At TEXTOR 94 the installation of a Dynamic Ergodic Divertor (DED) is foreseen to investigate the influence of a perturbation field at the high field side of the tokamak. The preliminary design has been described previously [1]. The DED can be operated with a rotating field at frequencies from 50 Hz to 10 kHz or a stationary field (DC operation). In the preliminary design the thickness of the graphite tiles (target plates) was not finally defined. Thicker tiles can sustain higher heat loads but at high frequency operation of DED the perturbation field is weakened due to the skin effect in the graphite tiles. Between the back side of the target plates and the field coils an optional thermal insulation was foreseen.

To find operational limits of DED due to overheating, the temperature of the target plates and the energy flux from the back side of the target plates to the coil system is calculated for the highest heat fluxes that can be expected. In the case of a rotating field, the heat is distributed poloidally almost uniformly and a one dimensional heat diffusion calculation is appropriate. From the cooling time between discharges an 'equilibrium state' is found showing the heating and cooling of the target plates for longer sequences of equal discharges. These calculations have been performed for the different options in the design: with and without thermal insulation and different tile thickness. From this tabulated results the possible operation sequences of DED can be compared for the different design options. The best thermal performance is reached with thermal insulation and the tiles as thick as possible: Discharges up to 10s at full power can be performed every 5 minutes. However, the maximum temperature of the graphite exceeds 900 C. This has to bee taken into account for the insulation material. Without thermal insulation full power discharges over 10 s lead to problems due to the radiation from the back side of the tiles to the water cooled coils.

At the DC operation of DED, the field causes 'hot stripes' on the DED divertor target plates due to increased heat fluxes compared to the rotating scenario. With a two dimensional model the maximum temperatures on both sides of the tiles are calculated. With a maximum temperature of about 900 C full power discharges can be performed for 7 s with cooling times of at least 15 minutes between the discharges.

[1] B. Giesen, proc. of 16th IEEE/NPSS Symposium on Fusion Engineering SOFE'95, (1995), Vol. 1, 712-715