Presented by Lane Carlson1

M. Tillack1, T. Lorentz1,N. Alexander2, G. Flint2, D. Goodin2, R. Petzoldt2

(1UCSD, 2General Atomics)

HAPL Project ReviewNRL, Washington D.C.October 30-31, 2007

Progress on Tracking & Engagement Demonstration

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 µm (1) engagement for all targets with ± 1.5 mm placement, (110 µm (1) with placement accuracy < ± 1 mm)• Prior reported engagement was 20% of targets in range of

verification system (150 µm).

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

crossingsensorsC2C3C1pulsed glint

laser (1064 nm)alignment & driver beam

(635 nm)

verification camera

retroreflectorcoincidence sensor/PSDPoisson

spot camera

fast steering mirror

f 2m focusing mirror

wedged dichroic mirror (front=long-pass filter, back=mirror)

chamber centerPoisson spot

laser (632 nm)spatial filter

collimating lens

drop toweraperturepellicle beam splitter

“Remember we have two scenarios…”

Wedged dichroic mirror compensates for glint/chamber center offset

Target at glint

location

Verification camera

Simulated wedged dichroic mirror

Target at chamber center

1 cm

glint beam

Co-axial glint return & driver beam

Glint return

simulated driver beam

Effort to improve engagement accuracy to 20 µm 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 µm

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)

• Most dominant uncertainty so far is deciphering the glint return …

Error contributions to engagement accuracy:

-Reading glint return off target ~50 µm

-Air fluctuations ~10 µm-Verification camera ~7 µm-Mirror pointing ~6 µm

Glint off target

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 Au-coated 4mm shell

Surface features & roughness are

important

“When we are getting to 20 µm…”

Surface roughness can be correlated to glint return repeatability

• As surface roughness improves, glint return on PSD is more repeatable (for a stationary target).

• Rotation of the target on a kinematic stalk introduces sphericity errors.

Light-weight shells require vacuum to

implement

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).

Grade 25 SS BB glint returnRMS roughness ~65 nm

Au-coated 4mm shell glint returnRMS roughness ~10 nm

=> Desire a smooth target for more

repeatable glint return

~1 mm *

* Glint return defocused to prevent PSD saturation

~1 mm *

---Glint returns------Glint returns---

Laser’s output energy and spatial profiles vary considerably

• Peak-to-peak energy ± 6% (consistent with laser spec’s)

Expanded glint beams immediately before overfilling target

~1 cm

• Spatial profile is inconsistent from shot-to-shot, thus depositing randomly-distributed energy on target.

---Glint beams------Glint beams---

Laser’s energy variation thought to be causing some apparent target motion

Same geometrical shape, yet hot

spots skew energy centroid

~1 mm

=> Probable cause of shot-to-shot position variation of 20-40 µm off rough targets, better for smoother.

Glint return off a stationary, Grade 25 (rough) target at PSD location

---Glint returns------Glint returns---

Improving glint laser’s output may improve glint return repeatability

All beams ~1 cm

• A more consistent, flat spatial profile may help improve glint return repeatability.

• Pointing stability may also be a concern.

Current profileImaging HomogenizerFlat-topped microlens

diffuser

Desire a smoother beam - more work to

be done.

alignment & driver beam

(635 nm)

verification camera

chamber centerspatial filter

collimating lens

Driver Beam & Engagement Verification Improvements

Target equally eclipsing beamlets

New simulated driver beam enables larger field of view

• Expanded observation range to 1mm.• Computes light centroid of inner and outer ring (i.e. “non-concentricity”)

• Limited observation range (150 µm).

• Non-linear calibration.• Computed light centroid of

obscured and un-obscured beamlets.

Driver beam overfilling target

target

Simulated driver beam

Verification algorithm post-processes snapshot to verify target engagement

• Post-processing algorithm can resolve 7 µm (1) engagement with 1mm range

Pre-processed image 4 mm target, 4.8 mm beam

• Triggered camera takes a snapshot as the simulated driver beam engages the target.

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.

wedged dichroic mirror

chamber center

1 mm placement disparity

glint location

False steering offset due to large wedge angle is corrected by Poisson spot system

• Solution: Use Poisson spot system to measure target’s Z-offset at glint location.

• Give one correction to FSM to modify steering.

• Wedge correction will not be an issue in a power plant due to long standoff.

Simulated dichroic wedge

“Z”

Poisson spot system gives one steering correction to FSM

X,Y position of Poisson

spot

Final location at glint

illumination

time

“Z”Target’s Z-position at glint location modifies mirror steering.

Optics In Motion fast steering 1” mirror

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 (< ±1mm).

Glint return on PSD

Commanded mirror position

Mirror response

Driver pulse

Alignment mode Mirror not settled in time

Mirror settled in time

5 ms

Driver pulse

Current Engagement Results

We have engaged moving targets with a simulated driver beam using the glint return

If injection accuracy < ±1 mm, engagement accuracy = 110 µm (1)

Engagement accuracy so far = 150 µm (1) (with injection accuracy of ±1.5 mm)

Stainless steel G25 BBsStainless steel G25 BBs

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.

Au/Pd-coated sapphire sphereWater-filled, Au-coated PAMS shell

Near-term effort focuses on completing demo, achieving 20 µm engagement goal

In summary: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 µm.

• 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:Long-term effort:

• Increase capabilities to mate with a prototypic injector in vacuum.

End of slideshow

Effort to improve engagement accuracy to 20 µm must address & minimize all uncertainties

• 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

• We are trying to systematically eliminate errors one-by-one.

- One means of quantifying progress is glint return stability.

Progress on Reducing Glint Errors

05

10152025303540

Base line

High energy (40 mJ)Gold-coated Target

Optics cleanedTable floating

Tent covering tableDefocused glint returnApertured glint return

Averaging return signalThermal drift removedExp. at thermal equlib.

Target motion (µ

m)

avg X,Y

Progress on Reducing Macroscopic Glint Errors(glint return repeatability off a stationary target)