Wendelstein 7-X: Project launched to build the world’s most powerful microwave heating system

IPP, KIT, the University of Stuttgart and Thales are developing 2-megawatt gyrotrons and methods for the reliable transmission of microwave beams of this intensity. The new technology will be used in the Wendelstein 7-X fusion experiment and lays the foundations for future fusion power plants. The German Federal Ministry of Research is funding the project with 6 million euros.

The Wendelstein 7-X fusion experiment in Greifswald aims to demonstrate the suitability of the stellarator principle for power plants. Essentially, researchers at the Max Planck Institute for Plasma Physics (IPP) aim to answer two questions: Can stellarators really operate continuously, as predicted by theory? And can sufficient thermal insulation with adequate particle confinement be achieved in the nuclear fusion plasmas of stellarators?

The latter is the fundamental prerequisite for a future functioning fusion power plant.

In recent years, researchers at the IPP have achieved important milestones along this path: in 2023, they generated a plasma with a pulse duration of 8 minutes and high energy turnover for the first time (meaning that a high heating power was introduced into the plasma and removed again over a period of 8 minutes).

The target pulse length set for the coming years is 30 minutes – with these pulse lengths, the key questions regarding stable operation in the physical equilibrium of the plasma and in the thermal equilibrium of the plasma facing components can be investigated and answered.

The foundations for continuous operation would then be in place. In 2025, Wendelstein 7-X also achieved, over a long pulse duration, an energy confinement time (a measure of the plasma’s thermal insulation) that had previously only been sustained over longer time by the best facilities based on the competing tokamak principle.

Microwaves are the primary heating method at Wendelstein 7-X

Without Electron Cyclotron Resonance Heating (ECRH), these successes would have been inconceivable. Although Wendelstein 7-X has been equipped with two further types of heating (ion cyclotron resonance heating ICRH and neutral beam injection NBI), ECRH remains the most important heating method. It heats the plasma using microwave radiation, achieving ion temperatures of up to 40 million degrees Celsius to date, and maintains such temperatures even over long periods (pulse durations). The central component of an ECRH is the microwave source, the so-called gyrotron (see below: How does ECRH work and why is it so crucial for fusion power plants?).

To achieve the next planned milestones in stellarator research with Wendelstein 7-X and thereby lay the foundations for future fusion power plants, four partners are preparing the next-generation Electron Cyclotron Resonance Heating: In addition to the IPP, these are the Karlsruhe Institute of Technology (KIT), the Institute of Interfacial Process Engineering and Plasma Technology (IGVP) at the University of Stuttgart, and the industrial partner Thales. The German Federal Ministry of Research, Technology and Space (BMFTR) is providing 6 million euros for the project, entitled HiPMiB (High Power Microwave Beams). HiPMiB was launched in January 2026, with the IPP acting as coordinator. The aim of the project is to develop gyrotrons capable of generating world-record heating powers of 2 megawatts at Wendelstein 7-X, thereby serving as the basis for heating systems in future fusion power plants. Such a gyrotron would then deliver the power of around 2,000 domestic microwaves at frequencies that are approximately 50 times higher.

In addition to generating high-power microwaves, the project also focuses on their transmission. Doubling the beam power to 2 megawatts also poses a challenge for the transmission system. Here, the existing Wendelstein 7-X transmission system is to be upgraded from 1 megawatt to 2 megawatts of single-beam power. As a project partner, the IGVP Stuttgart is contributing the experience it has gained in the past during the development of the transmission system for Wendelstein 7-X.

Why 2-megawatt gyrotrons?

Eleven gyrotrons are currently installed at Wendelstein 7-X, with output powers ranging from older prototypes dating from 1999 (approx. 0.6 megawatts) to the current record holder at 1.3 megawatts.

However, this is not sufficient for the facility’s ultimate goal – demonstrating that the optimised stellarator concept can maintain reactor-relevant plasma parameter stably. For physicists, the key factor here is the normalised plasma pressure, which is expressed as the so-called beta value in per cent.

beta is defined as the ratio between the thermal plasma pressure and the magnetic field pressure that must be applied from outside to hold the plasma together. Since the aim is to maintain the highest possible pressure for fusion and the externally generated magnetic field should remain as small as possible for cost reasons, a high beta value is generally desired.

For a power plant-relevant plasma in Wendelstein 7-X, values of 4 to 5 per cent must be achieved in order to confine fast ions. In a power plant, these are fast helium nuclei that are produced as a fusion product in the plasma. “We estimate that we will need a total heating power of up to 30 megawatts to achieve these beta values,” explains HiPMiB project manager Dr. Heinrich Laqua from the IPP. “The ECRH is expected to provide the majority of this, with neutral particle heating contributing a significantly smaller portion.”

As Wendelstein7-X has only 12 gyrotron positions, increasing the power per microwave beam to 2 megawatts is the only economically and technically viable way to approach the 30-megawatt target. “Each additional new gyrotron position would cost around ten million euros, regardless of its power output. We would need a new building, additional tunnels for transmission and a massive expansion of the high-voltage power supply. So we save a significant amount in costs if we instead develop more powerful gyrotrons for the available slots,” says Dr Laqua.

The collaboration between the four partners

The four partners will divide the tasks as follows:

• KIT possesses the greatest expertise in microwave technology in Europe and will therefore initially test and design a high-power gyrotron for short pulses in the laboratory. A key task here will be the cooling of the gyrotrons: the load on the resonator walls, where the microwaves are generated, is extremely high at around 20 megawatts per square metre and pushes the technical limits of water cooling for copper. The project is therefore working on microchannel cooling to improve heat transfer.

• On this basis, Thales will industrially manufacture a demonstrator with an output power of 2 megawatts, suitable for pulse durations of half an hour. The IGVP at the University of Stuttgart and the IPP are focusing on the technology for transmitting the microwaves from the gyrotron into the vacuum vessel of Wendelstein 7-X. In the past, long pulses lasting more than five minutes have caused problems due to overheated components and scorched sealing rings on vacuum valves. The transmission takes place quasi-optically via mirrors.
• To reduce the power densities and thus the stress on the components, the focal lengths of the mirrors are being optimised. This widens the microwave beams. In addition, the researchers aim to improve the cooling of the transmission lines using dry, cold air. Another task is the development of a special diagnostic system for the rapid detection of atmospheric arcing during wave transmission.

“A three-year timeframe is planned for this complex project, which is very ambitious,” says Heinrich Laqua. The aim is ultimately to develop microwave technology that will be indispensable for future fusion power stations. These technologies will enable Wendelstein 7-X to demonstrate the power plant viability of stellarators.

 

Wie How does the ECRH work and why ist it so crucial for fusion power plants?

The Electron Cyclotron Resonance Heating (ECRH) is a method for heating fusion plasmas using high-frequency microwave beams. At the heart of the ECRH is the gyrotron, a special microwave tube that acts as the microwave source. Inside the gyrotron, a ring-shaped electron beam is generated by an electron gun and directed into a so-called resonator, which is situated within a strong magnetic field. There, the electrons move in circular paths around the magnetic field lines (gyration).

 

The strength of the magnetic field in the resonator determines the resonance frequency, which corresponds to the circular frequency of the electrons. The electrons release part of their kinetic energy in the form of microwaves, thereby amplifying the wave in the resonator. This is comparable to the stream of air blown into a flute, which is amplified by resonance to produce an audible tone.

In order for the microwaves to heat the plasma in fusion facilities, their frequency must be tuned to the facility’s magnetic field. The rule of thumb is: 28 gigahertz per tesla of magnetic field strength, although operation at integer multiples of this – so-called higher harmonics or overtones – is also possible. At Wendelstein 7-X (2.5 tesla magnetic field strength), heating is carried out using the second harmonic resonance, i.e. 140 gigahertz. The second harmonic resonance describes the case, where the wave frequency is double the electron cyclotron frequency.

Compared to the other types of heating (neutral particle heating and ion cyclotron heating), electron heating is particularly relevant to power plants. This is because in a fusion power plant with a burning, i.e. self-heating, plasma, fast helium nuclei—which are produced in every fusion reaction—transfer their energy mainly to electrons. These, in turn, heat up the ions.

The ECRH thus creates conditions that are very similar to those in a power plant plasma.

 

Furthermore, there are technical reasons why it is suitable for a fusion power plant: the gyrotrons can be installed outside the reactor building, as microwave transmission is very efficient. The antennas in the plasma vessel can also withstand the harsh reactor conditions.

 

Frank Fleschner