Milestone in fusion construction

15 April 2008

The joining together of the first two modules of a new fusion reactor has begun at the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany. The Wendelstein 7-X stellarator should be complete in 2014.


Wendelstein shell segment 
One of five shell segments for
Wendelstein 7-X (Image: IPP)
When joined, the two modules will together represent one fifth of the reactor's inner core of superconducting magnetic coils. They will then be fitted with a superconducting bus, cryopiping and power connections. Construction work on the next two modules has already started.


When complete, Wendelstein 7-X will allow IPP researchers to study high energy plasma under stable conditions. Wendelstein is a stellarator fusion reactor - different to a Tokamak fusion reactor such as the Joint European Torus in the UK or the Iter device under contruction in France. A tokamak is based on a uniform toroid shape, whereas a stellarator twists that shape in a figure-8. This gets around the problems tokamaks face when magnetic coils confining the plasma are necessarily less dense on the outside of the toroidal ring.


IPP said that the early stages of construction were "hampered by quality problems" centred on the electrical insulation of the superconducting coils and additional engineering required to improve the coil support structure. Now, all 70 superconducting coils for Wendelstein 7-X have now been manufactured, while half have successfully passed cryo- and high voltage tests. The coils have been manufactured by Babcock Noell GmbH in Wuerzburg, Germany and and ASG Superconductors in Genoa, Italy. The support structure to keep the components in place despite huge magnetic fields has been half-completed by Ensa of Spain.


However, a revision in the construction schedule has put completion back to 2014. A consequence of time pressure has been the omission of 45 ports into Wendelstein's interior, and a start-up configuration without actively-cooled in-vessel components. IPP said that although the omission of the ports would reduce experimental flexibility, "none of the scientific objectives of Wendelstein 7-X will be affected."


Wendelstein 7-X is to operate for two years without active cooling, during which time it will be able to operate for about 50 seconds at 1 MWt, or at 8-10 MW for 5-10 seconds. The machine will then undergo an 18-month shutdown when it will be fitted with an actively cooled divertor for heat fluxes of up to 10 MWt per meter squared. This will bring Wendelstein 7-X to its full steady state capacity.