In-vessel components imaging systems: From the present experience towards ITER safe operation

Visual monitoring, control and inspection of in-vessel components are becoming more and more important for safe and reliable operation of existing fusion devices. It will be mandatory for ITER due to the low technological margin of in-vessel components to thermal overloads, high impact of potential damages and restrictions to in-vessel human access due to nuclear issues. In the recent years, imaging systems in infrared and visible light have been developed on existing facilities and in the perspective of ITER. They include complex optical systems that have to be integrated to the specific environment of fusion devices,with associated thermal, mechanical and neutron loads. At Tore Supra and JET, endoscopes have been installed to image plasma facing components (PFCs) during plasma operation for the purpose of real-time feedback and/or off-line analysis. Tore Supra has also recently been equipped with a remote inspection viewing system working under vacuum and high temperature, able to perform a survey of in-vessel components between plasma shots. Transposed to ITER constraints. the design of such imaging systems is very challenging. in particular as regards the neutron environment, the possible mirrors degradation and the overall active cooling in a reduced available space. The interpretation of the imaging is not trivial. There are for instance several issues in deriving in-vessel components surface temperature from an infrared imaging system, in particular in a metallic environment such as the one foreseen in ITER. Indeed, the temperature measurement depends on the emissivity of the surface which is not a priori known but also on the viewing direction, wavelength and physical state of the surface, the latter being subject to variations during plasma operation. Moreover, reflections from hot spots can also induce errors due to their parasitic contribution on the thermographic signal. In addition, due to the huge amount of data generated by these imaging systems, the use of advanced image processing techniques is required, starting from signal modeling, pattern recognition to heat flux estimate with on line identification of first wall material thermal features, possibly in real-time. The paper reports on the already achieved state of the art of all these issues in Tore Supra, JET, and explores the next steps towards a satisfactory implementation in ITER. (c) 2009 Elsevier B.V. All rights reserved.