1. Laser-IFE Final Optics Mini-Workshop
1. UCSD program on laser plasma and laser-material interactions
2. Damage-resistant optics research for IFE (including DP work)
UC San Diego 8 November 2000

2. Fusion Activities at UCSD
University of California, San Diego
Center for Energy Research

3. Part I. UCSD program on laser plasma and laser-material interactions
Contributors:
PI’s: M. Tillack, F. Najmabadi
staff: M. Zaghloul, T. K. Mau, S. S. and Bindhu Harilal
students: D. Blair, M. Cherry

4. Program elements Damage-resistant optics for IFE
Laser damage threshold of grazing-incidence metal mirrors
Environmental effects on laser optics
Beam propagation and breakdown physics
Breakdown physics at low gas pressure
Wavefront distortions beyond breakdown
Atmospheric effects
Damage resistant chamber materials
Thermomechanical modeling
Experiment planning
Z-machine proposal
X-ray source studies
Chamber physics
Modeling

5. Historical perspective
summer 1996: initial contact with LLNL
4/97-4/99: IUT with LLNL to explore IFE simulation experiments
7/99: start of grant with OFES, lab space obtained,
optics damage activity initiated
12/99: YAG laser installed
summer 2000: staff increased, collaboration with GA started
1/01: DP work planned on optics and chamber physics

6. Main laser parameters Spectra Physics Quantaray Pro 290
2J, 10 nm
700 nm
500 nm
300 nm
Peak power~1014 W/cm2
Nanolase dpssl:
10 mJ, <1 ns, 5 nm

7. Experiment Layout

8. Shack-Hartmann Sensor

9. Ring-Down Reflectometer
Jolin, Sanders and Turner, Boulder Damage Conference 1988.

10. Fabrication and surface analysis is provided by GA
Mirror Fabrication:
• Diamond turning
• Sputter coating
Surface Analysis:
• WYKO white light interferometer • SEM with energy dispersive x-ray analysis • Auger electron spectroscopy

11. Part II. Overview of damage-resistant optics research for IFE

12. Geometry of the final laser optics
(SOMBRERO values in red)
(30 m)
Prometheus-L reactor building layout

13. Threat Spectra 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* (~100 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

14. Mirrors and transmissive wedges are considered
Transverse energy J/cm2 possible
• Al at normal incidence ~0.2 J/cm2
• x10 due to cos q
• x10 due to increase in reflectivity
For 1.2 MJ driver w/ 60 J/cm2, each beam would be 0.4 m2
Grazing incidence metal mirror
Fused silica or CaF2 wedges

15. Mirrors vs. transmissive wedges
metal mirror
Fused silica wedge
Used in Prometheus-L and Sombrero
Tighter tolerances on surface finish
Low damage threshold larger optics
Used in DPSSL power plant study
Neutron damage
Increased absorption
B-integral effects

16. Operation of the fused silica wedges
Orth, Payne & Krupke, Nuclear Fusion 36(1) 1996.
Linear array used in DPSSL study, coupled to slab design of gain medium.
5˚ wedge, angled at 56˚
Amplifier slab
Key concern is laser absorption -- 8% after 1 hr. irradiation.
Operated at 400˚C for continuous annealing of defects
60 times worse at 248 nm vs. 355 nm

17. Why Aluminum is a Good 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

18. S-polarized waves exhibit high reflectivity at shallow angles of incidence

19. GIMM development issues*
Experimental verification of laser damage thresholds
Experiments with irradiated mirrors
Protection against debris and x-rays (shutters, gas jets, etc.)
In-situ cleaning techniques
Large-scale manufacturing
Cooling
Wavefront issues: beam smoothness, uniformity, shaping, f/number constraints
* from Bieri and Guinan, Fusion Tech. 19 (May 1991) 673.

20. Mirror defects and damage types

21. Fresnel Modeling of Reflectivity
metal substrate
Wave propagation in four layers of stratified
media is modeled. (Born & Wolf)
Each medium is homogeneous and characterized
by complex refractive index: n = n ( 1 + i k )
n = ( e/m )1/2; k = attenuation index
TE (S) polarization is assumed.
n4, k4
coating
n3, k3
n2, k2
contaminant
n1, k1 q1
Incident
medium
Refraction : n1 sin q1 = nj sin qj j = 2,3, ( Snell’s Law )
Reflection : ri,i+1 = (ni cos qi - ni+1 cos qi+1) / (ni cos qi + ni+1 cos qi+1) ( Fresnel )
Reflectivity is computed by repetitive usage of the 3-layer formula:
ri = [ri-1,i + ri+1 exp (i2bi)] / [1 + ri-1,i ri+1 exp (i2bi)]
where bi = (2p/lo) di ni cos qi , i = 2,3 and di is the layer thickness.

22. Auger electron spectroscopy surface analysis

23. Reflection of s-polarized (TE) waves including thin oxide coating
l=532 nm

24. 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
Carbon film thickness (nm)
Al2O3 coating thickness, d3/lo

25. Surface profile of undamaged Al-1100 mirror

26. SEM photos of damaged Al
50x
250x
1000x
4000x

27. Near-term experimental plans
Install Shack-Hartmann sensor and ringdown reflectometer
Examine source of grazing-incidence profile distortions
Test Al-1100 mirrors to damage fluence at 80-85˚ incidence; compare damage threshold and surface morphology with normal incidence exposure
Produce and test ultrapure Al and sputter-coated SiC mirrors
Perform tests in vacuum (chamber available after Jan. 2001)

28. Diffraction and Wavefront Distortions
Diffraction-limited spot size:
do = 4 l f M/pD
l = 1/3 mm
f = 30 m (distance to lens)
do = 200 mm (zoomed)
D = 1 m
M <16
“There is no standard theoretical approach for combining random wavefront distortions of individual optics” (ref: Orth)
Each l/3 of wavefront distortion translates into roughly a doubling of the minimum spot size (ref: Orth)

29. Neutron and gamma effects
Conductivity decrease due to point defects, transmutations, surface roughening
Estimated in Prometheus at ~0.5% decrease in reflectivity (ref: private conversation)
Differential swelling and creep
Swelling values of % per dpa in Al (ref. Prometheus)
The laser penetration depth is d=l/4pk where k>10, so the required thickness of Al is only ~10 nm. Swelling in Al can be controlled by keeping it thin. The substrate is the real concern.
Porous (10-15%) SiC is expected to have very low neutron swelling.
Absorption band at 215 nm in fused silica