uv signal lossesand straylight by chemicalcontamination
TRANSCRIPT
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UV signal losses and straylightby chemical contamination
Pierre ETCHETO, Delphine FAYE, CNES
Xueyan ZHANG, IAS
COMET RTS contamination, Toulouse Dec. 11th 2018
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Contamination issues for Space UV optics
UV losses and straylight by chemical contamination Dec. 11th 2018
� Molecular contamination very critical for UV instrumen ts
� Loss of reflection / transmission + spectral shift
� Straylight by scattering (drops, droplets..,)
� Ageing
� Condensation on optical components
� On ground : manufacturing, AIT, transport, storage, on launch pad. UV optics cannot be cleaned!
� During launch : outgassing + re-condensation on (cold) optics and detectors
� In flight : ageing (under Sun UV, radiations, atox, thermal cycling… ).
Few data available to build contamination requirements + cleanliness plans
⇒ CNES + IAS R&T (2014-2017) for Solar Orbiter / EUI :
Try to assess effects of molecular contamination on scatter and transmission in the EUV (17-30 nm) and FUV (100-200 nm)
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Outline of experiment(1) spectral losses a Lyman alpha(2) scatter in EUV
Samples tested : o (Lyman α) filters (MgF2 substrates) o (EUV) Mirrors (Al/Mo/Sic coating on SiO2 substrates)o (EUV) Filters (Al sheet on hexagonal grid)
Contaminants tested : outgassing products from typical Space contaminantso Scotchweld EC2216 epoxy resino RTV-566 Silicone elastomero M55J/RS-3C Carbon fiber-polycyanate resin composite honeycomb
Sequence :
o Measure all clean samples (transmittance and BRDF) + setup signature
o Contaminate samples with 2-3 levels of the 3 contaminants
o Measure samples again and check clean references
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UV losses and straylight by chemical contamination Dec. 11th 2018
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Contamination process
� Samples exposed in CNES oven
o 4 days at 10-4 mbar / 100°C
o Several quantities of chosen contaminants => outgassing + condensation
o Quantity assessment by micro-weighting before and after
� Contamination levels obtained
o Composite honeycomb : 10 and 20 µg /cm2 (mirrors).
o EC2216 : Below measurement error (disturbed by water adsorption / desorption)
o RTV Silicone : 20-120 µg /cm2
Contamination level control : major issue . All we have today are orders of magnitude
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UV losses and straylight by chemical contamination Dec. 11th 2018
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Molecular contamination aspect
Condensation of contaminants depends on :
o Sample surface state (Wettability)
o Sample temperature
o Sample exposure to evaporate contaminants…
⇒ Drops, droplets, RARELY uniform film
⇒ Strong effect on absorption (in drops) and on scatter
EC2216 contamination : Droplets and Drops =>
Silicone contamination : Droplets and drops =>
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UV losses and straylight by chemical contamination Dec. 11th 2018
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Lyman αααα Contaminated samples
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Lyman αααα transmittance facility
SOLEIL synchrotron, APEX line :
• energy range : 1 - 20 eV (60 - 300 nm)
• spectral resolution : 0.1 nm
• beam size : about 4 x 4 mm² (split in 2 spots)
• detector : IRD AXUV100 photodiode +transimpedance amplifier
• MgF2 window : to cut spectral harmonics below 115 nm
• Measurements at 2 locations on sample : � centre + 2 mm off centre (to check non-
uniform cibtmination)
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Exposure of contamination to flux
• 13 -14 h (EC2216 and composite) : below
• 2h30 (Silicone)
⇒ Notable change in aspect and drop of transmittance
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Effects of EC2216 at Lyman αααα
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Effects of honeycomb resin at Lyman αααα
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Effects of silicone at Lyman αααα
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EUV : Scatter vs peak transmission
Contaminated BRDF vs. Clean BRDF and signature
Scatter depends on both surface state an specular T/R :
o Direct comparison with signature shows component T/ R and actual scatter
o Comparison between contaminated and clean shows T/R loss and actual scatterchange
o Comparison between normalised clean and contaminated shows scatter to specular ratio, ie. straylight ratio
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EUV Scatter measurement facility� Soleil synchrotron « Metrology » XUV beamline : both EUV spectrum and BRDF
Angle-resolved scatter (BSDF) configuration
o Wavelength 17.4 nm (72 eV)
o Beam 0.1 x 0.2 mm, divergence negligible
o Detector : Si photdiode + 1.5 mm pinhle, on 287.5mm long arm
o Incidence 0° (for transmitting filters) / 6° (for mirrors)
o Measuring range : +/- 3°
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UV losses and straylight by chemical contamination Dec. 11th 2018
Vacuum chamber
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EUV References and signature
Checks :
o Check beam centering on samples : scan => uniform area (inside a stitch for grid filters)
o Measure and substract dark signal
o Signature (direct beam over +/-4°)
o Measure clean samples + compare with signature
Results :
o Signature : specular peak, width : +/-0.5° at 10-4, +/-0.7° at 10-5. Peaks 10-4 high at +/-3.5°, due to reflections on slitchamfers. Not critical : falls outside the samples => absorbed by sample holder
o Clean samples : specular peak + scatter. On mirrors, strong dispersion depending on sample roughness
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Signature and clean mirror samples
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EUV : Results on EC2216-contaminated mirrors
MA-1 (strong level, unknown) :
Scatter x10 scatter at all angles
MA-2 (slightly higher level, unknown) :
Contaminated scatter slightly higher than MA-1. Clean scatter very high
=> Significant scatter increase (even whenmasked by strong clean scatter)
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EUV : Results on composite-contaminated mirrors
MC-1 (10 µg/cm2) :
x10 scatter near specular, then drops to clean scatter (medium level)
MC-2 (20 µg/cm2) :
x20 scatter close to specular,then slightlyabove clean scatter (similar to MC-1)
⇒Significant scatter increase.
⇒Linear relationship with contam. Level?
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EUV : Results on EC2216-contaminated filters
=> Those levels of EC2216 contamination cause little scatter.
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FA-1 (7,35 µg/cm²) :
Scatter similar to clean scatter : few, small droplets
FA-2 (60 µg/cm²) :
Scatter similar to FA-1, but clean scatter much lower
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EUV : Results on composite-contaminated filters
=> No significant effect of contamination
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FC-1 (58 µg/cm²) :
Scatter similar to clean scatter (rather high)
FC-2 (52 µg/cm²) :
Scatter similar to clean scatter (medium level)
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EUV : Results on Silicone-contaminated filters
FS-1 (50 µg/cm²) :
Scatter not increased, but specular peaknarrowed
FS-2 (69 µg/cm²) :
Scatter slightly increased + specular peakmore narrowed
=> Non-scatter effect : focusing by drops?
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EUV : Results on heavily Silicone-contaminated filters
FS-3 ( 117 µg/cm 2) :
Measurements on droplets alone areas and aiming at large drop
Droplets :
• Slightly reduce transmission
• Specular shape unchanged
• Increased scatter
Drops :
• drastically reduce signal (high local contaminant thickness)
• Specular beam strongly narrowed (lenseffect ?)
• Scatter +/- similar to droplets
⇒Drops combined with droplets?
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ConclusionsFirst exploration of yet poorly known effects : It confirms how critical molecular contamination is fo r UV instruments.
Signal loss at Lyman α α α α (peak) :
o 40% (EC2216) => 60% (13h under flux)o 25-80% (composite, depending on droplet size) => 85% (14h under flux)o 75-97% (Silicone, depending on droplets) => opaque (2.5h under flux)
Signal loss at 17.4nm (peak) :
o 10-25% (EC2216 and composite)o 25-50% (Silicone) especially through large drops
Noticeable scatter increase:
o Especially on mirrors (EC2216 and composite), o Much less on filters (except heavy Silicone contamination)o Can be blurred by strong scatter of clean component (manufacturing dispersion)o Non-scatter effect : narrowing of specular beam by drops (lens effect ?)
Trouble in assessing contamination levels (EC2216) : o Water adsorption / desorption disturbs micro-weigh measurementso Non-reproducible behaviour between sampleso Comparison with Alu samplesmay not be relevant
⇒ More effort needed in assessing contamination levels a nd process
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UV losses and straylight by chemical contamination Dec. 11th 2018