Ten years after the contracts for seventy superconducting coils for the stellarator Wendelstein 7-X were signed, the last coil completed the functionality test at CEA Saclay. Professor Thomas Klinger, the scientific director of the project Wendelstein 7-X at IPP Greifswald, recalls the countless technical problems that needed to be overcome during these years of hard work. Now he is pleased that, for the last two years, IPP was able to keep to the mid-2014 target date for the end of construction. More than 15 years ago the 50 coils that form the confining magnetic field were assigned their special shapes as a result of calculations performed to optimise the stellarator design. Doubts had been expressed as to whether these coil shapes could be manufactured at all, but the test results show that optimised stellarators can indeed be built, points out Thomas Klinger.

The hardest test the coils had to pass was the so-called quenching, a process by which a material looses its superconductivity and all its magnetic energy is released at once. For this purpose, the test temperature of 5.7K was raised to 6.2K, forcing a quench. The tests show that the coils regained their superconductivity when they had cooled back down again, thus reaching the specified (electric) current levels. High voltages which may occur during the unlikely event of a fast shutdown are another critical issue. After reworking the insulation of some coils, all of them are now able to withstand the required voltage levels of up to 9k V.

At the inner edge of the stellarator vessel, the coils touch each other as a result of the limited space. This leads to the occurrence of so-called stickslip events in which the tension that builds up between two coils is suddenly released and the coils slip, setting vibration energy free. It was not clear, however, whether this would cause a quench in the superconducting cables that make up the coils. But mechanical testing showed that, even at crash energies which correspond to several simultaneous stick slip events, quenching could not be triggered. Now that all coils have arrived in Greifswald the assembly process is well underway.

Thanks to Thomas Klinger, IPP, for his input

More general information about Wendelstein 7-X and its superconducting coils can be found here: