On the choice of material for the first mirrors of fusion reactor plasma diagnostics

The rate of the reflectivity degradation for the nearest UV and visible regions was investigated depending on thickness of a sputtered layer of mirrors made of aluminum, copper, stainless steel(ss), tantalum and tungsten. The thickness of a sputtered layer was measured by the mass loss of mirror samples after every exposure to plasma with a multistep repetition of this procedure and with simultaneous measurements of reflectivity R. The ECR discharge in the H-D mixture or the reflex discharge in D-2 was used similar to ion sources. In the ECR discharge the effective energy spectrum of ions bombarding the mirror surface was spread between 0.1 and 1.0 keV or between 0.1 and 1.5 keV with the ion current density similar to 1 mA. In the reflex discharge the mean ion energy was between 0.65 and 0.8 keV with a half-width similar to 0.12 keV and approximately the same current density. The measurement results have shown: (1) The rate of a tested mirror degradation depends on the mirror material and on the energy spectrum of projectiles. (2) For metals under the test the difference in values of thickness of sputtered surface layer, after which the reflectivity begins to decrease (the range 0.1-0.6 mu m, excluding ss mirror), is much lower than the difference in the sputtering yields. (3) As the first step in the search for mirror material the R/Y criterion can be used in combination with the demand to have the mirror reflectivity and absorptivity values sufficient for the given method of plasma diagnostics.