Some problems of the material choice for the first mirrors of plasma diagnostics in a fusion reactor

Abstract

We present the results of simulation experiments on the effect of the fusion reactor environment on the optical properties of the first mirrors for spectroscopy and laser diagnostics. We found the greatest effect on the degradation of mirror optical properties was due to charge exchange atoms. These atoms can affect mirror quality in two ways: by sputtering and by redeposition of material sputtered from other inner components of the vacuum chamber. The degradation rates of mirrors made of different metals and subjected to long-term bombardment by ions from hydrogen or deuterium plasmas are compared. From analysis of all data, we concluded that special experiments will have to be conducted in order to make a correct choice of a first mirror material. These tests should include the following: (1) study the role of swelling on mirror surface modification; (2) find the multimillion shot laser-induced-damage threshold of metal mirrors; and (3) develop and test methods to protect the first mirrors from redeposition. In addition, a model should be developed for the charge exchange atom flux and energy distribution that represents real data from operating tokamaks.