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Progress on Tracking &
Engagement Demonstration
Presented by Lane Carlson1
M. Tillack1, T. Lorentz1,
N. Alexander2, G. Flint2, D. Goodin2, R. Petzoldt2
(1UCSD, 2General Atomics)
HAPL Project Review
NRL, Washington D.C.
October 30-31, 2007
Hit-on-the-fly experiment has demonstrated
engagement on moving target
1) Engaging moving targets (5 m/s) with a simulated driver beam by
using the glint return signal to steer a fast steering mirror.
2) Improved simulated driver beam and target engagement
verification system:
• 1 mm range
• 7 µm resolution
3) 150 Вµm (1пЃі) engagement for all targets with В± 1.5 mm placement,
(110 Вµm (1пЃі) with placement accuracy < В± 1 mm)
• Prior reported engagement was 20% of targets in range of verification
system (150 Вµm).
Final Key Requirement:
• 20 µm engagement accuracy in (x,y,z) at ~20 m (10-6)
We are continuing our effort on the “glint-only”
option table-top demo with help from Poisson spot
crossing
sensors
Poisson
spot
camera
alignment &
driver beam
(635 nm)
coincidence
sensor/PSD
C1
spatial
filter
C2
C3
drop
tower
pulsed glint
laser (1064 nm)
f 2m focusing
mirror
aperture
pellicle
beam
splitter
verification
camera
chamber
center
Poisson spot
laser (632 nm)
wedged
dichroic mirror
(front=long-pass
filter, back=mirror)
retroreflector
collimating
lens
fast
steering
mirror
Wedged dichroic mirror compensates for
glint/chamber center offset
1 cm
Target at
glint
location
Co-axial glint return
& driver beam
Target at
chamber
center
Verification
camera
Simulated
wedged
dichroic
mirror
Effort to improve engagement accuracy to
20 Вµm must address & minimize all uncertainties
• Initially effort focused on system integration and operation.
• Now, a more sophisticated control over the experiment is needed to
realize 20 Вµm goal.
• Working to understand and address all errors and uncertainties:
– Environment (air fluctuations)
– Sensors (speed, noise)
– Target (surface quality, sphericity)
– Glint laser/return (repeatability, stability)
Glint off target
Error contributions to
engagement accuracy:
- Reading glint return
target
~50 Вµm
- Air fluctuations
~10 Вµm
- Verification camera ~7 Вµm
- Mirror pointing
~6 Вµm
off
• Most dominant uncertainty so far
is deciphering the glint return …
Target surface quality and
glint laser energy output
Target’s surface roughness plays an important
part in glint return
• Two contributing errors:
– Glint laser’s pulse-to-pulse energy output variance
– Surface roughness causes certain features to reflect back to PSD.
• 25 µm patch off target propagates back
through optics to PSD
Grade 25 SS BB
Surface features &
roughness are
important
Au-coated 4mm shell
Surface roughness can be correlated to glint
return repeatability
• As surface
roughness
improves, glint
return on PSD is
more repeatable
(for a stationary
target).
Light-weight shells
require vacuum to
implement
• Rotation of the target on a
kinematic stalk introduces
sphericity errors.
Glint return on camera shows target’s surface
characteristics
• Surface characteristics are manifested by glint return on PSD.
• Rough targets may reflect light from a larger region, especially
when rotated (a different surface is presented).
---Glint returns--~1 mm *
Grade 25 SS BB glint return
RMS roughness ~65 nm
* Glint return defocused to
prevent PSD saturation
~1 mm *
Au-coated 4mm shell glint return
RMS roughness ~10 nm
=> Desire a smooth target for
more repeatable glint return
Laser’s output energy and spatial profiles vary
considerably
• Peak-to-peak energy ± 6%
(consistent with laser spec’s)
• Spatial profile is inconsistent from
shot-to-shot, thus depositing
randomly-distributed energy on
target.
~1 cm
---Glint beams---
Expanded glint beams immediately before overfilling target
Laser’s energy variation thought to be
causing some apparent target motion
---Glint returns--~1 mm
Same geometrical
shape, yet hot spots
skew energy centroid
Glint return off a stationary, Grade 25
(rough) target at PSD location
=> Probable cause of shot-to-shot
position variation of 20-40 Вµm off
rough targets, better for smoother.
Improving glint laser’s output may improve
glint return repeatability
• A more consistent, flat spatial profile may
help improve glint return repeatability.
• Pointing stability may also be a concern.
All beams ~1 cm
Imaging Homogenizer
Current profile
Flat-topped
microlens
diffuser
Desire a smoother beam more work to be done.
Driver Beam & Engagement
Verification Improvements
alignment &
driver beam
(635 nm)
spatial
filter
collimating
lens
verification
camera
chamber
center
New simulated driver beam enables larger
field of view
• Limited observation range (150 µm).
• Non-linear calibration.
• Computed light centroid of obscured
and un-obscured beamlets.
• Expanded observation range to
1mm.
• Computes light centroid of inner
and outer ring
(i.e. “non-concentricity”)
Simulated
driver
beam
target
Target equally eclipsing beamlets
Driver beam overfilling target
Verification algorithm post-processes
snapshot to verify target engagement
• Triggered camera takes a
snapshot as the simulated driver
beam engages the target.
Pre-processed image
4 mm target, 4.8 mm beam
• Post-processing algorithm can
resolve 7 Вµm (1пЃі) engagement
with 1mm range
Optic improvement yields clearer driver
beam, more precise verification
• Short-pass filter required for
glint return created striations.
• Replacing beam splitter and
filter with pellicle eliminated
interference.
False steering offset due to large wedge
angle is corrected by Poisson spot system
• Wedge correction will not be an issue in
a power plant due to long standoff.
“Z”
1 mm
placement
disparity
chamber
center
glint location
wedged
dichroic mirror
Simulated
dichroic
wedge
• Solution: Use Poisson spot
system to measure target’s
Z-offset at glint location.
• Give one correction to FSM
to modify steering.
Poisson spot system gives one steering
correction to FSM
time
“Z”
X,Y position of
Poisson spot
Final location
at glint
illumination
Target’s Zposition at
glint location
modifies
mirror
steering.
Improvements to mirror ensure it is is positioned
and settled in time for driver pulse
• Improvements include:
–
–
–
–
Alignment beam steering closer to PSD center.
Alignment gain improved.
Mirror hardware gain increased.
Dropping accuracy (< В±1mm).
Glint return on PSD
Optics In Motion fast
steering 1” mirror
Commanded
mirror position
5 ms
Driver pulse
Mirror response
Alignment mode
Mirror not settled in time
Driver pulse
Mirror settled in time
Current Engagement Results
We have engaged moving targets with a
simulated driver beam using the glint return
Engagement accuracy so far = 150 Вµm (1пЃі)
(with injection accuracy of В±1.5 mm)
Stainless steel G25 BBs
If injection accuracy < В±1 mm,
engagement accuracy = 110 Вµm (1пЃі)
Dropping water-filled PAMS, Au/Pd-coated sapphire
spheres expedites our way to real shells
• Au/Pd-coated sapphire spheres are heavier
and fall straighter in air than “real” shells.
• An expedient way to simulate higher-quality
targets before we go to vacuum.
Water-filled, Au-coated PAMS shell
Au/Pd-coated sapphire sphere
Near-term effort focuses on completing
demo, achieving 20 Вµm engagement goal
In summary:
• We are using a glint return off a falling target to steer a
simulated driver beam to hit it on-the-fly to nearly 100 Вµm.
• Verification system has 1mm range, 7 µm resolution.
• Table-top engagement demo honing in on 20 µm goal.
• Working on details of glint laser, glint return, and target
surface quality.
Long-term effort:
• Increase capabilities to mate with a prototypic injector in
vacuum.
End of slideshow
Effort to improve engagement accuracy to
20 Вµm must address & minimize all uncertainties
- One means of quantifying progress is glint return stability.
• Contributing errors identified:
– Target surface roughness
– Laser not at thermal equilibrium
– Room temperature & air
fluctuations, dirt, optics
– Spatial intensity variations in
glint beam
– Thermal drift of components
– Saturating PSD
– Asymmetric glint return
– FSM not settled
Progress
on on
Reducing
Glint Errors
Progress
ReducingMacroscopic
Glint Errors
T a rg et m o tion (Вµm )
35
30
25
20
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• We are trying to
systematically eliminate
errors one-by-one.
(glint return repeatability off a stationary target)
40
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