Preparations for operation of Wendelstein 7-X are progressing

Test of superconducting coils / first phase finished

Preparations for operation of the Wendelstein 7-X fusion device at Greifswald are progressing: Following the successful completion of the cooling system checks the magnet coils, representing the core of the device, are now being tested.

A wreath of 50 more than man-sized superconducting coils will later provide the magnetic cage for the hot plasma. Cooled with liquid helium to superconduction temperature close to absolute zero they consume hardly any energy. In order to vary the magnetic field, a second set of 20 planar, likewise superconducting coils is superposed on the stellarator coils.

All ten superconducting electromagnets of one type are supplied in series by one power supply. In the end of April, the tests started with the planar coils of type A and B. Then, one after the other, the five different types of the bizarrely shaped stellarator coils were examined: Cooled to 4 degrees Kelvin, i.e. minus 269 degrees Celsius, the coils circuits were first energized up to a low current of 500 amperes and the trigger levels of the quench detection units were adjusted. In the following, the currents were increased step by step – depending on the coil type – up to a maximum of 12.8 kiloamperes. During these up to four-hour-long pulses the tests focused not only on the coils but also on the cryosystem and the adjustment of numerous valves which let the coolant flow into the different cooling circuits. The sensitive current leads connecting the ambient and cryogenic temperature environments were tested and adjusted as well. Moreover, during the loading of the coils with current, the mechanical behavior of the coils – deformations and the mechanical stress on the coil cases due to the magnetic forces of the coils – was monitored and compared with the designed values.

The final evaluation showed that all coil circuit units passed the tests successfully. Using the same procedure now the entire wreath of coils can be tested. The first plasma in Wendelstein 7-X is expected in 2015.