Engineering a remote survey of JET's divertor structure under conditions of restricted access using digital photogrammetry

JET (Joint European Torus) Joint Undertaking is currently planning to insta ll its new Gasbox Divertor remotely. In line with JET's policy and philosop hy, it was decided to survey the divertor structure remotely to confirm its position, shape and integrity prior to installing the new tile configurati on. A survey to metrology standards with sub-millimetre accuracy is require d. While remote surveys have been carried out in the past, they have not be en to this level of accuracy. Digital photogrammetry (the evolution of photogrammetry, using CCD cameras) in conjunction with "targetless" software was selected as being the most s uitable technique. It was seen as the natural evolution of the survey techn iques already developed for use at JET, as well as having the potential to assist in preparations for future remote handling operations. Photogrammetr y requires a large number of retroreflective targets to ensure accurate res ults. Clearly it would not be practical to Jit targets remotely to the dive rtor structure. However, a technique has been developed at JET which uses a combination of targeted and targetless photogrammetry. A number of frames fitted with targets will be remotely positioned on the divertor structure. When surveyed these targets allow accurate determination of camera position s. Specially developed software allows the selection of untargeted features of components in the digital pictures. Their co-ordinates can be determine d by triangulation from the known camera positions. Trials have shown that accuracy of +/-0.6 mm is achievable. This paper will describe the development of this concept, the design of the necessary equipment, the resting to prove the accuracy and feasibility and the trials carried out in JET's In-vessel Training Facility. These include d remote handling of the six million pixel camera and other equipment, data handling and download via an ethernet link through the remote handling art iculated boom, development of survey techniques, optimization of analysis t echniques using coded targets for fast processing and finally the engineeri ng assessment of the divertor structure based on survey results.