

Fifty Coils, Five Shapes

The field comes from 50 twisted, non-planar superconducting coils, each about 3.5 m high, 2.5 m wide and 6 tonnes. Each module has ten, in pairs of the same shape mounted upside down, so there are only five shapes in all. Each coil has 108 turns of niobium-titanium cable: superconducting filaments in copper wires, braided into a cable inside an aluminium sheath, with liquid helium flowing through the gaps to cool it to about 4 K (−269 °C). In the full-system test of 2015 all coils together reached 12.8 kA.

Twenty planar coils (two types, 36 turns each) sit over them to adjust the field: they add or remove toroidal field and move ι up or down as a whole. Each coil type has its own power supply, seven circuits in all. In operation the field on the axis is 2.5 T; the design allows up to 3 T.

50 non-planar coils · 5 shapes · 20 planar coils · about 4 K · 12.8 kA

A Shape That Was Calculated

Early stellarators confined their field lines but not their particles: many particle orbits drifted out. The field of W7-X was calculated first. A theory group searched the possible configurations for more than ten years for one that met seven conditions at once, among them a field little disturbed by the plasma’s pressure, low heat losses, a very small bootstrap current, well-confined fast particles and modular coils that can be built. The calculation needed the supercomputers of the 1980s; the coils’ odd shapes are its result.

A paper in 2021 tested it. In a high-performance discharge of 2018 these “neoclassical” losses were about 30 % of the heating power; the same plasma in a less optimised stellarator would, by calculation, have lost more than all of its heating power, which is impossible. What remains is mostly lost to turbulence.

Seven conditions · 1980s supercomputers · neoclassical losses about 30 % (2021)

Cut Open: Cold and Vacuum

Cut open, it is shells within shells. Outermost is the 16 m outer vessel, the cryostat, pumped to a vacuum that insulates; the copper-coloured layer is the thermal shield. The coils and the ring-shaped central support that holds them are cooled to superconducting temperature together, a cold mass of 425 tonnes, by a 7,000 W helium refrigerator. The cut coils show their copper-coloured windings.

Inside the coils is the stainless-steel plasma vessel: 20 sections, twisted to follow the plasma, 35 tonnes, pumped to an ultra-high vacuum below 10⁻⁸ mbar. 254 port tubes pass between the cold coils to the outer wall, for diagnostics, fuelling and heating.

16 m across · cold mass 425 t · about 4 K · 254 ports · < 10⁻⁸ mbar

The Island Divertor

Heat and particles always leak to the edge of a plasma. At the edge of W7-X the field forms a chain of magnetic islands by itself: in the standard configuration ι is exactly 1 there, and the cross-section shows five islands (5/5). The target plates of ten divertor units, two per module, cut into those islands; heat and particles follow the islands onto the plates, where they are neutralised and pumped away. Unlike a tokamak, it needs no extra magnets to divert the edge.

In 2015⁠–⁠16 there were five graphite limiters, in 2017⁠–⁠18 an uncooled graphite test divertor. Since 2022 it has had a water-cooled divertor of carbon-fibre-reinforced carbon, designed for 10 MW per square metre, so far shown at 6 MW per square metre. The water pipes that cool the inner wall are 6.8 km long, in 657 separate circuits.

5/5 islands · 10 divertor units · water-cooled · 10 MW/m² (design)

Heated by Microwaves

The plasma is heated by waves and beams sent in from outside, not by an induced current. The main heating is electron cyclotron resonance: gyrotrons make 140 GHz microwaves, about 2 mm long, twice the frequency at which electrons circle in a 2.5 T field. Each gyrotron gives about 1 MW, the newest 1.3 MW, and eleven are installed. They stand in another building; the beams travel about 60 m through the air from one cooled copper mirror to the next and enter the plasma through a port.

Neutral-beam injection fires 55 keV hydrogen atoms into the plasma, up to 7 MW; ion cyclotron heating adds up to 1.5 MW. Fuel comes as pellets of frozen hydrogen: in the record discharge of 2025, about 90 went in over 43 seconds.

140 GHz · about 2 mm · about 60 m by mirrors · neutral beams up to 7 MW

Costs and Limits

It makes no power: it runs on hydrogen, not tritium, and was never meant to. The eight-minute discharge averaged 2.7 MW of heating; the design goal is 10 MW for 30 minutes, 18 GJ. Discharges longer than five minutes have run into overheated components and scorched sealing rings on vacuum valves. The stellarator record for the triple product, 1.126 × 10²⁰ keV·s·m⁻³ in 2024, is about 3.8 % of a power plant’s threshold (3 × 10²¹); the plasma pressure has reached almost 3 % of the magnetic pressure, where a power plant needs 4⁠–⁠5 %.

The investment from 1995 to 2021 was 460 million euros; with the Greifswald institute’s buildings, staff and running costs, 1.44 billion. Assembly took nine years and more than a million working hours. Most of its losses now are to turbulence. Its plasma is 30 cubic metres; ITER’s is 830. New companies, among them Proxima Fusion and Gauss Fusion in Germany, are designing stellarator power plants based on it.

2.7 MW of 10 · 480 s of 30 min · triple product about 3.8 % of the threshold

Taken Apart

The outer vessel with its ports, the 50 non-planar coils, the 20 planar coils, the plasma vessel, the divertor, the plasma and the neutral-beam injector, lifted apart layer by layer.

Rest the pointer on one (tap it on a phone) and it lifts on its own with its name; the rest fade.