The blanket surrounds the plasma inside the vacuum vessel. It has three functions: heat conversion, fuel breeding and sheilding. It is one of the most complex and essential systems in the reactor. Likely it is needs extensive development with a specially built test facility because because of the harsh environment it must survive in and its multiple functions.
Neutron from the fusion reactions in the plasma are stopped in the material of the blanket producing heat which is extracted by a circulation fluid.
There are numerous designs for breeder blankets with a variety of elements and heat extraction methods. The image below gives one example from a Japanese design.
The International Fusion Materials Irradiation Facility (IFMIF) is a neutron source intended as a test facility to develop new advanced materials for the plasma-facing components of fusion reactors. The device is based on a linear accelerator to generate neutron fluxes in the order of 1018m-2s-1 with an energy of 14.1 MeV as in a fusion reactor. The facility in under construction in Japan as part of the international Fusion for Energy consortium which is building ITER?
The Fusion Nuclear Science Facility (FNSF) is under consideration in the US where scientists and engineers consider a further R&D machine is necessary between ITER and DEMO in order to test materials and tritium breeder systems.