Author: Volker Philipps and Ralph Schorn

Research Centre Jülich (Association EURATOM /FZJ) – for a long time focussing on plasma-wall interaction and on testing plasma-facing structures – is contributing expertise to the ITER-like wall project at JET. R&D activities will address bulk tungsten components to be implemented into the divertor as load-bearing septum replacement plates.

Two options are being considered: the brazing of tungsten blocks on a Carbon Fiber Composit (CFC) substrate and stacks of tungsten lamellae. Both solutions are very challenging with respect to heat loads, joining techniques and electromagnetic loads. For the development of a brazed option, tests with different materials are carried out and evaluated at different temperatures. The brazing joint has to withstand operating temperatures of up to 1000 °C. Microstructural characterisation of the different CFC grades and of the brazing layers is currently being performed at Jülich.

The lamellae concept consists of stacks of tungsten and insulating spacers, held together by tie rods and being mounted onto a support structure. Differences in thermal expansion of the materials involved and mechanical pre-tension – being required to avoid detachment in case of electromagnetic loads due to disruptions – define tight limits for the design.

Electromagnetic loads are critical for both options. The new design thus has to minimise eddy current loads by segmentation of loops and halo current loads by defining an optimal electric path. One of the first important results was the need for a considerable modification of the present support structure – both in order to reduce the electromagnetic loads and to cope with the loads transferred from the tungsten tiles.

Comprehensive thermal and electromagnetic analysis – applying analytic as well as finite elements methods – is performed in parallel with the design process. In addition, heat load tests are prepared in Jülich at FZJ’s electron beam facility JUDITH and at the neutral beam test bed MARION. Extensive exposure to actual plasmas with relevant loads will take place in the TEXTOR tokamak.