Development and tests of B4C-covered heat shields for TJ-II

The Neutral Beam Injection System for the Spanish Stellarator will start in 2001. This heating system consists of two beam lines and will inject about 2 MW into the plasma. The ‘groove region’ of the vacuum vessel (Proc. 18th Symp. On Fusion Technol. (1994) 771) will be exposed to thermal loads as high as 5 MW/m2 (Fus. Technol. Vol. 35, (1999)). A heat resistant material must cover the present stainless steel groove protection plates. Plasma-sprayed boron carbide (B4C) seems to fulfil all the requirements, in addition needs little room and therefore does not reduce the size of the plasma. The development of the B4C-covered plates was done in two steps. In the first one, stainless steel plates with simplified geometry were covered with B4C and tested at the JET Neutral Beam Test Bed. To analyse damages after the tests, surface analysis by means of SEM, ESCA and Auger techniques have been done at Ciemat. The second step comprises the plasma spraying of stainless steel plates with the real geometry. In order to achieve a sufficiently good adhesion of the B4C-layer an intermediate layer of NiCr was sprayed. Standard tests of these plates at the manufacturer have successfully been terminated. The heat load cycling remains to be done in the near future.