1. Final Optics Update
Jeff Latkowski, Ryan Abbott, Brad Bell, and Tom Felter
HAPL Program Workshop
Rochester Laboratory for Laser Energetics
November 8-9, 2005
Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.

2. Overview
Ions cannot be allowed to strike the final optic at high fluxes/fluences
Laser damage testing to begin for (highly) neutron-irradiated fused silica
Neutrons and gammas can cause significant damage, even at the penultimate optic and/or vacuum windows
Three types of optics are under consideration:
Grazing incidence metal mirror (UCSD)
Thin Fresnel lens
Multilayer dielectrics

3. Ions will cause severe problems for either final optic concept
Aluminum GIMM will exfoliate at a rate of microns/day
Fused silica Fresnel will melt in single shot
0.5 MeV a
3 MeV a
5.2 mA-hr
10.3 mA-hr
15.5 mA-hr
13 mA-hr
9.2 mA-hr
4.4 mA-hr
Equivalent to 3 days for IFE final optic at 26 m
Implantation at 90º (previous work suggests exfoliation more rapid at grazing angles)

4. Either final optic will need to rely upon ion deflection to survive
Ion flux reduction of 104× is the goal (reduces exfoliation to <l/10 in 1 year)
Helmholtz coil pairs would be placed ~13 meters from chamber center:
Field strengths of <0.1 T (in the center of the pair) would be needed for optic at 26 meters
Normal conductors should suffice and shielding looks doable
Without deflection and only 10 mTorr of xenon, majority of ions reach the final optic
With deflection, the ion flux is reduced by more than 104×

5. Basecase ion deflection parameters have been developed for the various options
All cases assume final optic stand-off of 26 meters
All cases use central field strength of 0.75 kG (0.075 T)
For material cost of $10/kg, all coil sets <$1M
Electrical cost of 5¢/kW-hr assumed
Copper current density limit sets minimum coil cross sections
Y. Iwasa (1994) reported Cu resistivity increase of 2.5% after fast neutron fluence of 2 × 1017 n/cm2. Significant ( ×) shielding will be needed.

6. It may be possible to reduce fields with alternate coil and/or beam orientations
Gunslit option for ion mitigation (e.g., use 4 x 1 array of beamlets & deflect along the short side)
Already may be desirable for GIMMs, where mirror ends up with 11:1 aspect ratio due to grazing angle
Alternate coil configurations (e.g., non-Helmholtz) will be considered
Deflect ions along short angle

7. The fused silica final optic story is taking shape
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs

8. The fused silica final optic story is taking shape, (Cont’d.)
Laser-induced damage in SiO2 has been studied extensively
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs

9. In 1995, SiO2 samples were irradiated to dose of 1011 rads (~3 months IFE equivalent)
Optical absorption looks acceptable at 2, 3w; thermal and “radiation” annealing studied; make thin (~0.5 mm) Fresnel lens for final optic; 4w absorption is too high due to E’ color center in SiO2; still may be material that can be annealed to work at 248nm.
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs

10. Additional annealing runs have been completed
Previous neutron-induced absorption confirmed by annealing remaining samples at 380ºC
Higher temperatures used for ½ of samples:
Additional improvement in 2, 3 transmission
With higher temperature, even 4 transmission has improved (E’ color center appears to anneal at 500ºC)
l (nm)
T (%)
248 56
351 96
532 97 72

11. Laser damage testing of neutron- irradiated fused silica will occur in 2006
Samples have been annealed and polishing will be completed in November. ES&H approval on testing plans in place. Laser system (2, 3) identified. Laser damage testing of the same samples will begin around first of year.
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs

12. Fabrication of fresnel lenses that meet IFE specifications will be demonstrated
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs
Holographic technology has been demonstrated at 1mm thickness. Adapting this to 0.5mm is doable. Lithographic approach (needed for equatorial beams) needs to be demonstrated. 4-6” diameter samples will be fabricated.

13. Two-beam interference lithography can be used to fabricate the off-axis Fresnel lens segments
Laser, 351 nm
Launch
optics d2
TCC
Diverging beam d1
Fringe
locking
components
Collimated beam d3
Exposure set up at LLNL
Schematic illustration of the Fresnel lens pattern

14. Schematic of the lithographic process for the Fresnel lens panel fabrication
Photoresist
Substrate
Substrate
Photoresist
UV light
Develop
Photoresist
mask 1 (schematic)
mask 2 (schematic)
Substrate
Wet etch or dry etch
Schematic of the masks
Substrate
Schematic of the lithographic process
The lithographic approach is more appropriate for fabricating the equatorial plane HAPL final optics

15. Laser damage testing of Fresnel lenses will be conducted at 2 and/or 3
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs
Fresnels fabricated in previous step will be damage tested on Mercury

16. A target chamber will be installed on Mercury during FY2006

17. First Mercury experiments support HAPL: Damage testing of aluminum GIMMs
Goal: To validate GIMM mirrors for 5 4 ns IFE operating conditions
Experimental conditions:
Angle of Incidence: 85º  0.1º
Polarization: S
Wavelength: 1w, 2w, or 3w
Fluence: up to 10 4 ns
Vacuum level required: < 1 mtorr
Substrate sizes: 2” round, 4” square
Scatter/Incandescence detection camera
Beam Dump
GIMM
To beam diagnostic
(Energy, pulsewidth, Nearfield modulation)
To darkfield damage detection system

18. Fused silica Fresnel lenses could be segmented and irradiated
Given fused silica’s rapid saturation of defect concentrations, a facility such as ORNL’s HFIR might be able to provide relevant neutron doses.
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs
Fresnels (or a portion of them) will be irradiated and the optical absorption will be verified.

19. Irradiated SiO2 Fresnels could be tested on Mercury to provide the ultimate admiral’s test
The same Fresnels then could be damage tested in Mercury using the same methods utilized in the second step of our optics testing plan.
Laser damage threshold is sufficient
Optical absorption (with neutrons) is reasonable
Thin Fresnel lenses can be made with required specs
Success?

20. DPSSL basecase design uses only a single optic in the target bay
TC, 18m ID, 20m OD
20m
“Neutron pinhole”
Telescope brings beams to focus near center of target bay shielding wall
Final optic (Fresnel) focuses and deflects beam towards chamber center

21. Other designs must consider radiation damage to more than just the final optic
Previous work with 3-D SOMBRERO neutronics model showed that neutron scattering was quite important:
Penultimate optic dose might only be reduced by ~50x compared to final optic
Can use shielded beamtubes (see Sawan work on these)
Damage testing needed for dielectric, multilayer mirrors in addition to GIMMs and Fresnels
Given importance of scattered neutrons, even vacuum windows at bottom of building may be a concern:
They must be quite thick
Even low neutron doses (e.g., Mrads) darken SiO2, especially at 4

22. We would like to begin radiation studies for dielectric materials
Given large variety of materials & wavelengths, suggestion made (by Bayramian?) to test individual layer materials as first step:
Consider Al2O3, CaF2, SrF2, SiO2, etc.
Literature search on any known rad effects and unirradiated properties (index of refraction, optical transmission, CTE, etc.)
Irradiate 4-8 materials to moderate dose (Mrads on HFIR?)
Complete annealing runs, testing optical properties at each step
Downselect to most rad-resistant materials; identify best material combinations
Produce multilayers of best candidates at 2-4
Laser damage test virgin multilayers
Downselect again and irradiate best multilayers; Repeat damage testing
Given work on irradiated fused silica, LLNL ready to perform optical testing of rad materials
However, overall HAPL cuts make it unlikely we can start work this year

23. Synthesis High ion fluxes must be kept off the final optics
Ions can be deflected away from the final optics with reasonable magnetic fields and at a reasonable cost
We are working towards an integrated, self-consistent design for a fused silica Fresnel lens as the final optic:
Damage testing of irradiated fused silica and production of prototypical Fresnel lenses are the next steps
Laser damage testing of both final optic candidates will be performed on Mercury this fiscal year
Other optics, if used within the target bay, must be radiation tested as well