Laser Induced Damage Threshold LIDT of Grazing Incidence Metal Mirrors

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Laser Induced Damage Threshold (LIDT) of
Grazing Incidence Metal Mirrors

• Mark S. Tillack
• T. K. Mau
• Mofreh Zaghloul
• (S. S. and Bindhu Harilal)

Laser-IFE Program Workshop February 6-7,
2001 Naval Research Laboratory
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Statement of Purpose and Deliverables
Statement of purpose Our research seeks to
develop improved understanding of damage
mechanisms and to demonstrate acceptable
performance of grazing incidence metal mirrors,
with an emphasis on the most critical concerns
for laser fusion. Through both experimen-tation
and modeling we will demonstrate the limitations
on the operation of reflective optics for IFE
chambers under prototypical environmental
conditions.
Deliverables Measure LIDT at grazing incidence
with smooth surfaces. June 30, 2001 Model
reflectivity and wavefront changes of smooth
surfaces. Aug. 30, 2001 Measure effects of
defects and surface contaminants on
reflectivity, LIDT and wavefront. Jan. 31,
2002 Model reflectivity and wavefront changes
due to defects and Jan. 31, 2002 contamination.
Budget 330k
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1. Background
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Reference Geometry of the Final Optics
(20 m)
(SOMBRERO values in red)
(30 m)
Prometheus-L reactor building layout
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Damage Threats
Two main concerns

• Damage that increases absorption (lt1)
• Damage that modifies the wavefront
• spot size/position (200mm/20mm) and spatial
  uniformity (1)

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Mirror Parameters
Grazing incidence metal mirror
Al at normal incidence (1/3 or 1/4 mm) 0.2
J/cm2 x10 due to cos q x10 due to increase in
reflectivity Transverse energy 20 J/cm2 is
possible
For 1.2 MJ driver w/ 60 beams _at_5 J/cm2, each beam
would be 0.4 m2
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Aluminum is the 1st choice for the GIMM
Lifetime of multi-layer dielectric mirrors is
questionable due to rapid degradation by
neutronsAl is a commonly used mirror
material usually protected (Si2O3, MgF2,
CaF2), but can be used bare easy to
machine, easy to deposit

• Good reflectance into the UV
• Thin, protective, transparent oxide
• Normal incidence damage threshold0.2 J/cm2
  _at_532 nm, 10 ns

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GIMM development issues are well established

• Experimental verification of laser damage
  thresholds
• Wavefront issues beam smoothness, uniformity,
  shaping, f/number constraints
• Experiments with irradiated mirrors
• Protection against debris and x-rays (shutters,
  gas jets, etc.)
• In-situ cleaning techniques
• Large-scale manufacturing
• Cooling

from Bieri and Guinan, Fusion Tech. 19 (May
1991) 673.
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2. Experiments
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Fabrication capabilities are important to enable
us to optimize mirror performance
Substrates considered Bulk Al (cheap) CVD
SiC (550 for 3-cm disk, l/50, lt2Å)
superpolished fused silica (2, l/10 , 335)
30-cm Si wafers (free)
Mirror Fabrication Diamond turning Sputter
coating
Rohm Haas SiC l/50 flat, lt2Å
Si wafers (TBD)
1.5 x 15 cm diamond-turned Al flat
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Surface Analysis
50x
Surface Analysis WYKO white light
interferometer SEM with energy dispersive
x-ray Auger electron spectroscopy
1 mm
SEM photos of damaged Al
1000x
20 mm
Surface profile of undamaged Al mirror
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The UCSD laser lab is used to test GIMMs
Spectra Physics QuantaRay laser 2J, 10 ns _at_1064
nm 700, 500, 300 mJ _at_532, 355, 266 nm Peak
power1014 W/cm2
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A ring-down reflectometer is used to obtain
accurate measurements of reflectivity
specimen
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A Shack-Hartmann sensor is used to measure
wavefront changes
Spherical wave from a pinhole 144 mm spatial
resolution l/50 sensitivity
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2. Modeling
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Tools for modeling effects of damage on beam
characteristics
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Fresnel Modeling of Reflectivity

• Wave propagation in four stratified layers of
• media is modeled, each with complex n
• Refraction n1 sin q1 nj sin qj j
  2,3,4
• Reflection ri,i1 (ni cos qi - ni1 cos
  qi1) /
• (ni cos qi
  ni1 cos qi1)
• Reflectivity for 3 layers
• ri ri-1,i ri1 exp (i2bi) / 1
  ri-1,i ri1 exp (i2bi)

metal substrate
n4, k4
coating
n3, k3
n2, k2
contaminant
n1, k1
q1
Incident medium

• where bi (2p/lo) di ni cos qi , i 2,3
  and di is the layer thickness.
• Overall intensity reflectance R r22

Tasks Examine effects of coating material and
contaminant on mirror optical properties, and
compare with experiment Assess importance of
transmutation on optical properties of coating
and substrate
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Example Effect of Surface Contaminants

• Surface contaminants (such as carbon) on
  mirror protective
• coatings can substantially alter
  reflectivity, depending on
• layer thickness and incident angle.
• Uniform film thickness is assumed.

d20 q1 80o
d20 q1 0o
80o
60o
reflectivity
reflectivity
d22 nm q1 80o
40o
20o
lo 532 nm Al2O3 coating (10 nm) Al mirror
lo 532 nm Carbon film Al mirror
d22 nm q1 0o
q1 0o
Al2O3 coating thickness, d3/lo
Carbon film thickness (nm)
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Ray Tracing

• When surface defect d gt l, the effect on beam
  propagation can be assessed using a ray tracing
  approach.
• ZEMAX-EE optics design software will be used
• - User-defined surfaces (shape, optical
  properties)
• - Complete polarization ray tracing
• - Nonlinear model of thermal effects on index
• of refraction and material expansion.

Tasks - Evaluate surface deformation from
expected loads. - Quantify allowable surface
deformation (shape and size) to meet
beam propagation requirements (spot
size/location, intensity uniformity, absorption)
.
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Scattering Theory

• When surface deformation d lt l, scattered wave
  is composed of specular
• and diffuse components.
• Two analysis approaches
• - Perturbation theory (Raleigh-Rice) d
  ltlt l
• - Physical optics (Kirchoff) d lt l
• Surface roughness characterized by surface
  height distribution, d(r).
• For Gaussian d(r), overall scattered
  intensity is in the form
• Isc Io e-g Id
• where Io scattered intensity from flat
  surface,
• g f (s/l, q1, q2), s rms
  height, q1, q2 incident, reflected angle
• Id scattered diffuse intensity

Tasks - Characterize surface damage using
measured data and/or modeling - Evaluate
scattered intensity onto targets in terms of wave
front distortion and depolarization, using
analytic (and numerical) models.
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Final Optic Threats and Planned Research
Activities
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Final Optics Program Plan






FY
2001 FY 2002 FY 2003 FY 2004 FY2005
RADIATION DAMAGE (neutron and gamma effects)
Scoping Tests Irradiation PIE (incl. annealing)
Extended testing of prime candidates
Damage modeling
LASER-INDUCED DAMAGE
LIDT scoping tests for GIMM, materials development
System Integration
Laser damage modeling, 3w data from NIF
CONTAMINATION THREATS
Modeling
Test simulated contaminants
Mitigation
System Integration
X-RAY ABLATION
Scoping tests (laser-based x-ray source)
Mitigation
System Integration
Modeling
ION SPUTTERING
Calculate sputtering, gas attenuation
Mitigation
System Integration
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Final Optic Threats and Planned Research
Activities