Download PPT

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
(20 m)
(30 m)
Prometheus-L reactor building layout
(SOMBRERO
values in red)
Mirrors vs. transmissive wedges
metal mirror
Fused silica wedge
•
Used in Prometheus-L and Sombrero
•
Used in DPSSL power plant study
•
Tighter tolerances on surface finish
•
Neutron damage concerns:
•
Low damage threshold larger optics
(tends to result in less sensitivity to defects)
•
–
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
Thin, protective, transparent oxide
100
Reflectivity, %
Good reflectance into the UV
Normal incidence reflectivity of metals
95
90
Al
85
Ag
Au
Normal incidence damage threshold
~0.2 J/cm2 @532 nm, 10 ns
80
250
500
750
Wavelength, nm
1000
S-polarized waves exhibit high
reflectivity at shallow angles of incidence
Al2O3 reflectivity at 532 nm
Aluminum reflectivity at 532 nm
1
1
s-polarized
0.75
Reflectivity
Reflectivity
0.95
0.9
p-polarized
0.85
0.5
0.25
0.8
0
0.75
0
10
20
30
40
50
60
Angle of incidence
70
80
90
0
10
20
30
40
50
60
angle of incidence
70
80
90
Reflection of s-polarized (TE) waves
including thin oxide coating
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˚
Brewster's angle
Amplifier slab
62 cm
•
•
•
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
57 cm
Threat Spectra
Two main concerns:
• 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
Nonuniform sputtering by ions
Wavefront distortion of <l/3* (~100 nm)
(6x108 pulses in 2 FPY:
2.5x106 pulses/atom layer removed
Defects and swelling induced
by g-rays and neutrons
Absorption loss of <1%
Wavefront distortion of < l/3
Contamination from condensable
materials (aerosol and dust)
Absorption loss of <1%
>5 J/cm2 threshold
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=1m
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)
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
(unknown)
Ne gas
continuous anneal
Contamination
gas jet/shutter,*
cleaning system
mechanical shutter,
plasma closure
not addressed
*per Bieri
Laser damage threshhold of GIMM’s
• If damage threshold scales as (1-R), then we should be able to obtain
J/cm2 at 85˚.
• With cos q=0.0872, the transverse energy is >20 J/cm2
• For a 1.2 MJ driver energy and 60 beams, each beam is ~1 m2
85ϋ
1m
stiff, lightweight, actively cooled, neutron transparent substrate
11.5 m
2
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)
10~15 m
coverage fraction ~1%
(60 beams)
1m
1 Torr
25 cm
1 mTorr
pulsed or
steady
gas puff
chamber
• Possible solution: evaporation/recondensation
• Reduce pressure difference (e.g., 10 mTorr --> 100 mTorr)
vacuum vessel
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=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
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
h
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
ARIES-AT
ARIES-ST
He-cooled Ferritic Steel
18
10
3.5
SiC
ARIES-RS
232
Pb83Li17
18
250