Wendelstein 7-X progresses

One of the most important industry commissions in building the fusion plant Wendelstein 7-X was completed in May 2010: the production of the fifty superconducting solenoid magnets. The commission for this technologic core part of the experiment currently being developed at the Part-Institute Greifswald of the Max Planck Institute for Plasma Physics (IPP) was the hitherto largest single procurement in the Institute’s history. “All other major components are also completed”, says Professor Dr. Robert Wolf: “The production phase hence is finalised, the assembly of the largescale experiment is in full swing.” In addition to the fifty larger than man-sized, superconducting solenoid magnets and twenty additional coils producing the magnetic cage for the plasma, all other components are completed at the IPP: the massive supporting structure holding the coils in position, the twenty parts of the plasma vessel and its over 250 connecting pieces as well as ultimately the ten individual parts of the outer vessel – steel half-shells of 14 tons each. Once assembled, they will form a circular tube of 16 metres in diameter. In its 4.4 metre wide interior, this heat-insulating cool box later encases the entire coil ring cooled down to low temperature, in the interior of which in turn lies the plasma vessel with the many million degrees hot plasma.

Wendelstein 7-X, after completion the world-wide most modern fusion plant of the stellarator type, is intended to investigate this model’s suitability as a power station, in particular its capability for permanent operation. The goal is a power station obtaining energy from the fusion of atomic nuclei - similar to the sun. For this, one needs to succeed in encasing the fuel, a hydrogen plasma, within magnetic fields and heat it to temperatures of above a hundred million degrees.

“In the assembly process, the single parts of the stellarator plant first are pre-assembled to make five almost identical modules”, Robert Wolf explains: “These are then put together in a circle inside the experimental hall.” In the meantime, work progresses at all five modules at the same time; two are already completed and stand in their final location on the machine foundations.

“Everything began in parallel at two pre-assembly stands”, explains Robert Wolf, “where a special gripper carefully fed one of the six ton heavy solenoid coils over merely millimetre-broad spaces onto one part each of the plasma vessel. Then a second plasma vessel sector could be welded on one at a time and the thermal insulation at the seam could be closed. This super-insulation separates the low-temperature solenoid magnets from their warm environment. After this, six more coils were fed one at a time onto the vessel part, aligned and screwed together with one segment each of the supporting ring. After many control measurements, the first two semi-modules were then ready and both lifted to a third assembly stand.”

While the next two semi-modules were assembled at the now vacated assembly stands, the first two module halves were joined together at the third assembly stand. Now the conductors for the electric interconnection of the coils was attached – “a rather difficult operation”, according to Robert Wolf.

The stiff, metre-long superconductors, which are prepared for fitting at the Forschungszentrum Jülich, are already bent into their right shape. In parallel to this – by now in cramped conditions – the pipe-work for the helium cooling system of the coils was carried out. “Everything had to be tested as regards leaktightness. Then sensors and measuring cables had to be laid, before the first and then the second module could leave the assembly stand in direction hall”, Wolf remembers.

While these assembly steps are now repeated for the remaining modules, the already completed modules are encased in their outer shell at the hall and outfitted with 50 connecting pieces each, which connect the plasma and exterior vessels through the cold coil section. Once all modules are connected to form a ring on the machine foundations, the interior fitting inside the plasma vessel follows. In parallel to this, the systems for heating up the plasma are being built, amongst these the microwave heating, supervised by the Karlsruhe Centre of Technology, the supply installations for electric energy and cooling, the machine control as well as the numerous measuring instruments, which are to diagnose the behaviour of the plasma. Robert Wolf: “If everything continues to go according to plan, Wendelstein 7-X will be completed in some four years and brought on line.”