Aric Tibbitts

Design and construction of a Gamma reaction history diagnostic for the National Ignition Facility

Gas Cherenkov detectors have been used to convert fusion gammas into photons to record gamma reaction history measurements. These gas detectors include a converter, pressurized gas volume, relay collection optics, and a photon detector. A novel design for the National Ignition Facility (NIF) using 90° off-axis parabolic mirrors efficiently collects signal from fusion gammas with 8-ps time dispersion. Fusion gammas are converted to Compton electrons, which generate broadband Cherenkov light (response is from 250 to 700 nm) in a pressurized gas cell. This light is relayed into a high-speed detector using three parabolic mirrors. The relay optics collect light from a 125-mm-diameter by 600-mm-long interchangeable gas (CO2 or SF6) volume. The parabolic mirrors were electroformed instead of diamond turned to reduce scattering of the UV light. All mirrors are bare aluminum coated for maximum reflectivity. This design incorporates a 4.2-ns time delay that allows the detector to recover from prompt radiation before it records the gamma signal. At NIF, a cluster of four channels will allow for increased dynamic range, as well as different gamma energy thresholds.

Design, assembly, and testing of the neutron imaging lens for the National Ignition Facility

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 intensifiers 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.

Ejecta Particle-Size Measurements in Vacuum and Helium Gas using Ultraviolet In-Line Fraunhofer Holography

An Ultraviolet (UV) in-line Fraunhofer holography diagnostic has been developed for making high-resolution spatial measurements of ejecta particles traveling at many mm/μsec. This report will discuss the development of the diagnostic including the high-powered laser system and high-resolution optical relay system. In addition, the system required to reconstruct the images from the hologram and the corresponding analysis of those images to extract particles will also be described. Finally, results from six high-explosive (HE), shock-driven Sn ejecta experiments will be presented. Particle size distributions will be shown that cover most of the ejecta velocities for experiments conducted in a vacuum, and helium gas environments. In addition, a modification has been made to the laser system that produces two laser pulses separated by 6.8 ns. This double-pulsed capability allows a superposition of two holograms to be acquired at two different times, thus allowing ejecta velocities to be measured directly. Results from this double pulsed experiment will be described.

Design, assembly, and testing of a high-resolution relay lens used for holography with operation at both doubled and tripled Nd:YAG laser wavelengths

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 windows 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.

Alignment and testing of a telecentric zoom lens used for the Cygnus x-ray source

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.

Overview of the Gamma Reaction History Diagnostic for the National Ignition Facility (NIF)

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.

Imaging shielded configurations using near-horizontal and near-vertical trajectory cosmic-ray muons

This work will describe the proof-of-concept research applying muon tomography technologies based on drift tube systems to create images using near-horizontal trajectory muons. To date, the majority of imaging studies using cosmic-ray muons have used near-vertical trajectory muons. This work compares imaging results using near-vertical trajectory muons with results using near-horizontal trajectory muons. The muon flux is much lower for the near-horizontal trajectory muons, requiring longer imaging times, but the average muon energy is higher, so the horizontal results are expected to better differentiate high-Z materials. The muon tracking system is easily configurable and can be oriented to capture near-vertical trajectory or near-horizontal trajectory cosmic-ray muons. The software can track each muon passing through the system, and generate 3D images of the scene. The experimental design and preliminary results will be presented, including the comparisons of detection efficiency, image resolution, and integration times.

Development of a microchannel plate based gated x-ray imager for imaging and spectroscopy experiments on z

This paper describes the development of a microchannel plate (MCP)-based gated x-ray imager developed by National Security Technologies, LLC (NSTec) and Sandia National Laboratories over the past several years. The camera consists of a 40 mm × 40 mm MCP, coated with eight 4 mm wide microstrips. The camera is gated by sending subnanosecond high-voltage pulses across the striplines. We have performed an extensive characterization of the camera, the results of which we present here. The camera has an optical gate profile width (time resolution) as narrow as 150 ps and detector uniformity of better than 30% along the length of a strip, far superior than what has been achieved in previous designs. Reliability and reproducibility of the imager are also greatly enhanced. The spatial resolution is on the order of 40 microns for imaging applications and a dynamic range of between ∼100 and ∼1000. We also present results from a Monte Carlo simulation code developed by NSTec over the last several years. Agreement between the simulation results and the experimental measurements is very good.

Dynamic Characterizations of an 8-frame Half-Strip High-speed X-ray Microchannel Plate Imager

High-speed microchannel plate (MCP)-based imagers are critical detectors for x-ray diagnostics employed on Z-experiments at Sandia National Laboratories (SNL) to measure time-resolved x-ray spectra and to image dynamic hohlraums. A multiframe design using eight half strips in one imager permits recordings of radiation events in discrete temporal snapshots to yield a time-evolved movie. We present data using various facilities to characterize the performance of this design. These characterization studies include DC and pulsed-voltage biased measurements in both saturated and linear operational regimes using an intense, short-pulsed UV laser. Electrical probe measurements taken to characterize the shape of the HV pulse propagating across the strips help to corroborate the spatial gain dependence

Large Format X-ray Pinhole Camera

Summary form only given. National Security Technologies, LLC has successfully implemented many scientific and engineering innovations in the Large-Format Pinhole Camera (LFPHC), which have dramatically increased the detection sensitivity and reliability of the camera in exotic locations, such as the Sandia National Laboratories Z-facility. The LFPHC uses 40 x 76 mm microchannel plates that provide larger magnification capabilities for X-ray imaging. Improvements of the LFPHC have been demonstrated in its fielding at Z, where high-quality images were recorded. A major improvement was the development of a new, user-friendly LFPHC camera back that would tolerate high levels of radiation, electromagnetic interference, and mechanical shock. Key modifications resulted in improved detection sensitivity-, spatial resolution, uniformity along the microchannel plate strip, and stability of the interframe timing and delay. Design considerations and improvements are discussed.