Transcript ppt

INTEGRATION OF
IFE BEAMLET ALIGNMENT,
TARGET TRACKING
AND BEAM STEERING
Graham Flint
March 3, 2005
BISTATIC TARGET TRACKING CONCEPT
Gaussian
beam
Zero-crossing sensor
Bistatic
reference #1
Optical
stop
Target
Electromagnetic target
accelerator
Optical
Doppler
sensor
Laser
IFE Chamber
Centroid
sensor
(Bistatic
reference #2)

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
PRACTICAL LAYOUT OF
TARGET INJECTION AND TRACKING SYSTEM

3–axis tracking confined to in-chamber trajectory
BASIC LAYOUT OF KrF DRIVE LASER
Oscillator
Amplifiers
Aperture
Target
Diffuser
Intensity profile at aperture
instantaneous
averaged
Intensity profile at target
instantaneous
averaged
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)
EACH MAIN BEAM DIVIDED INTO BEAMLETS
LONG PULSE AMPLIFIER
(~ 100's nsec)
Last Pulse
Demultiplexer
Array
(mirrors)
Multiplexer
Array
(beam splitters)
FRONT END
( 20 nsec)
Target
Only three beamlets shown for clarity
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)
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
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

3–axis 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)
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)
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
TARGET ILLUMINATION VIA COMMON FOOTPRINT
ON 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
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 target
location at t  -1 msec
Target tracking

Single bistatic sensor

3–axis 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)
SUMMARY & CONCLUSIONS

Target tracking and beam pointing with a precision of ±5 mm
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
TRANSVERSE TARGET TRACKER
VIEWS TARGET ALONG FLIGHT AXIS
FLIGHT
Typical values:  = 532 nm, a = 2 mm, 7m < z < 14 m
z=7m
Centroiding accuracy ~ 2.5mm
Consistent with CCD framing rate
of 2000 fps, 1024 x 1024 resolution
z = 14 m
Centroiding accuracy ~ 5mm
LONGITUDINAL TARGET TRACKING BY OPTICAL
DOPPLER ALSO VIEWS TARGET ON FLIGHT AXIS
FLIGHT
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Transmit / receive aperture
~ 10 mm
Maximum target range
~ 20 m
Laser wavelengths
488 nm, 515 nm
Subcarrier fringe resolution
4.7 mm
Fringe count rate
~ 43 MHz
Photoelectron count rate
~ 109W (s-1)
Laser power
0.2-0.4W
Range resolution (/4)
± 2.5 mm
SHARP POISSON SPOT ALLOWS PRECISE TIMING
OF ZERO-CROSSING SENSOR

Source/sensor separation
~ 0.25 m

FWHM spot diameter
~ 12.5 mm

Spatial resolution
< 1 mm