Wendelstein 7-X on the home stretch
In Greifswald, preparations are underway to put the world’s largest stellarator into operation
After years of crunching numbers, designing plans, manufacturing components and assembling modules, the fusion device is due to enter a new phase in May, bringing scientists another step closer to generating electricity using the same principle as the sun.
It all began in April 2005: slowly but surely, the special gripper slides and rotates a magnetic coil weighing six tonnes onto an unconventionally shaped steel vessel. The only sound to be heard in the assembly hall apart from the steering commands is the whirring of the crane. Under the watchful eyes of their colleagues, the assembly team threads the large coil onto the vessel, millimetre by millimetre, with merely a finger’s breadth of space between the two elements. After three hours the assembly test is complete: “The technology and the tools work, and the personnel is well-trained,” concludes Lutz Wegener, Head of the Assembly Department, brimming with satisfaction.
The solenoid and the vessel were the first components of the Wendelstein 7-X fusion device to come to Greifswald from the different manufacturing plants across Europe. Greifswald, a university town in Western Pomerania, is home to a sub-institute established by the MPI for Plasma Physics (IPP, located in Garching, Bavaria) in 1994 in the course of Forschungsaufbau Ost, a federal programme designed to foster scientific research in eastern Germany. Since then, both facilities have been conducting complementary research and working towards the same goal: to emulate the sun’s method of producing energy here on earth. The aim is to generate electricity by fusing together atomic nuclei in a fusion power plant.
Due to the fact that the fusion fire does not ignite until it reaches a temperature of over 100 million degrees, the fuel – a low-density hydrogen plasma – must not come into contact with material walls. Magnetic fields therefore keep the fuel suspended inside a vacuum chamber to prevent contact. This magnetic cage can take one of two forms: while the Tokamak ASDEX Upgrade in Garching is in operation, the Wendelstein 7-X stellarator is being constructed near the coast of the Baltic Sea in Greifswald.
As yet, tokamaks are still the leading technology in this field due to their simpler construction design. Only one tokamak – such as the ITER international test reactor – is currently thought to be capable of producing energy-supplying plasma. “However,” says project head Thomas Klinger, “we have reason to believe that the stellarator principle will prevail where its competitor shows weakness.” This is because, unlike the tokamaks, which operate in pulses, stellarators are suitable for continuous operation – thanks to the specially constructed magnetic system surrounding them.
Its structure is the result of sophisticated calculations and optimisation efforts by the “Stellarator Theory” Group, which spent ten years searching for a particularly stable and heat-insulating magnetic cage. “The aim of the Wendelstein 7-X is to put the quality of the plasma balance and confinement on par with that of a tokamak for the first time. The experiment is designed to show that stellarators are suitable for use in power plants, too,” says Klinger, Director of the IPP in Greifswald. And with discharges lasting 30 minutes, the stellarator is due to demonstrate its main asset: continuous operation.
The device consists of five virtually identical modules that were pre-assembled in an experiment hall and combined to form a large ring: 70 superconducting coils, threaded onto a steel plasma vessel, are encased by a toroidal hull. In the vacuum created inside the vessel, the solenoids are later cooled down to a superconducting temperature close to absolute zero using liquid helium, so that they use up hardly any energy anymore.
Apart from these large components, the assembly team also installed miles of cooling pipes, conductors and measuring cables as well as numerous observation ports and sensors, while constantly reviewing the measurements of the many thousands of weld seams and checking for any leaks. “In the case of a device as complex as this one, the industrial production and assembly are an experiment in their own right,” explains Lutz Wegener. In fact, the assembly did not take six years, as planned, but nine. “We had initially underestimated the enormity of the task,” admits Klinger: “The superconducting technology together with the demanding geometry of the components meant we were faced with extreme requirements.” Constructing and manufacturing, measuring and calculating – the complex shapes called for methods that the Institute and the industry could only develop once the project was already underway. The project master plan was revised in 2007. “Since then, Wendelstein 7-X is right on target in terms of both time and costs,” says Klinger.
To celebrate the fact that the researchers are preparing to put the device into operation, an official ceremony will be held in Greifswald on 20 May. In addition to fusion researchers from all across Europe, the event will also be attended by Max Planck President Peter Gruss, as well as by dignitaries from the field of politics, including Germany’s Federal Minister of Education and Research, Johanna Wanka. Once the preparations are in progress, all of the technical systems will be tested one by one: the vacuum inside the vessels, the cooling system, the superconducting coils, the magnetic field they create. “If everything works well, we can begin generating plasma in about a year from now,” says Klinger.