AntiReflection Coated Silica Windows for Electra

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Anti-Reflection Coated Silica Windows for Electra
Stuart Searles, John Sethian Naval Research
Laboratory Washington, D.C. Russell
Smilgys Science Applications International
Corporation McLean, Virginia November 1314,
2001
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Immediate Objective

• Acquire 30 cm Electra window with AR coating able
  to survive 5 hours of laser operation.
• Window Exposure
• 0.7 J/cm2 _at_ 5 Hz (100 ns pulse width)
• Radiation excited by 450 kV e-beam into
  laser gas
• Inside cavity 1 atm Ar, Kr 0.3
  fluorine, trace HF
• Outside cavity Air

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Electra Window Specifications

• Transmission at 248 nm (gt 99.5)
• Damage threshold 1.4 J/cm2 in 100 ns _at_ 5 Hz
• Surface quality
• 20-10 scratch-dig
• Maximum inclusion size 0.1 mm
• Survive highly corrosive environment
• - Long term exposure to fluorine
• - Exposure in combination with UV
• - Exposure in combination with radiation

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AR Coated Window Availability
Few vendors able and willing to coat large
windows. Current vendor supplies AR coated
fused silica windows 20 cm 20 cm for NIKE
amplifier. Survives hundreds of single laser
pulses _at_ 0.25 J/cm2 (100 nsec pulse duration).
Current vendor unproven to meet Electras
specifications.
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Electra Window Design
Fused silica windows (bare windows on order)
- Corning code 7980, KrF grade fused silica
Absorption _at_ 248 nm 0.001136/cm AR
(antireflection) coating on both surfaces
- Conventional AR coating on atmospheric side
Dielectric stack e.g. SiO2/MgF2
- Fluorine resistant AR coating on laser cavity
side Dielectric stack e.g. Al2O3/AlF3

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Technical Approach
Acquire 1 diameter coated windows - Coating
system at SAIC in McLean, Virginia - Commercial
vendors Test - Gas cell HF and F2 exposure
with and without UV - Lumonics laser laser
damage threshold Evaluate -
Spectrophotometry transmission, optical
constants - Microscopy optical, and SEM -
Interferometry ZYGO - Electra laser cavity
(planned) electrons, x-rays, UV

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Coating System at SAIC in McLean,VA
Equipped for electron beam evaporation with ion
beam assist (IBAD)
- Vacuum chamber 8 by 12 cylinder - Base
pressure 10-7 Torr - Up to four different coating
materials in a run - Accommodates sheets
up to 3060
- Fixture mounts six 1 substrates - Heats to
300C - Provision for cover sheet to block dust
during pump down
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Laser Damage Testing of Windows
Use Lumonics KrF laser to simulate Electra Laser
specification 30 ns pulse duration _at_ 10
Hz Spot size 2 3 mm Achieve up to 4
J/cm2 with focusing optics
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Exposure to Fluorine and HF

• Diatomic fluorine is highly reactive.
• Atomic fluorine is more reactive than diatomic
  fluorine.
• - Atomic fluorine created by UV
  photodisssociation.
• HF formed by reaction of fluorine with water
  vapor.
• - HF highly corrosive to optics.
• HF can be eliminated from Electra cavity by water
  vapor control.

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One Month Exposure to 5 F2

• Uncoated silica window
• - Strongly degraded surfaces
• Vendor C with AR coated surfaces
• - Low density of deep pits (up to 3 microns)
• - General surface unaffected (6 nm rms)
• Next test need to monitor and control HF
  levels

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ZYGO Oblique Plot, Vendor C

• One month exposure

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Combination of UV, Fluorine

• 4 J/cm2 600 pulses and 5 F2
• Vendor C window with AR coated surfaces
• - General surface unaffected
• - Three pits
• Vendor A window with AR coated surfaces
• - General surface unaffected

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ZYGO Oblique Plot, Vendor C
Exposure to UV, F2
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ZYGO Solid Plot, Vendor C
Exposure to UV, F2
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Plans
Extend tests to larger areas and develop test
procedures. Measure induced absorption in
fused silica rods to evaluate possible limitation
in the regime of millions of pulses. Proceed
with upgrade of coating system at SAIC to coat
windows up to 60 cm in size for Electra and NIKE.