On the fracture toughness and structural integrity assessment of MPA CVD diamond in nuclear fusion devices

Nuclear fusion power plants require electron cyclotron (EC) heating and current drive (H&CD) systems to deploy MW-class, mm-wave radiation, generated by the gyrotrons, to the plasma for heating and control of instabilities. Diamond windows are used as confinement and vacuum barriers on the torus side of these systems and as vacuum inserts only at the output of the gyrotrons. The optical grade polycrystalline diamond is produced by microwave plasma assisted (MPA) chemical vapor deposition (CVD) process and it is used in shape of disks with thickness between 1 and 2 mm and diameter in the range of 70 to about 110 mm. Failure due to crack propagation is the main failure mode of these large diamond disks, which have the primary safety function on the torus side to confine tritium and other hazardous materials such as radioactive dust. Microcracks are indeed generated in the disks by the CVD process. In addition, beyond operational loads, the torus windows may be subject to off-normal events like overpressure acting on the disk. This work shows the path to measure the fracture toughness of diamond to use in nuclear fusion and to check the structural integrity of the diamond disks by fracture mechanics. The experimental setup, the samples and their first characterization are presented. Considering the ITER EC torus diamond windows, numerical analyses have been carried out to provide the stresses generated in the disks for the worst load conditions. High resolution microscopy activity is ongoing to detect, measure and classify the microcracks in specific areas of about 60 ITER disks, produced by the same manufacturer and in the same growth conditions. The information on the microcracks size shall be combine with the stress values to check the structural integrity of the disks against failure to fracture.