Transcript ppt
INTEGRATION OF IFE TARGET TRACKING AND BEAM POINTING Graham Flint October 27, 2004 OBJECTIVE Develop a “common reference frame” concept that ties together the various tasks of: target injection target tracking laser beam steering in a manner which satisfies the demands of IFE target irradiation. Hit the Target center to within 20µm at 6 Hz rep. rate Beam Steering Stations GIMMS (X60) (X1, Y1, Z1, 1, 1, 1), (X2, Y2, Z2, 2, 2, 2), … Target Tracking Stations (XA, YA, ZA, A, A, A), (XB, YB, ZB, B, B, B), … These 60 to 70 local coordinate systems need to communicate to each other in one absolute coordinate system: the “Common Reference Frame” (CRF) THERE ARE AT LEAST TWO STRATEGIES FOR IMPLEMENTING A COMMON REFERENCE FRAME • Beam steering and target tracking stations continually measure their displacement from each other – Displacements from initial alignment are tracked for all time (x60 - 70) Common reference frame is a Mathematical construction derived from measured displacements •Beam steering and target tracking stations are locked to a physical structure that limits misalignment (~10µm) – After initial alignment stations can realign to fiducials on structure between each shot (x60 - 70) Stations bolted to physical Structure that serves as the Common reference frame Since it is passive, we will examine using a physical structure implementation for the common reference frame. DIFFERENT BEAM POINTING STRATEGIES MAY BE EMPLOYED (X’,Y’) (X,Y) MONOSTATIC Move to , BISTATIC Move to X’, Y’ (X,Y) STRAWMAN IFE ERROR BUDGET IS TIGHT Sources of error – Laser beam pointing – Target tracking accuracy – Strain fluctuations in reference structure – Thermal anomalies in reference structure Magnitude of errors – Assumed requirement (all sources) 20 m – Single source contribution 10 m Unlike current ICF experiments, the target is not available for a long time prior to shot to allow beam alignment to target EACH STATION CAN LOCK TO THE CRF USING EITHER ONE (MONOSTATIC) OR TWO POINTS (BISTATIC) Outer shell CFR Structure R30m Inner shell CFR Structure R10m Target Chamber MONOSTATIC • Beam pointing accuracy ~ 3.3x10-7 rad (10m at 30m) • • Separation of mirror supports ~1m Maximum structural deformation ~ 0.17 m BISTATIC • • • Maximum line-of-sight error at chamber center 10 m Separation of reference points ~ 40 m Maximum structural deformation Point A Point B ~ 40 m ~ 13 m Bistatic is much less sensitive to deformation of the physical CRF. THE BISTATIC REFERENCE STRUCTURE MUST BE VERY STIFF AND HAVE LOW THERMAL EXPANSION • • • • Geometry Thick-walled polyhedron Type of structure - Dual-skin / honeycomb - Tubular space frame Weight Several thousand tons Static rigidity (3-axis) ~ 108 N m-1 (~ 5x105 lb in-1) –Allows ~800 lbf quasi-static forces to be applied • • • Dynamic rigidity (3-axis) TBD Thermal stability < 0.35 K m-1 variation Structural material Invar / Super Invar CRF –At volume cost similar to stainless steel GIMM TRACKER So far, 10µm pointing stability of bistatic CRF appears feasible. Mirror steering forces still to be investigated. TARGET CHAMBER TRANSVERSE TARGET TRACKING VIEWS TARGET ON FLIGHT AXIS FLIGHT Typical values: = 532 nm, a = 2 mm, 7m < z < 14 m z=7m Centroiding accuracy ~ 5m z = 14 m Centroiding accuracy ~ 10m At IFE distances, viewing Poisson spot in capsule’s shadow can meet 10 µm along entire flight in chamber LONGITUDINAL TARGET TRACKING BY OPTICAL DOPPLER ALSO VIEWS TARGET ON FLIGHT AXIS FLIGHT • • • • • • • Transmit / receive aperture ~ 18 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 0.5-1.0W 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 10 m in all axes ● 3-axis tracking is provided throughout acceleration and in-chamber trajectory ● Minimizes angular acceleration of beam-steering mirrors SHARP POISSON SPOT ALLOWS PRECISE TIMING OF ZERO-CROSSING SENSOR • • • Source/sensor separation ~ 0.5 m FWHM spot diameter ~ 25 m Spatial resolution < 5 m BISTATIC ALIGNMENT OF BEAM STEERING MIRROR UTILISES A COALIGNED REFERENCE BEAM LASER AT A DIFFERENT WAVELENGTH Chamber • • • Main beam and Gaussian reference beam are dynamically aligned so as to be co-axial at all times Obscuration used previously for target tracker is replaced by a phase shift spot (2 at 1, ~ at 2) Phase shift spot can be ion-beam etched into surface of grazing-incidence mirror Poisson spot of reference beam allows steering mirrors to be realigned between shots. POISSON SPOT REFERENCE FOR BEAM STEERING IS GENERATED BY A PHASE SHIFT Step edge is extremely shallow • • • • • • • • • Angle of incidence 84 Step depth 1.19 m Phase retardation at 248 nm 360 Phase retardation at 488 nm 183 Projected step diameter ~ 35 mm Fresnel number at sensor 20.581 ± 0.057 FWHM Poisson spot diameter at 40 m 0.28 mm Equivalent spot at chamber center 0.21 mm Required centroiding accuracy ~ 5% • • • • • • SUMMARY Bistatic approach (both to target tracking and to beam steering) is more robust than monostatic approach by a factor ~ 100:1. A 3-axis target tracking system combines a dual-wavelength CW laser with three sensors: two of which employ Poisson spot centroiding; one of which is an optical Doppler sensor. Continuous target tracking minimizes angular acceleration of beamsteering optics. Each driver laser beam is augmented by a coaxially aligned reference beam, together with a Poisson spot sensor which is located on the opposite side of the chamber. A reference element located at the surface of each grazingincidence mirror is not expected to affect the mirror’s primary function. Implementation is by way of a polyhedral reference structure which is constructed of Invar / Super Invar and which is predicted to weigh several thousand tons.