The Karlsruhe Institute of Technology (KIT) is in charge of the development, construction and testing of two prototypes and 14 series current leads for the stellarator Wendelstein 7-X (W 7-X) which is under construction at the Max-Planck-Institut für Plasmaphysik (IPP) Greifswald, Germany. Tests of the prototypes show excellent results for current carrying capability, losses and stability and thus fully confirm the design and technical construction.

The W 7-X current leads connect the power supply, which is at room temperature, to the 4.5 Kelvin cold superconducting magnet coils. They carry a nominal current of 14 kiloamperes (kA) and a maximum current of 18.2 kA. Having successfully realised the 68 kA ITER HTS current lead demonstrator in collaboration with CRPP (Switzerland) in 2003, KIT designed the 2.5 metres long current leads for W 7-X in a similar way (see image). A clamp contact forms the connection to the superconducting bus bar. Since the high currents would produce too much heat if standard current leads were used, the lower temperature part of the lead is made of a High Temperature Superconductor (HTS) which covers temperatures between 4.5 and 60 Kelvin and which is cooled by heat conduction from the 4.5 Kelvin end. At the 60 Kelvin end, the HTS module is connected to a copper heat exchanger that is cooled with 50 Kelvin helium. This copper heat exchanger was optimised to cover the temperature gradient from 60 Kelvin to room temperature and to allow optimum cooling in the unusual upside down operating condition of W7-X. In this configuration, the warm end of the current lead is at the bottom, which causes the helium at the cold end to sink and the warmer helium to rise, thus counteracting the cooling efforts. The heat exchanger is designed in a way that it prevents this free convection.

For the test, the two prototypes were connected via a short circuit busbar built by IPP. In the TOSKA facility of KIT, this ensemble was installed in a test cryostat with the room temperature end sticking out at the bottom. After cool down, the test campaign started on June 10th with measurements at different current levels. The current lead was operated in a very stable manner in steady state conditions up to 20 kA. Both the measured 4.5 Kelvin heat load and 50 Kelvin helium mass flow rate for cooling the heat exchanger were as expected. The measurements show that no greater cooling is required due to the upsidedown orientation of the current leads. The quench temperature, the temperature at which the HTS material loses its superconductivity, was measured to be 85-90 Kelvin, i.e., more than 25 Kelvin above the operating temperature. A simulated loss of helium cooling flow relased by accident into the heat exchanger shows that a maximumcurrent of 18.2 kA could be sustained for more than 18 minutes. This demonstrates the excellent safety performance of the HTS current lead. The test campaign ended on June 30th, followed by the warm up of the test cryostat. In subsequent high Forschungszentrum Jülich combines energy and climate research The matter of fusion research at Forschungszentrum Jülich (FZJ) is embedded in a new organisational structure: On October 1st 2010 FZJ’s Plasma Physics department became part of the newly founded “Institute of Energy and Climate Research” (IEK). Here, scientists from the former Institute of Energy Research and the Institute of Chemistry and Dynamics of the Geosphere, more than 600 people in total, will set to work on the common goal of realising a sustainable supply of energy for the future. With this step FZJ shows that they consider energy and climate research to be closely linked and that we need to use an integrated approach. In addition to fusion, IEK research topics comprise fuel cells, photovoltaic technology, fission reactor safety, material studies and climate-relevant processes in the geosphere. Ralph P. Schorn, FZJ voltage tests, the electrical insulation of the current leads withstood 13 kV DC, even under pressures as would be reached during a cryostat leakage (socalled Paschen condition).

After this successful test, KIT will set up production for the 14 series current leads for W 7-X.

Reinhard Heller, KIT