JET has completed in 2011 a large enhancement programme that includes, among other things, the installation of a beryllium wall and a tungsten divertor.

An overview of the status of the JET ITER-like wall project is presented in Matthews et at.119 The material combination chosen for the wall and the divertor is that chosen for the DT phase of ITER and experiments in JET with the new wall configura­tion will provide the first fully representative test of material migration, material mixing, and consequent tritium retention under ITER relevant conditions.180 Equally important is the opportunity to develop fully integrated scenarios and control schemes for protect­ing the wall. The project will therefore provide essen­tial information for interpreting material behavior in ITER and a sound technical basis for guiding the development of ITER scenarios.

The design layout, the main engineering challenges, and the operational limits of the JET ITER-like wall are discussed elsewhere (see, e. g., Nunes et at.,181 Thompson et at.,182 Riccardo,183 and Riccardo et at.184). Figure 17 describes the design layout and the planned material layout. It must be noted that the existing JET wall relies on a series of discrete poloidal limiters whereas at the moment ITER relies on a plasma conforming wall. The elec­trical resistivity of Be « 0.08 p. Q m is more than a 100 times lower than that of CFC («10 p. Q m). Therefore, after replacing the CFC tiles in JET, the mechanical loads due to eddy currents associated with disrup­tions, which were negligible in the case of carbon tiles, have become dominant for Be tiles and this

Beryllium W-coated CFC Inconel + 8mm Be Bulk W

Saddle c

Upper dump plate

Mus

Restraint ring protections

Inner wall cladding

Poloidal limiters

Inner wall g

Inner wall guard limiters

IW cladding for pellets

Divertor

(b)

Figure 17 (a) View of the present Joint European Torus main chamber with indications of how the carbon-fiber composite tiles will be replaced (reproduced with permission from Riccardo, V. J. Nucl. Mater. 2009, 390-391, 895-899). (b) Cross-section with allocations of materials. Image courtesy of EFDA-JET.

has posed a significant engineering challenge. Calcu­lations for the main limiter tile types clearly show that a tile of the size of the existing CFC tiles would give unacceptable eddy torques, leading to the inves­tigation of several slicing options.181 The chosen design has vertical slices with a large central block and one to three side slices, depending on the toroidal extent of the tile assembly, supported on a carrier via pins (see Figure 18). The design is defined by the balance between conflicting requirements ofeddy cur­rents (avoidance of large low resistance loops) and power handling (minimum number of vertical cuts to be shadowed). The problems associated with the design of the JET beryllium tiles (power handling capacity and disruption induced eddy currents) are discussed in detail elsewhere (see, e. g., Thompson eta/.182).

The installation of the new ITER-like wall and the NB enhancement has been completed by the mid of 2011 and operation is now restarting to provide important information for ITER.