Prototype zoom lenses should be designed with flexibility. One never knows what the future use of a prototype as-build lens will be. In a prior design of a large-image-format zoom lens used for proton radiography, extra back focal distance was constrained in the design to allow for future insertion of extra lenses. These added lenses can change the magnification to a different camera system, having a smaller image size. Three single commercial lens elements were mounted into a commercial variable-length housing barrel that was attached to the back of the zoom lens using a 3Dprinted flange. This new design has been adapted to support neutron radiography; the new configuration collects light from a thick, blue-light-emitting scintillator. After the initial request was made, it took us only three weeks to design, assemble, and conduct imaging tests. The scintillator’s light travels 24 inches before entering into the zoom lens. A large pellicle is inserted into the optical path to keep the zoom lens and camera out of the neutron flux. Because of reduced resolution from the volume scintillator, a five-axis self-leveling alignment laser was sufficient to adjust the tilting of the scintillator, pellicle, zoom lens, and camera. The design process for picking suitable COTS lens elements will be discussed.
A zoom lens has been designed for proton radiography applications. Radiographic images are recorded at the end of an accelerator, where protons exit an aluminum vacuum window producing a shadowgraph image onto an LYSO (lutetium yttrium orthosilicate) scintillator. Emission from this 5-inch-square scintillator reflects off a pellicle and is then collected by a zoom lens located 24 inches away. Proton radiography can make high-speed, multi-frame radiographs or radiographic movies. This zoom lens provides 2X magnification for viewing different object sizes. The zoom lens incorporates eleven lenses, including a moving doublet that changes the magnification. Refocus of the camera is required when zooming. Only one moving doublet lens is required to change magnification. The stop was anchored to the moving doublet and its diameter is unchanged throughout magnification changes. The entire lens system is housed in a cylindrical tube. This lens will be used with a 10-frame camera with a 44 × 44 mm square image format and 1100 × 1100 pixel resolution. Stray light suppression is most important in this lens system. Radial compensation is controlled by two locking micrometers on element 9, which relaxes the mechanical tolerancing. A helical cam barrel using a linear rail controls the movement of the doublet. Alignment of the mechanical gears will be discussed.
Tomographic imaging of large objects from an x-ray source require a 240 mm square scintillator that is imaged onto a 90 mm square CCD with <0.01% distortion. A large pellicle and internal folding mirror keep the optical elements and the CCD camera out of the x-ray beam path to minimize shielding requirements.
This work explores quick predictive methods for calculating potentially risky stresses in cemented doublets underdoing temperature change that agree well with finite element analysis. We also provide guidelines for avoiding stress concentrations.
Material scientists have developed computational modeling to predict the dynamic response of materials undergoing stress, but there is still a need to make precision measurements of surfaces undergoing shock compression. Miniature photonic Doppler velocimetry (PDV) probes have been developed to measure the velocity distribution from a moving surface traveling tens of kilometers per second. These probes use hundreds of optical fibers imaged by optical relays onto different regions of this moving surface. While previous work examined large surface areas, we have now developed a PDV microscope that can interrogate 37 different spots within a field of view of <1 mm, with a standoff distance of 17 mm, to analyze the motion differences across grain boundaries of the material undergoing dynamic stress. Each PDV fiber interrogates a 10 μm spot size on the moving surface. A separate imaging system using a coherent bundle records the location of the PDV spots relative to the grain boundaries prior to the dynamic event. Designing the mounting structures for the lenses, fibers, and coherent bundle was a challenge. To minimize back reflections, the fibers are index matched onto the first relay lens, which is made of fused silica. The PDV fibers are aligned normal to the moving surface. The imaging probe views the surface at an 18° angle. The coherent bundle is tilted 11° to its optical relay. All components are assembled into a single probe head assembly. The coherent bundle is removed from the probe head to be used for the next dynamic event. Alignment issues will also be discussed.
A pulsed neutron source is used to interrogate a target, producing secondary gammas and neutrons. In order to make
good use of the relatively small number of gamma rays that emerge from the system after the neutron flash, our detector
system must be both efficient in converting gamma rays to a detectable electronic signal and reasonably large in volume.
Isotropic gamma rays are emitted from the target. These signals are converted to light within a large chamber of a liquid
scintillator. To provide adequate time-of-flight separation between the gamma and neutron signals, the liquid scintillator
is placed meters away from the target under interrogation. An acrylic PMMA (polymethyl methacrylate) light guide
directs the emission light from the chamber into a 5-inch-diameter photomultiplier tube. However, this PMMA light
guide produces a time delay for much of the light.
Illumination design programs count rays traced from the source to a receiver. By including the index of refraction of the
different materials that the rays pass through, the optical power at the receiver is calculated. An illumination design
program can be used to optimize the optical material geometries to maximize the ray count and/or the receiver power. A
macro was written to collect the optical path lengths of the rays and import them into a spreadsheet, where histograms of
the time histories of the rays are plotted. This method allows optimization on the time response of different optical
detector systems. One liquid scintillator chamber has been filled with a grid of reflective plates to improve its time
response. Cylindrical detector geometries are more efficient.
The telecentric zoom lens system (ZLS) has proven to be invaluable in flash x-ray field operations and recent successful
experiments pertaining to stockpile stewardship. The ZLS contains 11 custom-manufactured lenses, a turning mirror
(pellicle), and an x-ray-to-visible-light converting scintillator. Images are recorded on a fully characterized CCD. All
hardware is supported by computerized, programmable, electro-mechanical mounts and alignment apparatus. Seven
different glass material types varying in chemical stoichiometry comprise the 11 ZLS lenses. All lenses within the ZLS
are out of the path of direct x-ray radiation during normal operation. However, any unshielded scattered x-ray radiation
can result in energy deposition into the lenses, which may generate some scintillating light that can couple into the CCD.
This extra light may contribute to a decrease in signal-to-noise ratio (SNR) and lower the overall fidelity of the
radiograph images. An estimate of the scintillation generation and sensitivities for each of the seven types of glass used
as lenses in the ZLS is presented. This report also includes estimates of the total observed background decoupling that
each of the lens material types contribute.
When a zoom lens views a tilted finite conjugate object, its image plane is both tilted and distorted depending on magnification. Our camera image plane moves with six degrees of freedom; only one moving doublet lens is required to change magnification. Two lens design models were analyzed. The first required the optical and mechanical axes to be collinear, resulting in a tilted stop. The second allowed the optical axis to be tilted from the lens mechanical axis with an untilted stop moving along the mechanical axis. Both designs produced useful zoom lenses with excellent resolution for a distorted image. For both lens designs, the stop is anchored to the moving doublet and its diameter is unchanged throughout magnification changes. This unusual outcome allows the light level at each camera pixel to remain constant, independent of magnification. As-built tolerance analysis is used to compare both optical models. The design application is for proton radiography. At the end of an accelerator, protons exit an aluminum vacuum window producing a shadowgraph image onto an LYSO (lutetium yttrium orthosilicate) scintillator. The 5″ square scintillator emission reflects off a pellicle and is collected by the zoom lenses located 24″ away. Four zoom lenses will view the same pellicle at different alpha and beta angles. Blue emission from the scintillator is viewed at an alpha angle of –14° or –23° and beta angles of ±9° or ±25°. The pellicle directs the light backwards to a zone where adequate shielding of the cameras can be achieved against radiation scattered from the aluminum window.
Optical designers assume a mathematically derived statistical distribution of the relevant design parameters for their Monte Carlo tolerancing simulations. However, there may be significant differences between the assumed distributions and the likely outcomes from manufacturing. Of particular interest for this study are the data analysis techniques and how they may be applied to optical and mechanical tolerance decisions. The effect of geometric factors and mechanical glass properties on lens manufacturability will be also be presented. Although the present work concerns lens grinding and polishing, some of the concepts and analysis techniques could also be applied to other processes such molding and single-point diamond turning.
Optical designers assume a mathematically derived statistical distribution of the relevant design parameters for their Monte Carlo tolerancing simulation. Presented are measured distributions using lens manufacturing data to better inform the decision-making process.
During the fabrication of an aspherical mirror, the inspection of the residual wavefront error is critical. In the program of a spaceborne telescope development, primary mirror is made of ZERODUR with clear aperture of 450 mm. The mass is 10 kg after lightweighting. Deformation of mirror due to gravity is expected; hence uniform supporting measured by load cells has been applied to reduce the gravity effect. Inspection has been taken to determine the residual wavefront error at the configuration of mirror face upwards. Correction polishing has been performed according to the measurement. However, after comparing with the data measured by bench test while the primary mirror is at a configuration of mirror face horizontal, deviations have been found for the two measurements. Optical system that is not able to meet the requirement is predicted according to the measured wavefront error by bench test. A target wavefront error of secondary mirror is therefore analyzed to correct that of primary mirror. Optical performance accordingly is presented.
We have investigated scintillator efficiency for MeV radiographic imaging. This paper discusses the modeled detection efficiency and measured brightness of a number of scintillator materials. An optical imaging camera records images of scintillator emission excited by a pulsed x-ray machine. The efficiency of various thicknesses of monolithic LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) are being studied to understand brightness and resolution trade-offs compared with a range of micro-columnar CsI:Tl (thallium-doped cesium iodide) scintillator screens. The micro-columnar scintillator structure apparently provides an optical gain mechanism that results in brighter signals from thinner samples. The trade-offs for brightness versus resolution in monolithic scintillators is straightforward. For higher-energy x-rays, thicker materials generally produce brighter signal due to x-ray absorption and the optical emission properties of the material. However, as scintillator thickness is increased, detector blur begins to dominate imaging system resolution due to the volume image generated in the scintillator thickness and the depth of field of the imaging system. We employ a telecentric optical relay lens to image the scintillator onto a recording CCD camera. The telecentric lens helps provide sharp focus through thicker-volume emitting scintillators. Stray light from scintillator emission can also affect the image scene contrast. We have applied an optical light scatter model to the imaging system to minimize scatter sources and maximize scene contrasts.
Cygnus is a high-energy radiographic x-ray source. Three large zoom lenses have been assembled to collect images from
large scintillators. A large elliptical pellicle (394 × 280 mm) deflects the scintillator light out of the x-ray path into an
eleven-element zoom lens coupled to a CCD camera. The zoom lens and CCD must be as close as possible to the
scintillator to maximize light collection. A telecentric lens design minimizes image blur from a volume source. To
maximize the resolution of objects of different sizes, the scintillator and zoom lens are translated along the x-ray axis,
and the zoom lens magnification changes. Zoom magnification is also changed when different-sized recording cameras
are used (50 or 62 mm square format). The LYSO scintillator measures 200 × 200 mm and is 5 mm thick. The
scintillator produces blue light peaking at 435 nm, so special lens materials are required. By swapping out one doublet
and allowing all other lenses to be repositioned, the zoom lens can also use a CsI(Tl) scintillator that produces green
light centered at 540 nm (for future operations). All lenses have an anti-reflective coating for both wavelength bands.
Two sets of doublets, the stop, the scintillator, and the CCD camera move during zoom operations. One doublet has x-y
compensation. Alignment of the optical elements was accomplished using counter propagating laser beams and
monitoring the retro-reflections and steering collections of laser spots. Each zoom lens uses 60 lb of glass inside the 425
lb mechanical structure, and can be used in either vertical or horizontal orientation.
Cygnus is a high-energy radiographic x-ray source. The rod-pinch x-ray diode produces a point source measuring 1 mm
diameter. The target object is placed 1.5 m from the x-ray source, with a large LYSO scintillator at 2.4 m. Differentsized
objects are imploded within a containment vessel. A large pellicle deflects the scintillator light out of the x-ray
path into an 11-element zoom lens coupled to a CCD camera. The zoom lens and CCD must be as close as possible to
the scintillator to maximize light collection. A telecentric lens design minimizes image blur from a volume source. To
maximize the resolution of test objects of different sizes, the scintillator and zoom lens can be translated along the x-ray
axis. Zoom lens magnifications are changed when different-sized scintillators and recording cameras are used (50 or
62 mm square format). The LYSO scintillator measures 200 × 200 mm and is 5 mm thick. The scintillator produces blue
light peaking at 435 nm, so special lens materials are required. By swapping out one lens element and allowing all lenses
to move, the zoom lens can also use a CsI(Tl) scintillator that produces green light centered at 550 nm. All lenses are
coated with anti-reflective coating for both wavelength bands. Two sets of doublets, the stop, and the CCD camera move
during zoom operations. One doublet has XY compensation. The first three lenses use fused silica for radiation damage
control. The 60 lb of glass inside the 340 lb mechanical structure is oriented vertically.
A new fisheye lens design is used as a miniature probe to measure the velocity distribution of an imploding surface
along many lines of sight. Laser light, directed and scattered back along each beam on the surface, is Doppler shifted by
the moving surface and collected into the launching fiber. The received light is mixed with reference laser light in each
optical fiber in a technique called photonic Doppler velocimetry, providing a continuous time record.
An array of single-mode optical fibers sends laser light through the fisheye lens. The lens consists of an index-matching
positive element, two positive doublet groups, and two negative singlet elements. The optical design minimizes beam
diameters, physical size, and back reflections for excellent signal collection. The fiber array projected through the
fisheye lens provides many measurement points of surface coverage over a hemisphere with very little crosstalk. The
probe measures surface movement with only a small encroachment into the center of the cavity.
The fiber array is coupled to the index-matching element using index-matching gel. The array is bonded and sealed into
a blast tube for ease of assembly and focusing. This configuration also allows the fiber array to be flat polished at a
common object plane. In areas where increased measurement point density is desired, the fibers can be close packed. To
further increase surface density coverage, smaller-diameter cladding optical fibers may be used.
A novel fiber-optic probe measures the velocity distribution of an imploding surface along many lines of sight. Reflected
light from each spot on the moving surface is Doppler shifted with a small portion of this light propagating backwards
through the launching fiber. The reflected light is mixed with a reference laser in a technique called photon Doppler
velocimetry, providing continuous time records.
Within the probe, a matrix array of 56 single-mode fibers sends light through an optical relay consisting of three types of
lenses. Seven sets of these relay lenses are grouped into a close-packed array allowing the interrogation of seven regions
of interest. A six-faceted prism with a hole drilled into its center directs the light beams to the different regions. Several
types of relay lens systems have been evaluated, including doublets and molded aspheric singlets. The optical design
minimizes beam diameters and also provides excellent imaging capabilities. One of the fiber matrix arrays can be
replaced by an imaging coherent bundle.
This close-packed array of seven relay systems provides up to 476 beam trajectories. The pyramid prism has its six
facets polished at two different angles that will vary the density of surface point coverage. Fibers in the matrix arrays are
angle polished at 8°to minimize back reflections. This causes the minimum beam waist to vary along different
trajectories. Precision metrology on the direction cosine trajectories is measured to satisfy environmental requirements
for vibration and temperature.
The National Ignition Facility (NIF) has a need for measuring gamma radiation as part of a nuclear diagnostic program.
A new gamma-detection diagnostic uses 90° off-axis parabolic mirrors to relay Cherenkov light from a volume of
pressurized gas. This nonimaging optical system has the high-speed detector placed at a stop position with the
Cherenkov light delayed until after the prompt gammas have passed through the detector. Because of the wavelength
range (250 to 700 nm), the optical element surface finish was a key design constraint. A cluster of four channels (each
set to a different gas pressure) will collect the time histories for different energy ranges of gammas.
The National Ignition Facility will begin testing DT fuel capsules yielding greater than 1013 neutrons during 2010.
Neutron imaging is an important diagnostic for understanding capsule behavior. Neutrons are imaged at a scintillator
after passing through a pinhole. The pixelated, 160-mm square scintillator is made up of 1/4 mm diameter rods 50 mm
long. Shielding and distance (28 m) are used to preserve the recording diagnostic hardware. Neutron imaging is light
starved. We designed a large nine-element collecting lens to relay as much scintillator light as reasonable onto a 75 mm
gated microchannel plate (MCP) intensifier. The image from the intensifier's phosphor passes through a fiber taper onto
a CCD camera for digital storage. Alignment of the pinhole and tilting of the scintillator is performed before the relay
lens and MCP can be aligned. Careful tilting of the scintillator is done so that each neutron only passes through one rod
(no crosstalk allowed). The 3.2 ns decay time scintillator emits light in the deep blue, requiring special glass materials.
The glass within the lens housing weighs 26 lbs, with the largest element being 7.7 inches in diameter. The distance
between the scintillator and the MCP is only 27 inches. The scintillator emits light with 0.56 NA and the lens collects
light at 0.15 NA. Thus, the MCP collects only 7% of the available light. Baffling the stray light is a major concern in the
design of the optics. Glass cost considerations, tolerancing, and alignment of this lens system will be discussed.
The National Ignition Facility and the Omega Laser Facility both have a need for measuring prompt gamma radiation as
part of a nuclear diagnostic program. A new gamma-detection diagnostic using off-axis-parabolic mirrors has been built.
Some new techniques were used in the design, construction, and tolerancing of this gamma ray diagnostic. Because of
the wavelength requirement (250 to 700 nm), the optical element surface finishes were a key design consideration. The
optical enclosure had to satisfy pressure safety concerns and shielding against electromagnetic interference induced by
gammas and neutrons. Structural finite element analysis was needed to meet rigorous optical and safety requirements.
The optomechanical design is presented. Alignment issues are also discussed.
The design and assembly of a nine-element lens that achieves >2000 lp/mm resolution at a 355-nm wavelength
(ultraviolet) has been completed. By adding a doublet to this lens system, operation at a 532-nm wavelength (green) with
>1100 lp/mm resolution is achieved. This lens is used with high-power laser light to record holograms of fast-moving
ejecta particles from a shocked metal surface located inside a test package. Part of the lens and the entire test package are
under vacuum with a 1-cm air gap separation. Holograms have been recorded with both doubled and tripled Nd:YAG
laser light. The UV operation is very sensitive to the package window's tilt. If this window is tilted by more than 0.1
degrees, the green operation performs with better resolution than that of the UV operation. The setup and alignment are
performed with green light, but the dynamic recording can be done with either UV light or green light. A resolution plate
can be temporarily placed inside the test package so that a television microscope located beyond the hologram position
can archive images of resolution patterns that prove that the calibration wires, interference filter, holographic plate, and
relay lenses are in their correct positions. Part of this lens is under vacuum, at the point where the laser illumination
passes through a focus. Alignment and tolerancing of this high-resolution lens are presented. Resolution variation across
the 12-mm field of view and throughout the 5-mm depth of field is discussed for both wavelengths.
A gated spectrometer has been designed for real-time, pulsed infrared (IR) studies at the National Synchrotron Light
Source at the Brookhaven National Laboratory. A pair of 90-degree, off-axis parabolic mirrors are used to relay the light
from an entrance slit to an output IR recording camera. With an initial wavelength range of 1500-4500 nm required,
gratings could not be used in the spectrometer because grating orders would overlap. A magnesium oxide prism, placed
between these parabolic mirrors, serves as the dispersion element. The spectrometer is doubly telecentric. With proper
choice of the air spacing between the prism and the second parabolic mirror, any spectral region of interest within the
InSb camera array's sensitivity region can be recorded. The wavelengths leaving the second parabolic mirror are
collimated, thereby relaxing the camera positioning tolerance. To set up the instrument, two different wavelength
(visible) lasers are introduced at the entrance slit and made collinear with the optical axis via flip mirrors. After
dispersion by the prism, these two laser beams are directed to tick marks located on the outside housing of the gated IR
camera. This provides first-order wavelength calibration for the instrument. Light that is reflected off the front prism
face is coupled into a high-speed detector to verify steady radiance during the gated spectral imaging. Alignment
features include tick marks on the prism and parabolic mirrors. This instrument was designed to complement singlepoint
pyrometry, which provides continuous time histories of a small collection of spots from shock-heated targets.
Shock waves passing through a metal sample can produce ejecta particulates at a metal-vacuum interface. Holography
records particle size distributions by using a high-power, short-pulse laser to freeze particle motion. The sizes of the
ejecta particles are recorded using an in-line Fraunhofer holography technique. Because the holographic plate would be
destroyed in an energetic environment, a high-resolution lens has been designed to relay the scattered and unscattered
light to a safe environment where the interference fringes are recorded on film. These interference fringes allow particles
to be reconstructed within a 12-mm-diameter, 5-mm-thick volume. To achieve resolution down to 0.5 μm, both a high-resolution
optical relay lens and ultraviolet laser (UV) light were implemented. The design and assembly of a nine-element
lens that achieves >2000 lp/mm resolution and operates at f/0.89 will be described. To set up this lens system, a
doublet lens is temporarily attached that enables operation with 532-nm laser light and 1100 lp/mm resolution. Thus, the
setup and alignment are performed with green light, but the dynamic recording is done with UV light. During setup, the
532-nm beam provides enough focus shift to accommodate the placement of a resolution target outside the ejecta
volume; this resolution target does not interfere with the calibrated wires and pegs surrounding the ejecta volume. A
television microscope archives images of resolution patterns that prove that the calibration wires, interference filter,
holographic plate, and relay lenses are in their correct positions. Part of this lens is under vacuum, at the point where the
laser illumination passes through a focus. Alignment and tolerancing of this high-resolution lens will be presented, and
resolution variation through the 5-mm depth of field will be discussed.
The National Ignition Facility (NIF) requires optical diagnostics for measuring shock velocities in shock physics experiments. The nature of the NIF facility requires the alignment of complex three-dimensional optical systems of very long distances. Access to the alignment mechanisms can be limited, and any alignment system must be operator-friendly. The Velocity Interferometer System for Any Reflector (VISAR) measures shock velocities and shock breakout times of 1- to 5-mm targets at a location remote to the NIF target chamber. A third imaging system measures self-emission of the targets. These three optical systems using the same vacuum chamber port each have a total track of 21 m. All optical lenses are on kinematic mounts or sliding rails, enabling pointing accuracy of the optical axis to be systematically checked. Counter-propagating laser beams (orange and red) align these diagnostics to a listing of tolerances. Floating apertures, placed before and after lens groups, display misalignment by showing the spread of alignment spots created by the orange and red alignment lasers. Optical elements include 1-in. to 15-in. diameter mirrors, lenses with up to 10.5-in. diameters, beam splitters, etalons, dove prisms, filters, and pellicles. Alignment of more than 75 optical elements must be verified before each target shot. Archived images from eight alignment cameras prove proper alignment is achieved before each shot.
A velocimetry experiment has been designed to measure shock properties for small cylindrical metal targets
(8-mm-diameter by 2-mm thick). A target is accelerated by high explosives, caught, and retrieved for later inspection.
The target is expected to move at a velocity of 0.1 to 3 km/sec. The complete experiment canister is approximately
105 mm in diameter and 380 mm long. Optical velocimetry diagnostics include the Velocity Interferometer System for
Any Reflector (VISAR) and Photon Doppler Velocimetry (PDV). The packaging of the velocity diagnostics is not
allowed to interfere with the catchment or an X-ray imaging diagnostic. A single optical relay, using commercial lenses,
collects Doppler-shifted light for both VISAR and PDV. The use of fiber optics allows measurement of point velocities
on the target surface during accelerations occurring over 15 mm of travel. The VISAR operates at 532 nm and has
separate illumination fibers requiring alignment. The PDV diagnostic operates at 1550 nm, but is aligned and focused at
670 nm. The VISAR and PDV diagnostics are complementary measurements and they image spots in close proximity on
the target surface. Because the optical relay uses commercial glass, the axial positions of the optical fibers for PDV and
VISAR are offset to compensate for chromatic aberrations. The optomechanical design requires careful attention to fiber
management, mechanical assembly and disassembly, positioning of the foam catchment, and X-ray diagnostic field-of-view.
Calibration and alignment data are archived at each stage of the assembly sequence.
The National Ignition Facility (NIF) requires optical diagnostics for measuring shock velocities in shock physics experiments. The velocity interferometer for any reflector measures shock velocities at a location remote to the NIF target chamber. Our team designed two systems, one for a polar port orientation, and the other to accommodate two equatorial ports. The polar-oriented design requires a 48-m optical relay to move the light from inside the target chamber to a separately housed measurement and laser illumination station. The currently operational equatorial design requires a much shorter relay of 21 m. Both designs posed significant optomechanical challenges due to the long optical path length, large quantity of optical elements, and stringent NIF requirements. System design had to tightly control the use of lubricants and materials, especially those inside the vacuum chamber; tolerate earthquakes and radiation; and consider numerous other tolerance, alignment, and steering adjustment issues. To ensure compliance with NIF performance requirements, we conducted a finite element analysis.
Thermal imaging is an important, though challenging, diagnostic for shockwave experiments. Shock-compressed materials undergo transient temperature changes that cannot be recorded with standard (greater than ms response time) infrared detectors. A further complication arises when optical elements near the experiment are destroyed. We have designed a thermal-imaging system for studying shock temperatures produced inside a gas gun at Sandia National Laboratories. Inexpensive, diamond-turned, parabolic mirrors relay an image of the shocked target to the exterior of the gas gun chamber through a sapphire vacuum port. The 3000-5000-nm portion of this image is directed to an infrared camera which acquires a snapshot of the target with a minimum exposure time of 150 ns. A special mask is inserted at the last intermediate image plane, to provide dynamic thermal background recording during the event. Other wavelength bands of this image are split into high-speed detectors operating at 900-1700 nm and at 1700-3000 nm, for time-resolved pyrometry measurements. This system incorporates 90-degree, off-axis parabolic mirrors, which can collect low f/# light over a broad spectral range, for high-speed imaging. Matched mirror pairs must be used so that aberrations cancel. To eliminate image plane tilt, proper tip-to-tip orientation of the parabolic mirrors is required. If one parabolic mirror is rotated 180 degrees about the optical axis connecting the pair of parabolic mirrors, the resulting image is tilted by 60 degrees. Different focal-length mirrors cannot be used to magnify the image without substantially sacrificing image quality. This paper analyzes performance and aberrations of this imaging diagnostic.
Optical diagnostics are currently being designed to analyze high-energy density physics experiments at the National
Ignition Facility (NIF). Two line-imaging Velocity Interferometer System for Any Reflector (VISAR) interferometers
have been fielded to measure shock velocities, breakout times, and emission of targets sized from 1 to 5 millimeters. A
20-cm-diameter, fused silica triplet lens collects light at f/3 from the targets inside the 10-meter-diameter NIF vacuum
chamber. VISAR recordings use a 659.5-nm probe laser. By adding a specially coated beam splitter at the interferometer
table, light at wavelengths from 540 to 645 nm is split into a thermal-imaging diagnostic. Because fused silica lenses are
used in the first triplet relay, the intermediate image planes for different wavelengths separate by considerable distances.
A pair of corrector lenses on the interferometer table reunites these separated wavelength planes to provide a good
image. Streak cameras perform all VISAR and thermal-imaging recording. Alignment techniques are discussed.
Optical diagnostics are currently being designed to analyze high-energy density physics experiments at the National Ignition Facility (NIF). Two independent line-imaging Velocity Interferometer System for Any Reflector (VISAR) interferometers have been fielded to measure shock velocities, breakout times, and emission of targets having sizes of 1-5 mm. An 8-inch-diameter, fused silica triplet lens collects light at f/3 inside the 30-foot-diameter NIF vacuum chamber. VISAR recordings use a 659.5-nm probe laser. By adding a specially coated beam splitter to the interferometer table, light at wavelengths from 540 to 645 nm is spilt into a thermal-imaging diagnostic. Because fused silica lenses are used in the first triplet relay, the intermediate image planes for different wavelengths separate by considerable distances. A corrector lens on the interferometer table reunites these separated wavelength planes to provide a good image. Thermal imaging collects light at f/5 from a 2-mm object placed at Target Chamber Center (TCC). Streak cameras perform VISAR and thermal-imaging recording. All optical lenses are on kinematic mounts so that pointing accuracy of the optical axis may be checked. Counter-propagating laser beams (orange and red) are used to align both diagnostics. The red alignment laser is selected to be at the 50 percent reflection point of the beam splitter. This alignment laser is introduced at the recording streak cameras for both diagnostics and passes through this special beam splitter on its way into the NIF vacuum chamber.
The National Ignition Facility (NIF) requires diagnostics to analyze high-energy density physics experiments. A VISAR (Velocity Interferometry System for Any Reflector) diagnostic has been designed to measure shock velocities, shock breakout times, and shock emission of targets with sizes from 1 to 5 mm. An 8-inch-diameter fused silica triplet lens collects light at f/3 inside the 30-foot-diameter vacuum chamber. The optical relay sends the image out an equatorial port, through a 2-inch-thick vacuum window, and into two interferometers. A 60-kW VISAR probe laser operates at 659.5 nm with variable pulse width. Special coatings on the mirrors and cutoff filters are used to reject the NIF drive laser wavelengths and to pass a band of wavelengths for VISAR, passive shock breakout light, or thermal imaging light (bypassing the interferometers). The first triplet can be no closer than 500 mm from the target chamber center and is protected from debris by a blast window that is replaced after every event. The front end of the optical relay can be temporarily removed from the equatorial port, allowing other experimenters to use that port. A unique resolution pattern has been designed to validate the VISAR diagnostic before each use. All optical lenses are on kinematic mounts so that the pointing accuracy of the optical axis can be checked. Seven CCD cameras monitor the diagnostic alignment.
The National Ignition Facility (NIF) requires diagnostics to analyze high-energy density physics experiments. As a core NIF early light diagnostic, this system measures shock velocities, shock breakout times, and shock emission of targets with sizes from 1 to 5 mm. A 659.5 nm VISAR probe laser illuminates the target. An 8-inch-diameter fused silica triplet lens collects light at f/3 inside the 33-foot-diameter vacuum chamber. The optical relay sends the image out an equatorial port, through a 2-inch-thick vacuum window, and into two VISAR (Velocity Interferometer System for Any Reflector) interferometers. Both streak cameras and CCD cameras record the images. Total track is 75 feet. The front end of the optical relay can be temporarily removed from the equatorial port, allowing for other experimenters to use that port. The first triplet can be no closer than 500 mm from the target chamber center and is protected from debris by a blast window that is replaced after every event. Along with special coatings on the mirrors, cutoff filters reject the NIF drive laser wavelengths and pass a band of wavelengths for VISAR, for passive shock breakout light, or for thermal imaging light (bypassing the interferometers). Finite Element Analysis was performed on all mounting structures. All optical lenses are on kinematic mounts, so that the pointing accuracy of the optical axis can be checked. A two-color laser alignment scheme is discussed.
The National Ignition Facility (NIF) requested an optical diagnostic for measuring shock velocities, shock breakout times, and shock emission of objects with sizes of 1 to 10 mm. For the polar port of the target chamber, an 8-inch triplet lens collects light at f/3 inside a 30-foot-diameter vacuum chamber and uses an optical relay to send the image into two interferometers located at a distance of 160 feet. Light propagates through a VISAR (Velocity Interferometry System for Any Reflector) interferometer employing a Mach-Zehnder configuration. After exiting the interferometers the images are recorded, both by streak cameras and CCD gated imagers. Discrete magnification changes are accomplished by swapping out optical elements. Large dove prisms are used to rotate the image to align a selected region of the object with the slits of the streak cameras. Unique mounting structures are required to remotely control the alignment of the optical axis. Finite Element Analysis (FEA) was performed on all mounting structures. The first 8-inch triplet can be no closer than 500 mm from the target chamber center and is protected by a blast window that has to be replaced after every event. The first several lens groups have to be fused silica for radiation resistance. A frequency-doubled Nd:YAG laser, operating at 659.5 nm, is used to illuminate the moving object. The VISAR laser wavelength had to be different than the first, second, and third harmonics of the NIF drive lasers.
Holographic data are acquired during hydrodynamic experiments at the Pegasus Pulsed Power Facility at the Los Alamos National Laboratory. These experiments produce a fine spray of fast-moving particles. Snapshots of the spray are captured using in-line Fraunhofer holographic techniques. Roughly one cubic centimeter is recorded by the hologram. Minimum detectable particle size in the data extends down to 2 microns. In a holography reconstruction system, a laser illuminates the hologram as it rests in a three- axis actuator, recreating the snapshot of the experiment. A computer guides the actuators through an orderly sequence programmed by the user. At selected intervals, slices of this volume are captured and digitized with a CCD camera. Intermittent on-line processing of the image data and computer control of the camera functions optimizes statistics of the acquired image data for off-line processing. Tens of thousands of individual data frames (30 to 40 gigabytes of data) are required to recreate a digital representation of the snapshot. Throughput of the reduction system is 550 megabytes per hour (MB/hr). Objects and associated features from the data are subsequently extracted during off-line processing. Discrimination and correlation tests reject noise, eliminate multiple-counting of particles, and build an error model to estimate performance. Objects surviving these tests are classified as particles. The particle distributions are derived from the data base formed by these particles, their locations and features. Throughput of the off-line processing exceeds 500 MB/hr. This paper describes the reduction system, outlines the off-line processing procedure, summarizes the discrimination and correlation tests, and reports numerical results for a sample data set.
In-line Fraunhofer holography has been developed and implemented at the Los Alamos National Laboratory to measure particle distributions of fast moving particles. Holography is a unique diagnostic that gives unambiguous information on the size and shapes of particle distribution over a 3D volume. Currently, the capability of measuring particles two microns in size which travel many mm/microsecond(s) ec has been demonstrated in hydrodynamic experiments at the Pegasus Pulsed Power Facility. Usually, for setting up an in-line holography experiment for measuring particles a few microns in size, the holographic film would be placed less than one centimeter from the particles. However, due to the high kinetic energy associated with the dynamic experiment, an optical relay system is used to relay the interference pattern 35 cm so that the glass hologram will survive. After the hologram has been recorded the data must be extracted. A spatially filtered laser is used to reconstruct a real image which is a projection of the particles over a 3D volume. Planes of data from this volume are digitized via a CCD camera by moving the hologram with a three axis actuator. After the data has been digitized it is then analyzed with intelligent image processing algorithms.
An optical lens system with varying magnification has been designed for a 70-mm image format. Twenty optical
elements were needed to provide for the 345- to 1050-mm focal length zoom range as well as the proper color correction
over the visible spectrum. A 4-in diameter port window limits thef/# of the optical lens system. Operation of the lens
system is done by actuating stepping motors through a MacPlus computer.
This lens system was designed for Los Alamos National Laboratory Group M-8, because no commercial zoom lens
existed that would change its reduction from 1/15 to 1/4 at a focusing range of 5 meters. Additionally, we required a larger,
non-standard image size that could be recorded by a rotating mirror streak camera. A Nikkor lens sales manual does offer a
long focal length, 35-nun lens only upon special order. The closest focusing range of this Nikkor lens is 6 meters.
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