Proposal to Use Metallic Mirrors for MSE Continued February 10, 2003 S. D. Scott PPPL

MSE metal mirrors.ppt Optical Properties of Candidate Materials at 0.66 Microns n (real) k (imaginary) Aluminum Silver Gold Iron Moly Austenitic 832 MV

MSE metal mirrors.ppt Silver has excellent reflectivity and superb S/P reflectivity Ratio Aluminum Silver Gold SS832 M2, M3 angles (guess only) Reflectivity for s and p states

MSE metal mirrors.ppt Aluminum Provides Least Difference in Phase Aluminum Silver Gold SS832 M2, M3 angles (guess only) Phase difference between Reflected s and p

MSE metal mirrors.ppt Phase of S and P states Aluminum Silver Gold SS832 M2, M3 angles (guess only)

MSE metal mirrors.ppt Aluminum Perturbs Measured Angle the Least Aluminum Silver Gold SS832 M2, M3 angles (guess only) Assume 5 o input polarization angle

MSE metal mirrors.ppt Serendipity: S and P Polarizations are Reversed at M2 and M3 Mirrors P S S P M2 45 o P S n n … so to first order, the S/P reflectivity ratio and phase shift at a metal mirror dont affect the relative amplitude or phase of the incident EM wave. There will be higher-order terms due to the variation of the S/P reflectivity ratio and phase shift with angle of incidence, since not all rays strike the mirrors at 45 o. incident emergent incident

MSE metal mirrors.ppt Compute Distribution of Angles-of-Indcidence with Ray- Tracing Code (thanks to Bravenec)

MSE metal mirrors.ppt There is Little Correlation Between AOI on M2 and M3

MSE metal mirrors.ppt Compute Net Reflectivity and Phase Shift for Each Ray Net result: fairly wide distribution of phase shifts among the various rays, but the average phase shift is reduced from a single mirror at 45 o.

MSE metal mirrors.ppt Compute Net Reflectivity and Phase Shift for Each Ray Net result: compared to aluminum, Silver has greater range of phase shifts and a larger average phase shift.

MSE metal mirrors.ppt Comparison of Single Mirror to 2-Mirror Systems Phase Shift (degrees) S/P Reflectivity Ratio 1-mirror 2-mirrors 1-mirror 2-mirrors Aluminum Silver

MSE metal mirrors.ppt Preliminary Conclusions Metal mirrors would solve our mechanical problems, and their optical properties dont necessarily preclude their use in MSE. Aluminum (solid), or Aluminum, Silver, or Gold on Stainless look attractive: –All have high reflectivity –Aluminum perturbs apparent angle the least –Gold might not require a protective coating against oxidation. We need to evaluate carefully: –Effect of change in polarization –Effect of two mirrors –Actual angles-of-incidence (ray tracing) –Requirements for alignment. –Compatibility of various substrate and coatings –Effect of unavoidable oxide coating and/or deliberate protective coating (e.g. SiO 2 ). –Possible use of dielectric coating to reduce phase shift.

MSE metal mirrors.ppt Motivation Solid metallic mirrors (no glass component) would be immune from the fracturing that has plagued us with the current glass components. TFTR successfully used aluminum-coated (glass substrate) mirrors. We have the option to lay down a film of silver, gold, aluminum etc (or maybe dielectric coatings?) if the optical properties of the metallic substrate are not ideal. Have a list of vendors for metallic mirrors courtesy Dave Johnson. Have software to compute the reflectivity and phase shift for s & p courtesy of Fred Levinton.

MSE metal mirrors.ppt Issues Metallic surfaces have different reflectivities for s- and p- incident polarizations affects inferred pitch angle (effect can be calibrated out) Note: we have two mirrors, whereas TFTR had only one. Metallic surfaces introduce a phase shift linearly polarized light becomes partially circularly polarized. Loss of signal. Reflectivity: silver and gold are excellent, aluminum is very good, stainless steel is fair. Oxide layers may form. Need to consider their optical properties Must consider possible warping introduced by stresses induced when solid mirror is bolted into place. Calculate differences in reflectivity and phase shift as a function of angle-of-incidence (about 45 o for C-MOD MSE).