Transcript No Slide Title

Proposal to Use Metallic Mirrors for MSE
Continued
February 10, 2003
S. D. Scott
PPPL
Optical Properties of Candidate Materials at
0.66 Microns
n (real)
Aluminum
Silver
Gold
Iron
Moly
Austenitic 832 MV
k (imaginary)
1.55
0.07
0.14
3.48
3.56
7.9
4.2
3.37
3.86
3.01
2.6
4.5
MSE metal mirrors.ppt
Silver has excellent reflectivity and superb S/P reflectivity
Ratio
Silver
Reflectivity for s and p states
Aluminum
Gold
SS832
M2, M3 angles
(guess only)
MSE metal mirrors.ppt
Aluminum Provides Least Difference in Phase
Aluminum
Phase difference between
Reflected s and p
M2, M3 angles
(guess only)
Gold
Silver
SS832
MSE metal mirrors.ppt
Phase of S and P states
Aluminum
SS832
Silver
Gold
M2, M3 angles
(guess only)
MSE metal mirrors.ppt
Aluminum Perturbs Measured Angle the Least
SS832
Aluminum
Assume 5o input polarization
angle
Silver
Gold
M2, M3 angles
(guess only)
MSE metal mirrors.ppt
Serendipity: S and P Polarizations are Reversed at M2
and M3 Mirrors
M2
emergent
P
S
n
S
P
45o
45o
incident
n
incident
… so to first order, the S/P reflectivity ratio and phase
shift at a metal mirror don’t affect the relative
amplitude or phase of the incident EM wave.
S
P
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
45o.
MSE metal mirrors.ppt
Compute Distribution of Angles-of-Indcidence with RayTracing 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 45o.
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
1-mirror
2-mirrors
2-mirrors
Aluminum
9.6
3.0-4.1
0.86
0.974-0.980
Silver
20.0
5.7-7.5
0.977
0.996
MSE metal mirrors.ppt
Preliminary Conclusions
•
Metal mirrors would solve our mechanical problems, and their optical properties
don’t 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.
SiO2).
– 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 45o for C-MOD MSE).
MSE metal mirrors.ppt