After Wendelstein 7-X generated a record plasma in February 2023 (lasting 8 minutes with a power output of 1.3 gigajoules), the stellarator at the Max Planck Institute for Plasma Physics (IPP) in Greifswald was shut down as planned. Since then, the Wendelstein-7-X team has extensively maintained the machine and added new modules.

W7-X will now start the new experiment phase OP2.2 on 10 September 2024, having been significantly optimised. Numerous extensions, improvements and repairs were made to Wendelstein 7-X itself, the control and data acquisition systems, the heating systems and the almost 50 different plasma diagnostics. An important goal was to significantly improve the availability and reliability of the systems, for which a systematic failure mode analysis was carried out. In addition, the scientific capabilities of all systems were significantly expanded and important new observation instruments were added.

Two of the most important enhancements:

• A heating element (gyrotron) is now available, which can generate significantly more than 1 megawatt of power into the plasma via microwaves. This Electron Cyclotron Resonance Heating (ECRH) method emits microwaves into the plasma at the exact frequency at which the electrons in the plasma circulate on helical paths around the magnetic field lines. Previously, Wendelstein 7-X had ten such modules, whose output over several minutes of operation was less than 1 megawatt, respectively. A total of 7.5 megawatts was possible over several minutes. The new module can feed up to 1.5 megawatts into the plasma at peak times. Twelve such elements are to be available for future operating phases. This will significantly increase the maximum ECRH output. The ECRH is the most efficient and most important type of heating in long-pulse operation. The new ECRH module was developed by the Karlsruhe Institute of Technology (KIT) and the Munich-based company Thales. It is the most powerful gyrotron in the world. The W7-X team also has the Ion Cyclotron Heating (ICRH) and the Neutral Particle Beam Heating (NBI) at its disposal.

• The new steady-state pellet injector will also be used for the first time. It was built at the Oak Ridge National Laboratory, a research centre of the US Department of Energy (DOE), especially for Wendelstein 7-X and is a world leader in its category. It is used to ensure the supply of hydrogen particles into the plasma - an important step on the way to a nuclear fusion power plant. The pellet injector produces long rods of frozen hydrogen, from which small pellets are regularly cut off at intervals of fractions of a second in order to shoot them into the plasma at high pressure, as in a blowpipe.

Aims of the new experimental campaign

The extensive scientific programme of the new measurement phases OP2.2 and OP2.3 is primarily concerned with gradually increasing the performance parameters for the generated plasmas. In the last measurement phase OP2.1, it was possible to briefly heat the ions in the plasma to around 35 million degrees Celsius (plasma physicists express this in terms of 3 kiloelectronvolts) and exhaust the heat energy in a controlled manner via the divertor (the most heat-resistant component). In future, this should be possible over periods of several minutes at higher plasma temperatures. "We are gradually approaching higher heating powers," says IPP Director Prof Thomas Klinger. "On the one hand, the aim is to carefully test the heat load limits on the carbon walls of W7-X. On the other hand, we want to understand turbulence-controlled transport processes in the plasma and the exhaust of heat and particles."

Unlike in the last phase of the experiment, the W7-X team is not aiming for new records for the plasma duration, but aim to increase the energy throughput "It would be possible, but of little scientific value, to generate long plasma pulses at low power values now," explains Prof Klinger. "The aim is to achieve long pulses at high plasma temperatures. And that's what we're working on right now."

The current schedule for the next experimental phases of Wendelstein 7-X:

OP2.2: September 2024 - December 2024
OP2.3: February 2025 - May 2025
Maintenance: June 2025 - August 2026
OP2.4: August 2026 - December 2026
OP2.5: February 2027 - May 2027