The last open seam on the steel outer cover of the Wendelstein 7-X fusion device was brazed shut in May. The core of the stellarator – its basic skeleton – is thus ready and can go into operation in Greifswald, Germany in 2014.

When completed, Wendelstein 7-X will be the world’s largest fusion device of the stellarator type. It is a ring-shaped device being installed as five almost structurally identical modules: Each of the five sections of the plasma vessel, along which 14 magnet coils are strung, is enclosed by a steel outer sheath, weighing altogether 120 tons. Assembled like slices of cake on the machine’s foundation, the five modules form a steel ring from which protrude numerous connection ports – inlets for measurement systems, heating facilities and pumps.

The 254th and last port was brazed in between the plasma vessel and outer vessel with millimetre precision on 28 May 2013. The elaborate port installation lasted two years. This was preceded by an equally long test phase – “a huge training session” as installation head Dr. Lutz Wegener put it – during which the methods for exact placement and connection of the variously configured ports to the bizarrely shaped plasma vessel were developed. One of the many challenges: As stainless steel inevitably shrinks at the seam when it is brazed, the components are distorted and change position. Yet, all instruments fed through the ports must act at precisely defined spots inside the plasma.

Before installation of Wendelstein 7-X is completed in 2014, there are still a few tasks to be done, such as linking the magnets to their power and helium supplies and doing the interior fittings of the plasma vessel. This will be accompanied by provision of the systems for heating the plasma, the supply facilities for electric power and cooling, machine control and finally the numerous measuring instruments for diagnosing the behaviour of the plasma.

Isabella Milch, IPP