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

2. Statement of Purpose and Deliverables
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

3. 1. Background

4. Reference Geometry of the Final Optics
(SOMBRERO values in red)
(30 m)
Prometheus-L reactor building layout

5. Damage Threats Two main concerns: Final Optic Threat Nominal Goal
Damage that increases absorption (<1%)
Damage that modifies the wavefront –
spot size/position (200mm/20mm) and spatial uniformity (1%)
Final Optic Threat Nominal Goal
Optical damage by laser >5 J/cm2 threshold (normal to beam)
Nonuniform ablation by x-rays Wavefront distortion of <l/3 (~80 nm)
Nonuniform sputtering by ions (6x108 pulses in 2 FPY:
2.5x106 pulses/atom layer removed
Defects and swelling induced Absorption loss of <1%
by g-rays and neutrons Wavefront distortion of < l/3
Contamination from condensable Absorption loss of <1%
materials (aerosol and dust) >5 J/cm2 threshold

6. 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 J/cm2, each beam would be 0.4 m2

7. Aluminum is the 1st choice for the GIMM
Lifetime of multi-layer dielectric mirrors is questionable due to rapid degradation by neutrons Al 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 threshold ~0.2 nm, 10 ns

8. 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.

9. 2. Experiments

10. • superpolished fused silica (2”, l/10 , $335)
Fabrication capabilities are important to enable us to optimize mirror performance
Mirror Fabrication:
• Diamond turning
• Sputter coating
Substrates considered:
• Bulk Al (cheap)
• CVD SiC ($550 for 3-cm disk, l/50, <2Å)
• superpolished fused silica (2”, l/10 , $335)
• 30-cm Si wafers (free)
Rohm & Haas SiC:
l/50 flat, <2Å
Si wafers (TBD)
1.5 x 15 cm diamond-turned Al flat

11. SEM photos of damaged Al Surface profile of undamaged Al mirror
Surface Analysis
Surface Analysis:
• WYKO white light interferometer • SEM with energy dispersive x-ray
• Auger electron spectroscopy
50x
1 mm
SEM photos of damaged Al
1000x
Surface profile of undamaged Al mirror
20 mm

12. The UCSD laser lab is used to test GIMMs
Spectra Physics QuantaRay laser:
2J, 10 nm
700, 500, , 266 nm
Peak power~1014 W/cm2

13. A ring-down reflectometer is used to obtain accurate measurements of reflectivity
specimen

14. A Shack-Hartmann sensor is used to measure wavefront changes
Spherical wave from a pinhole: 144 mm spatial resolution l/50 sensitivity

15. 2. Modeling

16. Tools for modeling effects of damage on beam characteristics

17. 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,i+1 = (ni cos qi - ni+1 cos qi+1) /
(ni cos qi + ni+1 cos qi+1)
Reflectivity for 3 layers:
ri = [ri-1,i + ri+1 exp (i2bi)] / [1 + ri-1,i ri+1 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 = |r2|2
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

18. 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.
d2=0
q1 = 80o
d2=0
q1 = 0o
80o
60o
reflectivity
reflectivity
d2=2 nm
q1 = 80o
40o
lo = 532 nm
Al2O3 coating (10 nm)
Al mirror
20o
lo = 532 nm
Carbon film
Al mirror
d2=2 nm
q1 = 0o
q1 = 0o
Al2O3 coating thickness, d3/lo
Carbon film thickness (nm)

19. Ray Tracing
When surface defect d > 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).

20. Scattering Theory
When surface deformation d < l, scattered wave is composed of specular
and diffuse components.
Two analysis approaches:
- Perturbation theory (Raleigh-Rice): d << l
- Physical optics (Kirchoff): d < 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.

21. Final Optic Threats and Planned Research Activities

22. Final Optics Program Plan
FY 2001 | FY 2002 | FY | 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

23. Final Optic Threats and Planned Research Activities