WISER Stellarator launched to study reactor-relevant plasma conditions

The modular coil geometry visualized here is what WISER’s similarity-scaling principles must replicate at reduced scale to remain predictive of full-reactor behaviour

(Image courtesy of E. Sánchez, CIEMAT)

Stellarator development faces a problem tokamaks have not had to solve in the same way: no stellarator has ever operated at the combined field strength, scale, and plasma conditions a fusion reactor demands, and EUROfusion researchers say at least one intermediate device is required between today’s Wendelstein 7-X and a first-of-a-kind stellarator reactor to close that gap. Spain’s answer is Project WISER, a roughly €500 million stellarator programme launched by CIEMAT, CDTI, and Técnicas Reunidas on April 30, designed to test reactor-relevant plasma physics at reduced scale before anyone commits to building full size.

What WISER is actually built to prove

WISER stands for WInd-tunnel for a StEllarator Reactor, and the wind tunnel framing is literal rather than marketing language. CIEMAT’s National Fusion Laboratory developed two results that made the project possible: optimized stellarator configurations that reach the confinement quality a fusion reactor requires, and a rigorous set of geometric and dynamic similarity principles that define the parameters of a smaller device capable of accurately predicting full-scale reactor behaviour. The device, internally designated WISER-5 in EUROfusion programme documentation, is specified at a major radius of 5.0 metres and a magnetic field of 1.9 tesla, built around a quasi-isodynamic confinement concept and designed as a non-nuclear facility.

That scale matters because it sits close to Wendelstein 7-X’s own dimensions, the device Europe currently relies on as its main stellarator research platform. WISER is not attempting to leapfrog W7-X in size. It is attempting to operate at the specific combination of plasma conditions, particularly collisionality and beta, that EUROfusion researchers identify as the most demanding simultaneous test of any reactor-candidate magnetic configuration, and one no stellarator built to date has attempted.

Why an intermediate step exists at all

EUROfusion’s own stellarator work package, WP STEL, frames WISER as one of only two intermediate projects its beneficiaries are currently pursuing on the path to a stellarator first-of-a-kind reactor, alongside Wendelstein-Alpha, a separate device being developed jointly by Proxima Fusion and IPP Greifswald in Germany. Programme documentation lists three specific physics gaps an intermediate device needs to close before a FoAK stellarator becomes credible: alpha particle confinement and fast-ion physics under reactor-relevant parameters, plasma operation at high magnetic fields, and all-metal plasma-facing components under high particle and heat fluxes.

WISER’s planned start of operation is listed as 2036 or later in the same EUROfusion roadmap material, which places it on a longer timeline than the headline announcement alone would suggest. The similarity-scaling approach is the project’s central commercial argument. By predicting full-scale reactor behaviour from a smaller device, CIEMAT and its partners are aiming to reduce the financial, scientific, and technological risk of committing to a full-size stellarator reactor before the physics is proven at reactor-relevant conditions.

Where WISER sits in Spain’s fusion infrastructure

WISER will be the fourth fusion device built by CIEMAT’s National Fusion Laboratory, following TJ-I, TJ-IU, and TJ-II, the flexible Heliac stellarator that has operated at the laboratory since 1997 and that has already played a role in Europe’s broader stellarator physics research. The project also benefits from its proximity to IFMIF-DONES, the international neutron irradiation facility under construction in Escúzar, Granada, where the main accelerator building began construction in 2026. IFMIF-DONES tests materials for fusion reactor conditions rather than plasma confinement itself, but the two facilities sitting within the same country gives Spain a combined materials testing and reactor-scale physics capability that few national programmes currently have in one place.

WISER’s results will not be known for years, but the project gives Europe a second funded route, alongside Wendelstein-Alpha, toward resolving the physics questions that currently stand between today’s stellarators and a working reactor design.

Stay ahead in the fusion revolution explore more breakthroughs from leading innovators in clean energy technology.