One of the main challenges for ITER, and for fusion reactors, is the compatibility between a reactor-grade plasma and the first wall. The first wall requirements are that it must not contaminate the plasma and degrade performance, while the plasma must neither damage the wall nor shorten its lifetime. Limited use can be made of CFC (carbon fibre reinforced carbon), which is a robust material under high heat loading, as it could lead to high tritium retention, and the ITER tritium inventory needs to be strictly controlled.

The ITER design comprises a beryllium-clad first wall in the main chamber (to suppress carbon migration which codeposits tritium in the divertor region), CFC at the divertor strike points (to handle the highest heat loads), and tungsten for the rest of the divertor (to handle higher heat loads than beryllium can, while minimising carbon sources).

The combination of materials proposed for the ITER first wall (see details in the box) has never been tested in a tokamak, let alone in one with ITER-relevant geometry and plasma parameters. Recently, approval has been given to conduct such a test on JET, with the installation of an ITER-like first wall in 2008.

Two designs of the ITER-like wall are under consideration for JET. The reference design consists of a beryllium wall in the main chamber and an alltungsten divertor. Another option is to use CFC tiles at the divertor strike points, which would make the material choices identical to those in the ITER design. A final choice between these two options will be made in 2006. Preparations for installation of the new wall have been progressing rapidly, following final approval of the project in April 2005.

For tungsten plasma facing components, tungsten-coated CFC tiles could be appropriate. A 3.5 μm coating technology has already been tested in situ, but this may not be sufficient for the full range of experiments envisaged. A research programme is currently underway to test the suitability of coatings of various thicknesses, exposed to power loading in a neutral beam test bed. The outcome will determine whether tungsten-coated CFC is suitable, and what thickness will be required. Should coated CFC not be deemed suitable, the alternative would be to use solid tungsten tiles. Tungsten will also be used for neutral beam shine-through protection tiles. These tiles may also be modified for improved inertial or interpulse cooling, to extend Advanced Tokamak operation to 20 s pulses. During the one year installation period, extensive use of remote handling technology will be made to implement the new first wall and divertor.

The ITER-like wall programme on JET will benefit from expertise gained at ASDEX Upgrade on tungsten coating of carbon tiles. There is also a synergy between the objectives of the two programmes: ASDEX Upgrade is exploring the viability of a tungsten first wall (tungsten is considered the long-term front runner as a material for fusion reactors), while JET will be looking at more immediate ITER needs. Both experiments will confront the challenge of operation with an all-metal wall but tackled from two different directions, low Z (beryllium) and high Z (tungsten), providing highly complementary data.

Following installation in 2008, 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 scenarios will be determined, and detritiation techniques will be  tested. 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 neutral beam heating power. This level of heating power is expected to produce ELMs and disruptions which could cause melt damage to the first wall. The performance and life of the wall will be studied in the presence of such events, and mitigation techniques for ITER will be demonstrated and optimised.