Shutting the lid – core of Wendelstein 7-X complete

Last major component of the fusion device in place / thermal-insulation cryostat mounted

The objective of fusion research is to derive energy from fusion of atomic nuclei, just as happens in the sun. To ignite the fusion fire, a future power plant must confine the fuel, a hydrogen plasma, in magnetic fields and heat it to temperatures exceeding 100 million degrees. Wendelstein 7-X, the world’s largest fusion device of the stellarator type, when finished, is intended to investigate the suitability of this configuration for a power plant. Discharges lasting up to 30 minutes are to demonstrate the device’s essential property, its capability for continuous operation. The core of the device comprises 50 large superconducting magnet coils. Their bizarre shapes are owed to sophisticated optimisation computation: They are to produce a highly stable and thermally insulating magnetic cage for the plasma.

The circular device is assembled from five structurally almost identical modules. Each module comprises a section of the plasma vessel, its thermal insulation, ten of the superconducting stellarator coils and four planar coils and the connections linking them, the piping for cooling the coils, and a section of the support ring, making for a total weight of about 120 tonnes per module.

All five modules are meanwhile finished and, enclosed in a steel outer shell 16 metres in diameter, are in place at their final position on the machine’s foundation. All that is missing to complete the ring is the “lid” on the last module, the final section of the thermally insulating outer shell. The component, weighing about 14 tonnes, is scheduled for installation on 21 December 2011. “As we have already done this four times, the 70-millimetre manoeuvring gap on either side within which the crane has to place the large component meanwhile strikes us as extremely generous”, states installation head Dr. Lutz Wegener. Once this is done, the device will then appear in its final form, viz. a steel ring with numerous ports projecting from it. “It’s just a pity”, states Dr. Hans-Stephan Bosch, Associate Director in the Wendelstein 7-X project, “that there will then be nothing more to be seen of the machine’s interior, particularly of its hallmark, the coils.”

There is plenty else to be done: Already installed are four-fifths of the ports linking the apertures in the plasma vessel through the cold coil region into the outer vessel – about 45 of them per module. These five major components have yet to be connected up: The brazing areas of the support ring, plasma vessel and outer vessel have to be closed, and the magnets be connected with the power and helium supplies. Then come the main power lines, cooling pipes, interior facilities in the plasma vessel and repeated control measurements and leakage tests: The base machine will then be ready. In parallel, the systems for heating the plasma will be incorporated. Then there are the supply facilities for electric power and cooling, the machine control and, lastly, the numerous measuring instruments for diagnosing the plasma’s behaviour. After several years of strict adherence to its time schedule and budget, Wendelstein 7-X should be finished in 2014.

Isabella Milch