The installation of a Dynamic Ergodic Divertor (DED) is foreseen for the tokamak TEXTOR-94 in order to active influence transport parameters in the plasma edge and to study the resulting effects on effective heat exhaust, edge cooling, impurity screening, plasma confinement and stability. The paper concerns the engineering design of a multi polar helical coil system mounted on the inboard side of the TEXTOR vessel (temperature 250 C) and its power supply system. The coils are energized by a 4-phase current at 50 Hz and 7 selected frequencies in the band from 1 kHz to 10 kHz causing rotation of the perturbation field and possibly also a rotation of the plasma surface. DC operation is also possible by an additional rectifier allowing direct comparison of improvements with respect to the radiation efficiency of impurities and impurity screening with the Ergodic Divertor of TORE SUPRA..

In order to cope with the skin effect and eddy currents, to fulfil the cooling requirements and to make optimal use of the available space, a special conductor has been developed for the 18 invessel coils. Each coil has a length of approximately 10 m, has only one turn and is sheathed vacuum-tight by a corrugated stainless steel tube. The coils are actively cooled. The layout of coils and support structure take into account requirements for conditioning of the outer tube and the vessel at 250 C, as well as thermal expansions (conductor, insulation, tube and vessel) and protections against disruptions (halo currents). All coil terminals are transferred by coaxial feedthroughs to the outside of the vessel allowing for different coil configuration necessary for operation at different modes. By counter serial connection of coils 1 and 3 as well as coils 2 and 4 the 4-phase rotating field can be generated by only two sets of power units (frequency convertors) having a phase shift of 90 degrees. A controlled rectifier supplies the DC intermediate circuit of the frequency convertors and a 4-quadrant bridge with IGBT s produces the AC output voltage of 600 V and the output current of 1.5 kA.

The paper describes the engineering design of the main components. Results of calculations for conductor cooling and high frequency AC-losses are discussed both for different options.