Transcript ppt

Prometheus-L Reactor Building Layout
Two Main Options for the Final Optic
(1) SiO2 or CaF2 wedges
85ϋ
40 cm
stiff, lightweight, actively cooled, neutron transparent substrate
4.6 m
(2) Grazing incidence metal mirror
Neutrons and g-rays create defects in SiO2
which result in photon absorption
MeV n0
Si
Oxygen Deficient Center
(ODC, 246 nm)
Si
MeV g-rays
Si+
Normal Site
O
+
Si
Si
Non-Bridging Oxygen Hole
Center (NBOHC, 620 nm)
O- O-
Si
O2-
Si
annealing
annealing
Si
E’ Center (213 nm)
Si
O
Normal Site
Si
SiO2 samples irradiated at LANSCE
for 1011 rad
Final Optic Damage Threats
Two main concerns:
• Damage that increases absorption (<5%)
• 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)
Sputtering by ions
Ablation by x-rays
(~25 mJ/cm2, partly stopped by gas)
Wavefront distortion of <l/3 * (~100 nm)
(6x108 pulses in 2 FPY:
2.5x106 pulses/allowed atom layer removed)
Defects and swelling induced by
g-rays (~3) and neutrons (~18 krad/s)
Absorption loss of <1%
Wavefront distortion of < l/3 *
Contamination from condensable
materials (aerosol and dust)
Absorption loss of <1%
>5 J/cm2 threshold
* “There is no standard theoretical approach for combining random wavefront distortions of individual optics.
Each l/3 of wavefront distortion translates into roughly a doubling of the minimum spot size.” (Ref. Orth)
Si2O irradiated at 105˚C, thermally
annealed at 380˚C
MeV g / n° irradiation of CaF2 at room temperature
also leads to the formation of color centers
60
Mrad g-rays
11 MRad
g-rays(60
( Co),
Co),post-annealing
post-annealing
o
766 kRad
kRad n˚
766
n
0.4
-1
Abs.Coeff. (cm )
Absorption Coefficient (cm-1)
0.5
0.3
0.2
0.1
o
Annealed at 385
385˚C
Annealed
C
0.0
200
300
400
(Goal ~0.01/cm)
500
600
700
800
900
1000
Wavelength (nm)
•
•
•
•
10 MRad g-irradiation (60Co) yields no color centers for virgin sample
Absorptions at 335, 410, and 540 nm due to color centers
• Color center is a “missing fluorine” that captures an electron
Annealing removes absorption due to n0 induced color centers
CaF2 is “softened” by n0 (g-irradiation induces color center of annealed sample)
GIMM development issues*
• Experimental verification of laser damage thresholds
• Protection against debris and x-rays (shutters, gas jets, etc.)
• Wavefront issues: beam smoothness, uniformity, shaping,
f/number constraints
• Experiments with irradiated mirrors
• In-situ cleaning techniques
• Large-scale manufacturing
• Cooling
* from Bieri and Guinan, Fusion Tech. 19 (May 1991) 673.
Normal incidence reflectivity of various metals
1
0.8
Reflectivity
0.6
0.4
0.2
Ag
Al
Cu
W
Au
Hg
Mo
0
200
400
600
Wavelength, nm
800
1000
Shallow angle reflectivity measurements of
undamaged surfaces
1
Reflectivity
0.95
no oxide
10 nm
0.9
20 nm
30 nm
Al 6061
Al 1100
0.85
0
10
20
30
40
50
Angle
60
70
80
90
Surface deformation leads to roughening
and loss of laser beam quality
Single Shot Effects on LIDT:
Laser heating generates point defects
Coupling between diffusion and elastic
fields lead to permanent deformation
 Progressive Damage in Multiple Shots:
 F1 varies from a few to ~ 10 J/cm2.
Thermoelastic stress cycles shear atomic
planes relative to one another (slip by
dislocations)
 LIDT is a strong function of material
& number of shots – it degrades up to
a factor of 10 after only 10000 shots
(survival to ~108 shots is needed).
Extrusions & intrusions are formed when
dislocations emerge to the surface, or by
grain boundary sliding.
 Uncertainty in saturation behavior
Laser Damage Experiments to Metal Mirrors
fluence
Spectra Physics YAG laser:
2J, 10 ns @1064 nm;
800, 500, 300 mJ @532, 355, 266 nm
Peak power density ~1014 W/cm2
Several kinds of Al surfaces have been
fabricated and characterized
75 nm Al on superpolished flat:
±2Å roughness, 10Å flatness
diamond-turned Al 6061
MgSi occlusions
Al 1100 showing grain boundaries
and tool marks
Surface deformation patterns after one
laser shot of intensity near LIDT
(Solution of continuum equations with defect diffusion in the self-consistent elastic field)
Focused Laser-induced Surface
Deformation (vacancy density
correlates with deformation)
Computer Simulation
Experiment
Uniform Laser-induced
Surface Deformation
Computer Simulation
(The model correctly predicts number of arms)
Focused laser-induced surface deformation (Lauzeral, Walgraef
& Ghoniem, Phys. Rev. Lett. 79, 14 (1997) 2706)
(Walgraef, Ghoniem & Lauzeral, Phys.
Rev. B, 56, 23, (1997) 1536)
Progressive damage in multiple shots is caused by
successive dislocation slip & grain boundary sliding
Low Density
High Density
A reflectometer accurately measures reflectivity
100 ppm accuracy
partially-reflective
spherical output coupler
photodiode
Damage to aluminum at grazing angles
Several shots in Al 6061 at 80˚, 1 J/cm2
1000 shots in Al 1100 at 85˚, 1 J/cm2
MgSi
Fe
Fe
1000x
1000x
Silicide occlusions in Al 6061 preferentially absorb light, causing explosive
ejection and melting at only 1 J/cm2;
Fe impurities appear unaffected
Exposure of Al 1100 to 1000 shots at
85˚ exhibited no damage up to 18 J/cm2
For more information….
http://aries.ucsd.edu/IFE
http://lasers.llnl.gov/lst/advanced.html
http://puma.seas.ucla.edu/web_pages
Damage Regimes for Al-1100
Damage Regimes for 99.999% pure Al
Damage to aluminum at grazing angles
10000 shots in Al 1100 at 85˚, 20 J/cm2
Exposure of Al 1100 to 10000
shots at 85˚ exhibits catastrophic
damage at fluence >20 J/cm2
Single pulse in pure Al at 85˚, 180 J/cm2
99.999% pure Al survives single
shot damage up to the melting limit