Target Engagement

1
Target Engagement
Graham Flint - General Atomics Tom Lehecka -
Penn State Electro-Optics Center Bertie Robson -
NRL
HAPL Project Review Oak Ridge National
Laboratory March 21-22, 2006
2
Overall Concept for Target Engagement in a
Semi-Rigid Environment
Coincidence sensors
Focusing mirrors
Target glint source
Segmented vacuum window
Dichroic mirror
Chamber N-S axis
Amplifier / multiplexer/fast steering mirrors
ASE Source
Offset Target
Alignment Laser
Cats eye retroreflectors

• Slow steering mirrors in the amplifier chain
  continuously center each alignment beamlet upon
  its coincidence sensor.

Wedged dichroic mirrors
Target injection / tracking subsystem
Grazing incidence mirrors

• Integrated injection / tracking subsystem places
  the target within a defined volume which is
  centered upon the chambers nominal center.

• Offset target is illuminated in flight by a
  Q-switched glint laser.
• Each coincidence sensor views the glint via the
  rear surface of a wedged dichroic mirror.
• Disparities between beamlet alignment and glint
  signals are used to reposition the fast-steering
  mirrors.
• Coincidence sensors verify pointing of redirected
  beamlets.

3
Geometric Correction associated with a Single
Glint Reference Source
Chamber axis
?
r
Glint offset
?/2
a
Target

• Beamlet displacement from chamber axis ?
• Glint offset a r sin ? /2
• For negligible offset error (as ? ? p) ?a lt 5
  ?m
• For IFE ( r 2.35 mm) ? r/r 0.2
• Current specification ? r/r 1.0

4
Null-Referenced In-Flight Measurement of Target
Diameter, Transit Time and Velocity
Detector mask
Target Trajectory
Detector 1
Signal 1
Time
Monochromatic Illumination Source
10X Telecentric Objective
Detector 2
Signal 2
Time
?t

• Assumed target velocity 100 ms-1
• Image velocity (10x) 1000 ms-1
• Mask slit width 25 ?m
• Transit time precision lt 25 nsec

• Corresponding diametric precision lt 0.1
• Requirement for glint offset correction 0.2
• Velocity determination precision
  (0.5 m separation) 10-5
• Target placement precision 100 ?m

5
Coincidence Sensor Views Target Glint via Common
Footprint on Grazing Incidence Mirror
Beamlet
Target glint source
Dichroic mirror
Grazing incidence mirror
Target at t 0
Wedge angle 1 mrad
Target at t -1.2 ms

• Dichroic mirror has long wavelength pass first
  surface, high reflecting rear surface.
• Mirror wedge angle compensates for combination of
  targetoffset and glint parallax.
• Except for monolithic dichroic mirror, main laser
  and coincidence sensor share a common optical
  path.
• Common path eliminates sensitivity to vibration
  at frequencies below 50 Hz.

Target injection / Tracking subsystem
6
Coincidence Sensor / Retroreflector Schematic
and Performance
Alignment laser beam
Vacuum window
Cats eye retroreflector
10X Microscope objective
Position Sensitive Detector
Virtual return from beamlet
LWP blocking filter
Coincidence Sensor
Target glint return
Main beamlet

• Sensor clear aperture 100 mm
• Sensor effective focal length
  15 m
• Sensor field at chamber center
  4.5 mm X 4.5 mm
• Target-to-beamlet error (1 ? )
  24 ?m
• 3.2 ? collective centroid error (48
  beamlets) 11 ?m

7
End-to-end Target Engagement Strategy
TARGET LAUNCH (MECH. / EM SLING SHOT)
DEMONSTRATED PERFORMANCE (MECHANICAL)
I
TARGET TRACKING
POISSON SPOT (X Y
AXES) CROSSING SENSOR/ OPTICAL
DOPPLER (Z AXIS)
II
ELECTROSTATIC FINE STEERING VOLTAGE
PREC - 10-2)
POST-STEERING TRAJECTORY PRECISION
COINCIDENCE SENSOR FIELD OF VIEW (PRECISION
-10-2) FAST STEERING MIRROR
COMMAND PRECISION
BEAMLET STEERING MAXIMUM EXCURSION (PREC.
-10-2)
BEAMLET POINTING PRECISION
III
SINGLE BEAMLET ENGAGEMENT
PRECISION COLLECTIVE
BEAMLINE CENTROID PRECISION
10-7
10-6
10-5
10-4
10-3
Precision
( 99.9 CONFIDENCE LEVEL )
1 ?
3.2 ?