Comprehensive First Mirror Test for ITER at JET with Carbon Walls
Metallic mirrors will be essential components of all optical spectroscopy and imaging systems for plasma diagnosis that will be used on the next-step magnetic fusion experiment, ITER. Any change of the mirror performance, in particular reflectivity, will influence the quality and reliability of detected signals. On the request of the ITER Design Team, a First Mirror Test (FMT) has been carried out at JET during campaigns in 2005-2007 and 2008-2009. To date, it has the most comprehensive test performed with a large number of test mirrors exposed in an environment containing both carbon and beryllium; the total plasma time (in 2005-2007 period) over 35h including 27h of X-point operation. 32 stainless steel and polycrystalline molybdenum flatfront and 45o angled mirrors were installed in separate channels of cassettes on the outer wall and in the Mk-II HD divertor: inner leg, outer leg and base plate under the load bearing tile. Post exposure studies comprised reflectivity measurements and surface analyses with microscopy, secondary ion mass spectrometry, ion beam analysis and energy dispersive X-ray spectroscopy. The essential results are: (i) on the outer wall high reflectivity (~90%) is maintained for mirrors close to the channel entrance but it is degraded by 30-40% deeper in the channel (ii) reflectivity loss by 70-90% is measured for mirrors placed in the divertor: outer, inner and base; (iii) deuterium and carbon are the main elements detected on all mirror surfaces and the presence of beryllium is also found; (iv) thick deposits show rough columnar structure and thickness is 1-20mm; (v) bubblelike structures are detected in deposits; (vi) the deposition in channels in the divertor cassettes is pronounced at the very entrance; (vii) photonic cleaning with laser removes deposits but the surface is damaged by laser pulses. In summary, reflectivity of all tested mirrors is degraded either by erosion with CX neutrals or by the formation of thick deposits. The implications of results obtained for first mirrors in next-step device are discussed and critical assessment of various methods for in-situ cleaning of mirrors is presented. The conclusion is that engineering solutions should be developed in order to install shutters or to implement a cassette with mirrors to replace periodically the degraded ones.