Fiberoptic in-vessel viewing system for the International Thermonuclear Experimental Reactor

A viewing system was designed and a prototype realized for the in-vessel inspection of the International Thermonuclear Experimental Reactor. The viewing is based on the line scanning principle, and the system consists of ten identical units installed on top of the reactor at 36° intervals. Each device contains a laser, beam steering mirrors, and viewing probe with insertion mechanics. The probe has an outside diameter of 150 mm and a length of 14 m. The illumination design applies frequency-doubled Nd: yttrium–aluminum–garnet lasers whose beams are guided through hermetically sealed windows into the vacuum vessel. The diffuser optics creates a vertically oriented light stripe onto the vessel surface that is viewed by the imaging optics, consisting of 16 modules altogether covering horizontal and vertical field-of-views of 2° and 162°. The optical images are transferred to charge coupled device cameras via coherent fiber arrays. The multifocus design uses stacked fiber rows whose ends are assembled into different axial positions. The viewing probes rotate at a constant angular speed of 1°/s and pictures are taken at 0.01° intervals. The complete picture of the vessel interior is generated in 6 min producing 5.8×10 9 image pixels. The image processing and analysis of possible defects in the vessel surfaces are performed off-line after the viewing procedure. A full-scale prototype of the viewing probe was constructed to demonstrate the feasibility of the design. Its illumination optics utilizes a diffractive optics element that transforms the collimated input beam into a rectangular output lobe with uniform intensity. The prototype has horizontal and vertical imaging optics field-of-views of 2° and 12°. The test results showed that the prototype can take pictures of good quality applying a continuously rotating probe having an angular speed of 0.08°/s. Under optimum conditions, the minimum resolvable feature size at a 3 m distance is smaller than 1 mm, which satisfies the requirement specification. Further development is needed to increase the illumination power to improve the imaging speed and to develop linear fiber arrays that are compatible with the vacuum and high-flux radiation environment of the primary vacuum vessel.