The final laser optic:options, requirements & damage threats

Mark S. Tillack

ARIES Project Meeting

Princeton, NJ

18-20 September 2000

Geometry of the final laser optics

Prometheus-L reactor building layout

(30 m)

(SOMBRERO values in red)

(20 m)

Mirrors vs. transmissive wedges

• Used in Prometheus-L and Sombrero

• Tighter tolerances on surface finish

• Low damage threshold larger optics (tends to result in less sensitivity to defects)

Fused silica wedgemetal mirror

• Used in DPSSL power plant study

• Neutron damage concerns:

– absorption, color centers

• B-integral effects

Why Aluminum is a Good Choice

Multi-layer dielectric mirrors are doubtful due to rapid degradation by neutrons

Al is a commonly used mirror material• usually protected (Si2O3),

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 J/cm2 @532 nm, 10 ns

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

Reflection of s-polarized (TE) waves including thin oxide coating

Operation of the fused silica wedges

• Linear array used in DPSSL study, coupled to slab design of gain medium.

• 5˚ wedge, angled at 56˚

62 cm

57 cm

Orth, Payne & Krupke, Nuclear Fusion 36(1) 1996.

Brewster's angle

• 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

Amplifier slab

Threat Spectra

Final Optic Threat Nominal Goal

Optical damage by laser >5 J/cm2 threshold (normal to beam)

Nonuniform ablation by x-rays Wavefront distortion of </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 -rays and neutrons Wavefront distortion of < /3

Contamination from condensable Absorption loss of <1%materials (aerosol and dust) >5 J/cm2 threshold

• Damage that increases absorption (<1%)

• Damage that modifies the wavefront –

– spot size/position (200m/20m) and spatial uniformity (1%)

Two main concerns:

Diffraction and Wavefront Distortions

Diffraction-limited spot size:

do = 4 f M/D

= 1/3 m

f = 30 m (distance to lens)

do = 200 m (zoomed)

D = 1 m

M <16

• “There is no standard theoretical approach for combining random wavefront distortions of individual optics” (ref: Orth)

• Each /3 of wavefront distortion translates into roughly a doubling of the minimum spot size (ref: Orth)

Proposed Design Solutions

Threat SOMBRERO Prometheus-L DPSSL study

Laser damage mirror size mirror size, coatings continuous anneal

X-ray ablation gas jet/shutter* Xe gas, plasma closure (1 Torr Ar)

Ion sputtering gas jet/shutter* Xe gas , plasma closure not addressed

Radiation damage lifetime limit Ne gas continuous anneal(unknown)

Contamination gas jet/shutter,* mechanical shutter, not addressedcleaning system plasma closure

*per Bieri

Laser damage threshhold of GIMM’s

• If damage threshold scales as (1-R), then we should be able to obtain 2 J/cm2 at 85˚.

• With cos =0.0872, the transverse energy is >20 J/cm2

• For a 1.2 MJ driver energy and 60 beams, each beam is ~1 m2

85˚

stiff, lightweight, actively cooled, neutron transparent substrate

1 m

11.5 m

1 m 25 cm

coverage fraction ~1%(60 beams)

pulsed or steady gas puff

1 Torr

1 mTorr

10~15 m

vacuum vessel

chamber

Gas protection of beamlines

• Beamline volume = 7.7 m3

• Mass @1 Torr = 60 g (7700 Torr-liters)

• A credible turbopump speed is 50 m3/s (50 Torr-l/s @1 mTorr)

• Possible solution: evaporation/recondensation

• Reduce pressure difference (e.g., 10 mTorr --> 100 mTorr)

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) -- need to check this

• Differential swelling and creep

– Swelling values of 0.05-0.1% per dpa in Al (ref. Prometheus)

– The laser penetration depth is d=/4 where >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

Final Optics Tasks

• Re-assess protection schemes in more detail– In previous studies, issues were identified and potential design

solutions proposed, but detailed analysis of phenomena was not performed

• Correlate damage mechanisms with beam degradation– Estimate defect and contamination rates from all threat spectra

– Analyze result of mirror defects and deformations on beam characteristics

• System integration– Flesh out the beam steering and alignment issues

– Integrate with target injection and tracking system

High conversion efficiency is achievable with wall temperatures under 1000˚C

First wall material TFW Tcoolant

ARIES-RS vanadium alloy 700˚C 610˚C 45%

ARIES-ST ODS ferritic steel 600˚C 700˚C 45%

ARIES-AT SiC/SiC 1000˚C 1100˚C 59%

Blanket designs for high efficiency

• Use neutrons (80% of power) to maximize outlet temperature• Segment radially and optimize routing• Use thermal insulation if necessary• Optimize conversion cycle

18

232

3.5

250

18

10

Pb83Li17

SiC

He-cooled Ferritic Steel

ARIES-RS

ARIES-ST

ARIES-AT