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Magnet tests on Wendelstein 7-X successfully completed

Important step in the preparation for operation / first plasma in the fusion device one step nearer

July 07, 2015

With the successful completion of the testing of all seventy magnetic coils, the functionality of the key technological components for the Wendelstein 7-X fusion experiment is assured. The superconducting coils, exceeding the height of a man, will create the magnetic field that confines the fusion plasma at a temperature of many million degrees. Currently running in the Max Planck Institute for Plasma Physics in Greifswald are the preparations for operation of Wendelstein 7-X. Later this year, the device should produce its first plasma.

Coil tests: In the control room, the measured data from all test series are brought together and evaluated. Zoom Image
Coil tests: In the control room, the measured data from all test series are brought together and evaluated.

The objective of fusion research is to develop a power source that is friendly to climate and the environment and, similarly to the sun, harvests energy from the fusion of atomic nuclei. To light the fusion fire in a future power station, the fuel – a hydrogen plasma – must be confined in magnetic fields and heated to a temperature of over 100 million degrees. Wendelstein 7-X, which, once complete, will be the largest stellarator-type fusion device in the world, will not produce energy but will investigate the suitability of this type for a power station. With discharges lasting up to 30 minutes, it should demonstrate its significant property, its ability to operate continuously.

A ring of 50 superconducting coils, some 3.5 metres high, is the key part of the device. Cooled with liquid helium to the superconducting temperature, not far from absolute zero, once switched on, they consume very little energy. Their special shapes are the result of refined optimisation calculations: their task is to create a particularly stable, thermally insulating magnetic cage for the plasma. To be able to change the magnetic field, a second set of 20 planar, superconducting coils enclose the stellarator coils.

The preparations for operation on Wendelstein 7-X have been running for a good year. One by one, the operation of each technical system is being tested. From the end of April 2015 it was the turn of the magnetic coils, the core piece of the device. All the coils had already been tested individually during manufacture. Investigated in particular was how the coils react to a so-called quench – the hardest test, that a superconductor must withstand: the coil suddenly loses its superconducting properties and becomes a normal conductor. With the tests, it was confirmed that the coils withstand the high level of stress undamaged and, once cooled again, are superconducting as well as before.

The superconducting magnet system of Wendelstein 7-X: 50 specially shaped stellarator coils (blue) und 20 planar coils (brown) produce the magnetic cage for the plasma. Zoom Image
The superconducting magnet system of Wendelstein 7-X: 50 specially shaped stellarator coils (blue) und 20 planar coils (brown) produce the magnetic cage for the plasma. [less]

During the operational preparations, it was now a question of the behaviour of the magnetic coils when assembled together: the superconducting electromagnets were first tested under current in groups. A start was made with the planar coils of type A, the type B coils followed and then – from May 2015 – the five different types of stellarator coils in turn: Cooled to 4 Kelvin, i.e. minus 269 degrees Celsius, the individual coil circuits were first tried out at a low current of 500 Ampere and the quench detection system was adjusted. The current was then increased in steps to as much as 12.8 kiloampere, depending on the coil type. In pulses lasting up to four hours, the cryogenic system was also tested and the many valves that steer the various cooling circuits were adjusted. The sensitive current leads between warm and supercooled areas were also tested and adjusted. In addition, the coils were subjected to a simulated quench to test whether the automatic detection system was triggered. Also to be recorded and compared with planed values were the changes in shape of the coils and the mechanical stresses in the housings induced by the magnetic forces of the coils. „Everything agrees well with the calculations“, Dr. Hans-Stephan Bosch, department head of “Wendelstein 7-X Operation” was able to state on 17th June.

The successful testing in individual groups was followed – with the same procedure – by the test of the complete set of coils. Now, when the current was switched on, all 70 coils exerted their magnetic forces on one another. This was the first time that all coils together were supplied with current up to the ultimate required value of 12.8 kiloampere. “The complete set of coils has withstood all technical tests”, summed up Dr. Bosch on 6th July the comprehensive series of tests: “This assures the required functionality of the primary components of the system. We can now take up the challenge of the next major step, the measuring out of the magnetic surfaces.” This will test whether the coils produce the plasma cage in the desired form and shape. Later this year, Wendelstein 7-X should produce its first plasma.

Isabella Milch

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