INTEGRATION OF IFE BEAMLET ALIGNMENT, TARGET TRACKING AND BEAM STEERING

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INTEGRATION OF IFE BEAMLET ALIGNMENT, TARGET
TRACKING AND BEAM STEERING Graham FlintMarch
3, 2005
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BISTATIC TARGET TRACKING CONCEPT

• Tracking error is estimated to be 5 mm or less
  in all axes
• 3-axis tracking is provided throughout
  acceleration and in-chamber trajectory
• Minimizes acceleration of beam-steering elements

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PRACTICAL LAYOUT OF TARGET INJECTION AND
TRACKING SYSTEM
l 3axis tracking confined to in-chamber
trajectory
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BASIC LAYOUT OF KrF DRIVE LASER
Oscillator
Amplifiers
Aperture
Target
Diffuser
Intensity profile at target
Intensity profile at aperture
averaged
averaged
instantaneous
instantaneous
The laser profile at the aperture is imaged
through the amplifiers onto the target If the
optical distortion is small, then the image
duplicates the aperture Concept of Induced
Spatial Incoherence (ISI)
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EACH MAIN BEAM DIVIDED INTO BEAMLETS
LONG PULSE AMPLIFIER ( 100's nsec)
Demultiplexer Array (mirrors)
Multiplexer Array (beam splitters)
FRONT END ( 20 nsec)
Target
Only three beamlets shown for clarity
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ASSUMPTIONS
Optimistic Scenario

• Chamber gas has negligible effect on target
  trajectory
• Target injection accuracy is 1 mm
• Shot-to-shot system drift is less than allowable
  beam/target alignment budget (200 msec)

Pessimistic Scenario

• Chamber gas has some effect on target trajectory
• Target injection accuracy is 5 mm
• Shot-to-shot beamlet co-alignment drift exceeds
  allowable beam/target alignment budget (5-10 msec)

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TARGET TRACKING AND BEAMLET STEERING CONCEPT
(OPTIMISTIC SCENARIO)

• Coincidence between each outgoing beamlet and
  target determined 10 ns before shot
• Error signals used to co-align beamlets prior
  to next shot
• All beamlets (in one beamline) collectively
  aligned via fast-steering aperture

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OPTIMISTIC SCENARIO APPROACH
Beamlet co-alignment

• Slow (10 Hz) alignment of each beamlet
• Each beamlet alignment update based on data from
  previous shot

Target tracking

• Single bistatic sensor
• 3axis tracking throughout in-chamber trajectory
• Precision better than 5 mm (all axes)

Beam steering

• Fast collective steering of all beamlets in a
  beam line
• Two-axis translation of a single lightweight
  element (20 mg)
• Precision better than 5 mm (2 axes)

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ASSUMPTIONS
Optimistic Scenario

• Chamber gas has negligible effect on target
  trajectory
• Target injection accuracy is 1 mm
• Shot-to-shot system drift is less than allowable
  beam/target alignment budget (200 msec)

Pessimistic Scenario

• Chamber gas has some effect on target trajectory
• Target injection accuracy is 5 mm
• Shot-to-shot beamlet co-alignment drift exceeds
  allowable beam/target alignment budget (5-10 msec)

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TARGET TRACKING AND BEAMLET STEERING CONCEPT
(PESSIMISTIC SCENARIO)

• Misalignment between each outgoing beamlet
  and target determined 2 msec before shot
• Individual beamlets directed via fast-steering
  mirrors

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TARGET ILLUMINATION VIA COMMON FOOTPRINTON
GRAZING INCIDENCE MIRROR

• Can combine selectable lead time with large
  target injection errors
• Steering mirror speed can be matched to
  alignment drift rate
• Allows wide range of target injection
  velocities

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PESSIMISTIC SCENARIO APPROACH
Beamlet co-alignment

• Probe beam interrogates entire beamline (t ? -2
  msec)
• Coincidence sensor/retroreflector in each beamlet
• Beamlets individually aligned to predicted
  targetlocation at t ? -1 msec

Target tracking

• Single bistatic sensor
• 3axis tracking throughout in-chamber trajectory
• Target position predicted to 5 mm at t ? -2
  msec (all axes)

Beam steering

• Fast steering of individual beamlets
• Two-axis steering mirror immediately ahead of GIM
• Coarse adjust commences at t ? -20 msec
• Fine adjust commences at t ? -2 msec
• Precision better than 5 mm (2 axes)

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SUMMARY CONCLUSIONS

• Target tracking and beam pointing with a
  precision of 5 ?m can be achieved
• Parts count changes little between optimistic
  and pessimistic scenarios
• Principle cost tradeoff exists between beamline
  stability and steering mirror response time
• Could be significant cost impact for fast
  versus slow steering mirrors
• Assessment of IFE vibration/drift environment is
  an important step in the system definition process

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TRANSVERSE TARGET TRACKER VIEWS TARGET ALONG
FLIGHT AXIS
FLIGHT
Typical values ? 532 nm, a 2 mm, 7m lt z lt 14
m
Consistent with CCD framing rateof 2000 fps,
1024 x 1024 resolution
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LONGITUDINAL TARGET TRACKING BY OPTICAL DOPPLER
ALSO VIEWS TARGET ON FLIGHT AXIS
FLIGHT

• Transmit / receive aperture 10 mm
• Maximum target range 20 m
• Laser wavelengths 488 nm, 515 nm
• Subcarrier fringe resolution 4.7 ?m
• Fringe count rate 43 MHz
• Photoelectron count rate 109W (s-1)
• Laser power 0.2-0.4W
• Range resolution (l/4) 2.5 mm

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SHARP POISSON SPOT ALLOWS PRECISE TIMING OF
ZERO-CROSSING SENSOR
l Source/sensor separation 0.25 m l FWHM
spot diameter 12.5 ?m l Spatial resolution lt
1 ?m