Scheme of in-situ sensitivity calibration for divertor impurity monitor in ITER

Sugie, Tatsuo; Costley, A. E.*; Ogawa, Hiroaki; Kasai, Satoshi; Walker, C.*

The Divertor Impurity Monitor will use mirrors as the first optical element and these mirrors will potentially be subject to damage from impurity deposition and dust. Measures to reduce/eliminate the effects of these depositions are being developed. However, it will not be possible to prevent the degradation completely. Therefore, in-situ and remote calibration methods, which will have to be installed in the strong radiation field, are required. So far an in-situ calibration system using a retro-reflector, which is installed in front of the first mirror, has been designed. However, a small amount of the incident light goes back to the detector due to the asymmetric optics. To solve this problem, a method using a micro retro-reflector array instead of the retro-reflector is studied here. A standard light is set behind the bio-shield or in the diagnostic room and the light is applied to the micro retro-reflector array through the same optics which is used in the plasma measurement. The reflected light is measured with a detector to estimate the sensitivity change of the optics. The signal to noise ratio has been estimated under the assumption that the plasma facing first mirror is made of molybdenum and other mirrors are made of aluminum. In addition, we assume the use of a xenon lamp for the light source and a back-thinned type CCD for the detector. As a result, sufficient signal to noise ratio will be obtained by this calibration system in the wavelength range of 200 - 1000 nm.