a Institute of Physics, University of Basel, Switzerland;

1. Influence of material choice on the deposition/erosion mechanisms affecting optical reflectivity of metallic mirrors G. De Temmermana, V.S. Voitsenyab, R.A. Pittsc M. Maurera, L. Marota, and P. Oelhafena a Institute of Physics, University of Basel, Switzerland; b Institute of Plasma Physics, NSC KIPT, Akademischa St. 1, 61108 Kharkhov, Ukraine; c Centre de Recherches en Physique des Plasmas, Association EURATOM, Conférédation Suisse, EPFL, 1015 Lausanne, Switzerland

2. Introduction • Experiment in Tore Supra with plasma facing mirrors showed strong differences in the sputtering yields of Mo, SS and Cu under similar exposure conditions • Surface of the SS mirrors appeared to have been protected from sputtering • A difference in the stickiness of carbon has been proposed to explain these findings (V. Voitsenya, ITPA-9) • Experiment was initiated in the Univ. Basel to clear up the behaviour of SS and Cu mirrors submitted to D2 glow discharge with controlled partial pressure of methane. • In parallel, tests of different candidate materials were made in TCV where samples are exposed in the divertor region • Differences in the deposition efficiency of carbon on different substrates were noticed

3. Laboratory experiments (Uni Basel) • Exposure of metallic mirrors to a low temperature deuterium plasma with controlled partial pressures of methane in the gas mixture • 2 substrate materials: copper and stainless steel prepared in IPP Kharkov, Ukraine U= -200 V Water cooled sample holder =0 / 1.8 / 3.5 % Samples characterization: In situ measurement of the reflectivity using laser reflectometry (532 nm) Weight measurement: determination of the eroded/deposited depth SEM: surface morphology Total and diffuse reflectivity (250-2500 nm); Spectroscopic ellipsometry (350-2300 nm)

4. Strong correlation between the carbon content in the plasma and the degradation rate of R All samples are "carbon free" Reflectivity during plasma exposure Stainless steel Copper For =3.5%, appearance of interferences typical from the growth of a:CH layer No carbon on the other samples (EDX measurements)

5. No significant effect of physical sputtering Carbon protection effect Deterioration of the reflectivity by an increase of the roughness Appearance of the crystallographic grains Evolution of the surface morphology Stainless steel Copper

6. Erosion/deposition measurements • Eroded/ deposited depth estimated both from mass loss measurements and profilometry Different behaviour of both substrates towards erosion/deposition

7. Reflectivity after exposure • Reflectivity measured with a UV-Vis-NIR spectrophotometer equipped with an integrating sphere Copper Stainless steel Degradation of the reflectivity due to absorption of light in the deposited layer Degradation of the reflectivity due to an increase of the surface roughness

8. Reflectivity of linearly polarized light • Measurement of the reflectivity of linearly polarized light using a spectroscopic elipsometer at various incidence angles. • Rs: E field perpendicular to the plane of incidence • Rp: E field parallel to the place of incidence • Wavelength range: 350-2300 nm

9. Reflection of polarized light (400 nm) Stainless steel Copper

10. Reflection of polarized light (800 nm) Stainless steel Copper Polarization of the light strongly affected by the carbon layer. A drastic increase of the surface roughness has only a slight effect on the polarization Deposition of impurities appears to be a more serious problem for diagnostics using polarized light

11. Exposure of mirrors in TCV (1) • TCV (Lausanne), 90% carbon coverage of the first wall Mirrors located in the divertor region and recessed below the surface of divertor tiles, no direct contact with the plasma

12. Exposure of mirrors in TCV (2) • No shutter installed, the sample manipulator is electrically insulated from the torus • Sample exposures integrated over short experimental campaign periods of few weeks including He glow discharge conditioning Magnetic equilibrium of the standard single null diverted discharge. The red arrow indicates the mirror location. • Mirrors exposed to a variety of diverted plasma configurations (many plasma configurations can be achieved at TCV

13. Deposition efficiency • Test of different materials and different distances Thickness determined by ellipsometry/SIMS/ profilometry Deposited layer consists mainly of carbon and deuterium Strong differences in the thickness measured on Si and Mo samples under similar exposure conditions

14. Summary/ Conclusions • Both laboratory experiments and sample exposures in the TCV tokamak have shown the material dependence of the erosion/deposition patterns affecting the reflectivity of metallic mirrors (with carbon as impurity) • Monte Carlo simulation (SDTRIMSP) have confirmed the differences observed for the various substrates (not shown here) • These different features are only of importance until a certain deposited thickness is reached (after this the deposition rate on the various metals is the same) • Further experiments are needed to test other materials The material choice not only influences the resistance of mirrors towards erosion but also their sensitivity to impurity deposition