Morris I. Kaufman

ICF gamma-ray reaction history diagnostics

Reaction history measurements, such as nuclear bang time and burn width, are fundamental components of diagnosing ICF implosions and will be employed to help steer the National Ignition Facility (NIF) towards ignition. Fusion gammas provide a direct measure of nuclear interaction rate (unlike x-rays) without being compromised by Doppler spreading (unlike neutrons). Gas Cherenkov Detectors that convert fusion gamma rays to UV/visible Cherenkov photons for collection by fast optical recording systems have established their usefulness in illuminating ICF physics in several experimental campaigns at OMEGA. In particular, bang time precision better than 25 ps has been demonstrated, well below the 50 ps accuracy requirement defined by the NIF. NIF Gamma Reaction History (GRH) diagnostics are being developed based on optimization of sensitivity, bandwidth, dynamic range, cost, and NIF-specific logistics, requirements and extreme radiation environment. Implementation will occur in two phases. The first phase consists of four channels mounted to the outside of the target chamber at ~6 m from target chamber center (GRH-6m) coupled to ultra-fast photo-multiplier tubes (PMT). This system is intended to operate in the 1013–1017 neutron yield range expected during the early THD campaign. It will have high enough bandwidth to provide accurate bang times and burn widths for the expected THD reaction histories (> 80 ps fwhm). Successful operation of the first GRH-6m channel has been demonstrated at OMEGA, allowing a verification of instrument sensitivity, timing and EMI/background suppression. The second phase will consist of several channels located just inside the target bay shield wall at 15 m from target chamber center (GRH-15m) with optical paths leading through the cement shield wall to well-shielded streak cameras and PMTs. This system is intended to operate in the 1016–1020 yield range expected during the DT ignition campaign, providing higher temporal resolution for the expected burn widths of 10–20 ps associated with ignition. Multiple channels at each phase will allow for increased redundancy, reliability, accuracy and flexibility. In addition, inherent energy thresholding capability combined with this multiplicity will allow exploration of interesting gamma-ray physics well beyond the ignition campaign.

Design of a Thermal Imaging Diagnostic Using 90-Degree, Off-Axis, Parabolic Mirrors

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.

Fielding of an imaging VISAR diagnostic at the National Ignition Facility (NIF)

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.

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.

Imaging VISAR diagnostic for the National Ignition Facility (NIF)

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.

Design of an imaging VISAR diagnostic for the National Ignition Facility (NIF)

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.

Design and assembly of a telecentric zoom lens for the Cygnus x-ray source

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.

Overview of the line-imaging VISAR diagnostic at the National Ignition Facility (NIF)

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.

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.

A fisheye lens as a photonic Doppler velocimetry probe

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.