Optically transmissive window on ITER port cell door

In the ITER Thomson Scattering Core LIDAR system there are currently three concepts for collecting scattered light from the plasma and delivering it to the diagnostic room. An option to use a broadband mirror in Cassegrain configuration has been explored. In this concept a large optical window, 1.5 m in diameter, is required in the port cell door. This could be manufactured in a number of ways, including using a single thick window or mosaic arrangement with a supporting structure. Challenges associated with this design include maintaining the secondary confinement barrier, withstanding a pressure differential of 0.5 bar across the window in foreseeable fault conditions, and maintaining the optical properties of the window. It must also be possible to protect the window when work is being carried out in the area. Fracture analysis for brittle material was carried out, for a flaw size with a length to depth ratio of 80/20, which gave a safety reserve factor of 4. The flaw size was based on a standard optical transmissive lens for collection light. A static analysis was carried out on both options of having a single thick glass window or a mosaic arrangement. The principal stress for a single window arrangement is 12 MPa and for a proposed mosaic arrangement is 12.5 MPa. The analyses have demonstrated that a single glass window with a thickness of 42 mm would be mechanically acceptable but expensive to manufacture and it would be difficult to polish glass of that size to lens quality. By comparison in the proposed mosaic window arrangement, the glass thickness is reduced by 50% with the aid of the supporting structure. As a result of the supporting structure the geometrical optical losses are 4%, which are mechanically and optically acceptable, whilst significantly reducing the cost of manufacturing.