The environment effect on operation of in-vessel mirrors for plasma diagnostics in fusion devices

First mirrors will be the plasma facing components of optical diagnostic systems in ITER. Mirror surfaces will undergo modifications caused by erosion and re-deposition. A vast experimental program to investigate this problem has been launched: stainless steel, Mo and Cu mirror samples have been set in different places inside large scale fusion devices. In this paper the main results obtained in LHD, T-10, Tore-Supra and Textor are analysed and some details of future experiments on Jet are described. We can draw the five following conclusions. First, deposition is probably impossible to avoid, the true question rather lies in the rate of deposition or in the ratio between deposition and sputtering rates. Secondly, only first mirrors maximally open to the hot plasma and fixed not too far from the last closed magnetic surface can remain free from deposition of contamination film due to the possibility that the CXA sputtering rate predominates over the rate of film deposition. Thirdly, secondary mirrors, shaded from the direct plasma view by obstacles, might be protected from the growing of the deposit. Fourthly, baffles or a long tube minimizing the viewing solid angle under which the mirror is seen can be used to avoid appearance of contamination. And fifthly, it is evident that the scale of experiments related to the behavior of in-vessel mirrors in fusion devices under operation must be enlarged. (A.C.)