1. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 1 Be deposition on ITER first mirrors: layer morphology and influence on mirror reflectivity G. De Temmerman, M. Baldwin, R. Doerner, D. Nishijima, R. Seraydarian Center for Energy Research, University of California at San Diego, USA K. Schmid, C. Linsmaier Max Planck Institut für Plasma Physik, Garching, Germany L. Marot Institute of Physics, University of Basel, Switzerland Work performed as part of US-EU Collaboration on ITER Materials Presented to LIDAR Cluster Meeting, 8/8/7

2. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 2 Motivation Deposition of impurities may strongly affect the First Mirror reflectivity Studies have focused on the effects of carbon deposition on the reflectivity Presence of Be in the divertor plasma may mitigate erosion of the graphite targets (and subsequent carbon migration) [1] Formation of Be-rich layers in direct line-of-sight locations from the plasma (no long-range migration for Be) [2] Until now, it was assumed that Be deposition will make the coated mirror behave like a Be mirror beyond a given deposited thickness [3] 1 M.J. Baldwin, R.P. Doerner, Nucl. Fusion, 46 (2006), 444; 2 M.J. Baldwin et al, J. Nucl. Mater.,337-339 (2005) 590 3 J.P. Allain et al, 12 th meeting of the ITPA TG on diagnostics, March 2007, Princeton, USA Aim of the experiments: Assess the effect of Be deposition on Mo and Cu mirror reflectivity Collect data on the “optical quality” of deposited Be layers

3. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 3 Experimental setup Magnetic field Metallic mirrors (Mo and Cu) exposed to a Be-containing D 2 plasma ion flux (target): 2.5-3.5·10 18 cm -2 s -1 T e ~ 6 eV n e ~ 2.5-3.5·10 18 m -3 target bias -50 V T target ~ 700°C (graphite target) Be concentration: 0.03-0.1% Only material eroded/reflected from the graphite target is collected by the mirror Experiments simulate the situation of mirrors looking at the ITER divertor targets (under the dome for example)

4. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Incidence angle  10° Wavelength range 450-950 nm (CCD spectrometer) Reflectivity measurements Reference: unexposed sample from the same material 4 Enclosure window Calibrated white light source Mirror Spectrometer with linear CCD array Optical fiber with focusing lens

5. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 5 Mirror temperature monitored by a thermocouple installed at the backside of the mirror Heating of the mirror arises due to plasma radiations 2 cases Deposited layer XPS depth profile on a W witness plate exposed with c Be =0.1% Deposited layer consists mainly of Be Film thickness  90 nm Deposition rate  0.005-0.02 nm/s

6. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Mirror initially at 240°C Target bias -50 V Case of a pure Be layer deposited on a Mo mirror Reflectivity of coated mirrors (Mo) Mirrors exposed to similar plasma conditions Mirror reflectivity strongly decreased by thin Be-rich layers Layers are quite dark… 4 Handbook of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991 6

7. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Layer morphology and reflectivity Mirror initially at 240°C 7 Well developed layer morphology. Layers deposited at RT exhibit the same aspect. 4 Handbook of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991

8. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Surface roughness Material Temperature (°C) Be film thickness (nm) Roughness (nm) W240-3209040.6 CuRT-1204111.3 CuRT-1204121.9 Cu240-3207516 Cu240-3205514.3 MoRT-120327.6 Does the layer roughness explain the reflectivity measurements ? Influence of surface roughness on reflectivity: Bennett’s formula In the present case, roughness of the coated mirrors does not significantly differ from the roughness of the mirrors before exposure. Roughness certainly plays a minor role here… 8

9. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Deeper look in the layer morphology Be layer 75 nm thick on Cu at 240-300 C ; P=5·10 3 torr, n e =3·10 12 m 3 9 High level of porosity Layer seems to be made of small-size crystallites separated by void How does it fit with the reflectivity measurements ?

10. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Influence of the porosity (1) Bruggeman effective medium approximation: polydispersed spheres in a continuous medium [5,6] ε eff calculated from this relation. Then n and k of the effective medium are calculated using Reflectivity of the porous Be coated surface calculated for the couples (n,k) 10 5 D.A.G. Bruggeman, Ann. Phys., 24 (1935) 636; 6 M.T. Othman et al, J. Appl. Phys., 99 (2006) 083503

11. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 Influence of the porosity (2) 75 nm of Be deposited on Cu (interface effects and roughness are neglected) Film porosity strongly decreases the layer reflectivity !! Good agreement between calculated and measured reflectivity 11 Simulations

12. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 T=RT-110°C, P=6mtorr T=250-320°C, P=6mtorr T=RT-110°C, P=1.5mtorr T=250-320°C, P=1.5mtorr P T Increasing the pressure during deposition favours the formation of porous layers Increasing the surface temperature (to 300 °C) enhances the crystallite growth In agreement with the Structure-Zone model for physical deposition J.A. Thornton, J. Vac. Sci. Tech., 11 (1974) 666 Formation conditions influence porosity

13. R. Doerner, ITPA SOL/DIV meeting, Avila, Jan. 7-10, 2008 13 Deposited layers are Be-rich and contain almost no carbon Reflectivity of the coated mirrors strongly decreased by Be deposition. Roughness, porosity  Porosity of the layers seem to explain the observations Deposition rates observed in PISCES-B experiments in the range 0.005-0.02 nm/s Formation of porous layers: According to the Structure Zone Model [7], high neutral pressure during deposition favours high levels of porosity Increasing the surface temperature acts the other way What should we expect in ITER ? 7 J.A. Thornton, J. Vac. Sci. Technol., 11 (1974) 666 Summary