An overview of a comprehensive First Mirror Test for ITER at JET

The test was performed with 32 stainless steel and molybdenum mirrors placed in pan-pipe shaped cassettes and exposed in JET in the divertor and on the main chamber wall for 127 000 s including 97 000 s of X-point operation. Surface composition and total reflectivity were determined afterwards. All mirrors from the divertor were coated with deuterated carbon deposits causing the reflectivity loss by a factor of 6-10 in the visible range. Flaking and exfoliation of deposits were observed in some cases. On the main chamber wall the deposition occurred mainly on mirrors located deep in cassette channels, whereas mirrors close to the channels entrances were free from deposits and retained fair reflectivity (∼90% of initial value) especially in the infra-red range. No significant differences in behaviour of steel and molybdenum were noted. The need for development of methods for mirror cleaning and/or protection in a reactor-class device is addressed.