3D model of the Helically Symmetric eXperiment (HSX) stellarator.

Stellarators are operated at research institutes around the world in a wide range of sizes and magnetic configurations. The largest operating stellarator is Wendelstein 7-X in Germany. Japan’s Large Helical Device (LHD), which completed experimental operation in 2025, was another major large-scale stellarator, while the smaller Helically Symmetric eXperiment (HSX) in the USA is important for research into quasi-helical symmetry.

Wendelstein 7-X

Wendelstein 7-X, the successor to Wendelstein 7-AS, is an experimental stellarator operated by the Max Planck Institute for Plasma Physics in Greifswald, Germany. It achieved its first plasma in December 2015 and is designed to demonstrate reactor-relevant plasma confinement and ultimately plasma discharges lasting up to 30 minutes. In 2025, Wendelstein 7-X achieved a record triple product for long plasma discharges and an energy turnover of 1.8 GJ during a 360-second plasma. Following major upgrades, Wendelstein 7-X was equipped with actively water-cooled plasma-facing components, including a high-heat-flux divertor. These systems allow much longer plasma discharges and are essential for the ultimate goal of demonstrating quasi-steady-state operation for up to 30 minutes.

Wendelstein 7-X stellarator.

Wendelstein 7-X is a modular stellarator with 50 non-planar superconducting NbTi coils that generate its optimised three-dimensional magnetic field. Each coil is about 3.5 metres high and weighs roughly 6 tonnes. The coils are cooled to about 4 K and form a magnetic configuration with five-fold symmetry and a field strength of up to about 3 T.

The magnetic configuration has five-fold symmetry, corresponding to the five similar modules from which the machine is assembled. Each module contains ten non-planar coils, and the 50 coils comprise five different coil shapes. The plasma cross-section changes continuously around the torus rather than remaining circular. The complete Wendelstein 7-X device is about 16 metres in diameter and 5 metres high and has a total mass of approximately 725 tonnes. Its outer cryostat encloses the superconducting magnet system and the complex plasma vessel.

The plasma volume is about 30 m³. Wendelstein 7-X uses electron cyclotron resonance heating (ECRH) as its principal heating system, supplemented by neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH). Its heating systems are designed to provide up to about 14 MW of plasma heating. Recent experiments have focused increasingly on maintaining high plasma performance for long durations rather than simply maximising stored plasma energy.

Large Helical Device

Interior of the Large Helical Device (LHD), showing the stainless-steel first wall.

The Large Helical Device (LHD), operated by Japan’s National Institute for Fusion Science in Toki, was one of the world’s largest stellarators. It operated from 1998 until the completion of its experimental programme in December 2025. LHD achieved an ion temperature of about 120 million kelvin and plasma durations of up to 54 minutes, making it one of the most successful long-pulse stellarator experiments. Its heliotron magnetic configuration was produced by a pair of superconducting helical coils supplemented by three pairs of poloidal field coils.

The resulting magnetic field confined plasma in a twisted three-dimensional configuration. LHD achieved its first plasma on 31 March 1998. It had a major plasma radius of about 3.9 metres and a plasma volume of about 30 m³. Plasma heating was provided by neutral beam injection, ion cyclotron range-of-frequency heating and electron cyclotron resonance heating.

The LHD machine weighed approximately 1,500 tonnes and measured about 13.5 metres in diameter and 9.1 metres in height. Its two massive superconducting helical coils formed the core of a magnetic system capable of fields of up to about 3 T. Because the precise geometry of these coils was critical, the vacuum vessel was assembled only after their installation; it was divided into 140 sections that were subsequently welded together.

Helically Symmetric eXperiment

The Helically Symmetric eXperiment (HSX) is an optimised stellarator operated by the College of Engineering at the University of Wisconsin-Madison, USA. Although much smaller than Wendelstein 7-X or LHD, it is unique because its magnetic field is designed to have quasi-helical symmetry, making it an important experiment for studying optimised stellarator confinement and turbulent transport.

Quasi-helical symmetry does not make HSX geometrically axisymmetric. Instead, the strength of its three-dimensional magnetic field varies in a nearly helically symmetric way around the torus. This reduces certain neoclassical particle losses and allows researchers to test how magnetic-field optimisation improves stellarator confinement. The field is generated by 48 specially shaped coils arranged with four-fold periodicity.

HSX has a plasma volume of about 0.44 m³ and a magnetic field of approximately 1 T. Electron cyclotron resonance heating is used to produce plasmas with temperatures above 10 million degrees. Its experiments focus particularly on turbulent transport, plasma flows and the effects of quasi-helical symmetry on confinement.