M. S. Tillack

Final Optic Research – Progress and Plans

HAPL Project Meeting, PPPL27-28 October 2004

Z. Dragojlovic, F. Hegeler, E. Hsieh, J. Mar, F. Najmabadi,

J. Pulsifer, K. Sequoia, M. Wolford

with contributions from:

Overview

1. Final optic program summary

2. New mirror fabrication and testing

3. Larger scale testing

4. Contaminant transport modeling

5. Gas puff modeling

The steps to develop a final optic for a Laser IFE power plant

(1 of 2)

1. “Front runner” final optic – Al coated SiC GIMM:UV reflectivity, industrial base, radiation resistance

2. Characterize threats to mirror:LIDT, radiation transport, contaminants

Key Issues:• Shallow angle stability• Laser damage resistance

goal = 5 J/cm2, 108 shots

• Contamination• Optical quality• Fabrication• Radiation resistance

3. Perform research to explore damage mechanisms, lifetime and mitigation

MicrostructureBonding/coating

q”=10 mJ/cm2Al: 20-500 nmSiC: 10 μm

Fatigue Ion mitigation

~50 cm85˚

6. Perform mid-scale testing5. Develop fabrication techniques and advanced concepts

The steps to develop a final optic for a Laser IFE power plant (2 of 2)

4. Verify durability through exposure experiments

10 Hz KrF laserUCSD (LIDT)

XAPPERLLNL (x-rays)

ion accelerator neutron modeling and exposures

Diamond-turned, electroplated mirrors survived 105 shots at 18 J/cm2 on a small scale

(mm2)

... and we would like to improve the high-cycle

fatigue behavior

Still, these mirrors ultimately fail due to grain

motions, ...

1.Relatively small grains (10-20 μm)2.Relatively dense, thick coating

35 μm “thick thin-film” mirror,

turned at Schafer Corp. and exposed to 104 shots at 5

J/cm2

no damage to elecroplated mirror (turned at GA)

under the same exposure conditions

Post-processing after thick (35-50 μm) thin-film deposition should provide good optical

quality with a damage-resistant microstructure

rough substrate

polish/turn coat final polish/turn

Ringdown reflectometry (now @266 nm) indicates somewhat high

absorption at 85˚

<1 nsnanolaserpolarizertest specimenphotodiodeoutput coupleroutput couplerlens

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are needed to see this picture.

reflectivity of 35 μm Schafer mirror

Diamond turning lines are too deep – 50 nm rms –

(A new Pacific Nanotechnolgy AFM has

been added to our surface analysis

capabilities)

Peaks grow during exposure (unlike earlier results which exhibited etching)

etching observed previously in diamond-turned polycrystalline

foils

It’s time to start making smoother mirrors

MRF systems are popping up all over the place(this one is at Edmund Optics)

Larger mirrors are being fabricated with increasing emphasis on surface

quality

2. Other improvements under consideration

• MRF surface finishing

• Hardening techniques• nanoprecipitate, solid solution hardening• friction stir burnishing (smaller grains)

1.Mid-scale 4” optics• Thick e-beam coatings• Electroplated Al

Scaled testing was initiated at Electra during late August

we spent 1 week assembling the optical path, developing test procedures, and exploring issues for large scale testing

Experimental Layout

43”12”

Lens

Beam Dump

Wave Plate

Cube

Mirror

Beam Sampler

Beam Profiler

UV Window

WindowCamera

Laser energy measurements showed dramatic energy loss along the beam

path

vacuumchamber

telescope

Nikemirror

periscope

1” aperture

1/2waveplate

polarizer cubes

3” lead aperture

2” graphite aperture Electra oscillator

0.57 J

5.2 J

3.9 J

0.14 J

p-polarized

1.04 J

10 cm

10 cm

12.8 J(measured with a

30cm x 30 cmcalorimeter)

14.2 – 15.3 J(measured with a30 cm x 30 cm

calorimeter)13.2 J

with a 2” dia.aperture

80 cm

0.14 J to 5.2 J(measured with a2” calorimeter)

We don’t see this with our Compex laser

1 2 3 4

1 = 86 mJ2 = 84 mJ

1

2

3 45

6

78

1 = 228 mJ2 = 119 mJ3 = 95 mJ4 = 92 mJ

5 = 13 mJ6 = 75 mJ7 = 58 mJ8 = 56 mJ

3 = 86 mJ4 = 85 mJ

An alternative idea for scaled testing:large-aperture uncoated FS window

@56˚

10” diameter, 6-m fl Nike lens

700 J blunderbuss

8” port

12” FS window($5250)

30 cm squareaperture

34˚

10” roundaperture

30 cm

6.7”

10”assume 700 J in 900 cm2 ~ 0.75 J/cm2

~25% of s-light reflected = 0.09 J/cm2

10” round on 6x12 rectangle ~ 362 cm2

35 Joules (polarized) available

beam dump

chamber

Another alternative idea for scaled testing:

Contrast is >100:1 over a 7˚ range

10” diameter, 6-m fl Nike lens700 J

blunderbuss

8” port

12” FS window

30 cm squarebeam with 9” round aperture

32˚

12”

6”

• assume 700 J in 900 cm2 ~ 0.75 J/cm2

• ~25% of s-light reflected = 0.09 J/cm2

• 9” round ~ 410 cm2

• 37 Joules (polarized) available

beam dump

chamber

Displacement field after 1st shot

• Net flow toward chamber center is predicted

– we need to include rad-hydro displacements

• Net flow toward optic?

Contamination transport from the chamber to the final optic was explored using Spartan

• 160 MJ NRL target

• 50 mTorr Xe @RT

• Bucky hand-off at 500 μs

Particles transport rapidly toward thefinal optic

• We need to run multiple shots to establish the long-term behavior

Test particle trajectories Pressure at 100 ms

Pa

1

2

3

4

Gas puffing was examined as a posssible optic protection

technique

• ~1 Torr-m may help reduce ion and x-ray damage

• Fast gas puff could be used immediately preceding implosions

• Might also help cool chamber gas

A gas puff sufficient to protect optics would increase the base pressure beyond

100 mTorr

Pump speed per duct 1.5x105 l/s

Duct diameter 75 cm

Duct length 3 m

Number of ducts 64

Orifice conductance 44 l/s/cm2

Target mass 4 mg

Rep rate 5 Hz

Chamber radius 7 m

It doesn’t look promising!

electroplatesuccess

5-yr plan and progress to date

2001 2002 2003 2005 2006

start KrF larger optics

initial promising results at 532 nm

attempts at thin film optics

Phase I evaluation

lower limits at 248 nm, chemistry control

new lab,cryopum

p

extended database,

mid-scale testing,radiation damage,

mirror quality, design integration

2004