ITER-like Wall Project

One of the main challenges for fusion reactors is the compatibility between a reactor-grade plasma and the materials facing the plasma (the "First Wall"). Most current tokamaks (including JET) use carbon composite (CFC) tiles for the First Wall, as does the Space Shuttle, which use it on the wings to withstand extreme heat fluxes. However, from JET's D-T experiments it is obvious that carbon composites are not suitable for the tritium operation due to high carbon migration, leading to tritium deposition in walls. Therefore the ITER design comprises a beryllium-clad First Wall in the main chamber, while use of carbon tiles is limited to the region where the edge plasma is deflected on to the wall ("divertor strike points") and tungsten tiles are to be used elsewhere on the divertor (see areas marked Be, C and W in Fig. 1). Tungsten is very resistant to high temperatures (melting only at 3695 degrees Celsius) but it is a heavy element (proton number 74) that can pollute plasmas considerably: it gets highly ionised in extreme plasma temperatures which causes immense energy losses due to plasma radiation, and dilutes the D-T fuel. Beryllium is a light element with a proton number just 4. However it melts at just 1284 degree Celsius. The combination of beryllium and tungsten has never been tested in a tokamak, let alone in one with ITER-relevant geometry and plasma parameters like JET.

During the one year installation period in 2008, extensive use of Remote Handling technology will be made in implementing the beryllium first wall and tungsten divertor. Following installation, the JET experimental programme will focus on optimising operating scenarios compatible with the ITER-like wall. The level of retained tritium and its dependence on plasma parameters will be determined. Plasma performance will be tested to show that the level of tungsten reaching the core is acceptably low. The lifetime of the wall will be studied with ITER-relevant power loading provided by increased heating due to Neutral Beam Enhancement Project. Also notice the synergy in the pan-European fusion research: while ASDEX Upgrade tokamak (Association Euratom-IPP Garching, Germany) is exploring the viability of an all-Tungsten first wall (tungsten is considered the long-term front runner as a material for fusion reactors), JET will be looking at more immediate ITER needs.