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”). Until 2011 JET used carbon composite (CFC) tiles for the First Wall.
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. 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.
The JET experimental programme focuses 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 the German Association Euratom-IPP Garching with ASDEX Upgrade tokamak 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.