4.19.6.1 PFC Design Considerations

Robust engineering solutions are needed for the com­ponents that directly surround the plasma in order to withstand the thermal and mechanical loads during normal and off-normal operation and contribute to protect the outer components from the effect of neu­trons, especially the vacuum vessel and the magnets. Considerations here are limited to the portion of the PFC surface that protects the main chamber (the so-called first wall), which for both JET and ITER is made of beryllium.

The strategy and the criteria adopted to design the PFCs of JET and ITER are substantially different. One of the main differences is the longer plasma pulse duration foreseen in ITER that has required the use of water-cooled structures to handle the power in steady-state conditions. This translates into design solutions that consist of tiles bonded to an actively cooled copper-alloy substrate that must withstand the thermomechanical loads with a good engineering margin and achieve the desired fatigue lifetime. An additional problem in ITER is the deg­radation, albeit limited, of thermal and mechanical properties and the properties of the joints due to neutron irradiation (see Section 4.19.4.4).

In contrast to ITER, technical constraints pre­vented the use of actively cooled first-wall struc­tures in JET and the first-wall protection relies on a series of discrete poloidal limiters. The tiles are inertially cooled, and their power handling perfor­mance is driven by the need to (1) avoid surface melting and (2) reduce thermally induced stresses to give an adequate fatigue lifetime. At 40 mm typi­cal thickness, the tiles are thermally thick for a typical 10 s pulse and will handle up to about 60 MJ m~2 without melting.

Another important driver for designing PFCs is the magnitude of the electromagnetic loads associated with plasma disruptions. In tokamaks, dis­ruptions produce large changes in magnetic field, dB/dt, which induce eddy currents in the conducting materials. The currents interact with the local mag­netic field, B, to produce a torque, which is strongly dependent on the geometry. The design of the PFCs has to manage this torques via a combination of the castellations of the tiles along with cuts, which will interrupt the eddy current loops. The tile assembly must also withstand electromagnetic forces due to halo currents which, during disruptions, pass between plasma and vacuum vessel via the tiles. Enormous attention has been paid at JET and ITER during the design phase to address this problem.

The problems associated with the design of the first wall at JET and ITER are briefly described in Section

4.19.6.1.1 and 4.19.6.1.2, respectively.