Numerical analyses of a Brewster-angle window featuring the largest diamond disk to date and an overview of integrity assessment against fracture failure

The diamond Brewster-angle window is a compact broadband window solution for high power, continuous wave (CW), frequency steering gyrotron operations. A large diamond disk with minimum diameter of 180 mm is required for a waveguide aperture of 63.5 mm, compatible with a mm-wave power of 2 MW-CW. In 2020, a first-of-its-kind disk was manufactured with 180 mm diameter and thickness of about 1230 µm at centre and in the range of 1100-1150 µm at the edge. The first part of this work shows thermal and structural analyses run to check the window performance with the large 180 mm disk at the minimum achieved thickness of 1.1 mm. The analyses were carried out using the experimentally measured loss tangent distribution over the disk as input and considering HE11 and Gaussian modes for the torus and gyrotron side of the electron cyclotron (EC) system, respectively. The window design was then verified for normal operation, considering also the off-normal event of overpressure on the torus side. The second part of the work deals with the path to assessing the integrity of the diamond disks on the base of fracture mechanics. Main failure mode of the disks is the failure to fracture. On one side, the fracture toughness of the diamond of interest shall be measured to check the resistance to crack propagation. On the other side, stresses acting on the disks and size and positions of the microcracks generated by the diamond manufacturing process shall be evaluated. The recent developments of this approach are reported with reference to the ITER EC torus diamond disks. These disks feature a diameter of 70 mm and thickness of 1.11 mm.