Why Germany’s Wendelstein 7-X just broke the fusion record that matters most
When most people think of fusion reactors, they picture a tokamak — the iconic donut-shaped device that confines superheated plasma in a toroidal magnetic field. Tokamaks, including the massive ITER project under construction in France, dominate fusion research funding and attention. But there is a rival design with a compelling advantage that tokamaks cannot match: the ability to run continuously. That design is the stellarator, and in 2025 it achieved the most significant milestone in its history.
Tokamaks vs. Stellarators
Both tokamaks and stellarators confine plasma in a toroidal (donut-shaped) chamber using powerful magnetic fields. The critical difference lies in how they generate those fields. Tokamaks use a combination of external magnetic coils and a large induced electrical current flowing through the plasma itself. This plasma current is essential for confinement, but it creates two problems: plasma instabilities driven by the current, and the inability to sustain the current continuously — tokamaks must operate in pulses. Stellarators, by contrast, use only external coils — but those coils are twisted into enormously complex three-dimensional shapes that generate the same stabilizing magnetic field geometry that a tokamak achieves with its plasma current. The result is a device that requires no plasma current and can, in principle, run indefinitely.
Wendelstein 7-X: The World’s Most Powerful Stellarator
The Wendelstein 7-X (W7-X), operated by the Max Planck Institute for Plasma Physics in Greifswald, Germany, is the world’s largest and most advanced stellarator. Completed in 2015 after more than 1 million assembly hours, it was designed to demonstrate that stellarators can achieve the plasma performance predicted by theory — and therefore qualify as a viable concept for future power plants. The device cost approximately €1.06 billion, with support from the European fusion consortium EUROfusion and a three-laboratory American consortium including Princeton and Oak Ridge National Laboratories.
The Record-Breaking 2025 Campaign
W7-X’s OP2.3 experimental campaign, which concluded on May 22, 2025, achieved a world record in the fusion triple product — the key metric that combines plasma density, temperature, and energy confinement time into a single figure of merit. When this product exceeds a critical threshold (the Lawson criterion), a plasma produces more energy from fusion reactions than the heating power invested, enabling the reaction to sustain itself. W7-X sustained this record triple product value for 43 seconds, surpassing previous long-duration records held by tokamak devices including JET and JT-60U. This is especially remarkable given that W7-X has three times less plasma volume than JET and five times less heating power. The same campaign increased W7-X’s total energy turnover to 1.8 gigajoules — lasting 6 minutes — up from the previous record of 1.3 gigajoules set in 2023. Plasma pressure relative to magnetic pressure reached 3% for the first time across the full plasma volume.
Why Long Pulses Matter
The triple product record at long durations is specifically the metric that matters for power plants. A real fusion power plant must confine high-performance plasma for hours, days, or years — not milliseconds or seconds. Tokamaks hold the record for short plasma bursts (fractions of a second to a few seconds), but they fundamentally cannot sustain this indefinitely. W7-X’s demonstration that it can maintain record performance for 43 seconds — and the scientists’ belief that these conditions could be sustained for weeks or months given that 30 seconds is long enough to see the plasma reach a stable state — is exactly what a power plant needs.
The Continuous Fueling Breakthrough
A key enabler of W7-X’s 2025 records was a new continuously operating pellet injector developed by Oak Ridge National Laboratory specifically for the experiment. Since September 2024, this device has continuously formed a 3-millimeter strand of frozen hydrogen, cutting it into small cylindrical pellets fired into the plasma at speeds of 300–800 meters per second — providing a steady, continuous fuel supply that is essential for long-duration operation.
The Road to a Stellarator Power Plant
W7-X will not produce electricity — it is a research device. But its results provide the data needed to design a power-plant-capable stellarator. The Proxima Fusion startup, which raised approximately $150 million in June 2025, is explicitly targeting a stellarator-based fusion power plant in the 2030s. The long-duration triple product record demonstrates that stellarators are not merely an interesting alternative to tokamaks — they may in fact be the more natural design for the continuous operation that commercial fusion power demands.