Chester L. Shepard

Four‐frame holographic probing system for plasma density measurement (invited)

An important upgrade of UV holographic interferometry is now in use. Many hundreds of laser fusion-type target plasmas have been probed at KMS Fusion, Inc. using the holographic technique. The data obtained with this diagnostic include high-resolution images and plasma electron density distributions at various probe times. The diagnostics system now produces a sequence of four interferograms of an evolving plasma. The times of the 20-ps pulses in the sequence are fully adjustable and known within 10 ps. The experiments to which this diagnostic has been adapted will be described along with details of problems and their solutions. Selected sets of recent interferograms of plasmas produced by irradiation of thin foils will be presented along with their inversions.

Determination of temperature in glass with a fluorescence method

Abstract We have developed a method for measurement of temperature in glass products with the following features: (1) non-contacting, (2) real time, and (3) through-thickness. The method is based on fluorescence emission. Many glass products include Fe2O3 as an additive in various amounts. Ferric (Fe3+) and ferrous (Fe2+) ions absorb light in the ultraviolet and infrared parts of the spectrum. Absorption of light by iron ions in glass results in a predictable fluorescence emission. The emission in turn depends on the glass temperature, and this dependence is the basis for our method of measuring temperature. We have measured the fluorescence emission lifetimes in several commercial automotive glass samples over a temperature range from 25°C to 550°C (about 300–825 K). Imaging the fluorescence emission from glass samples onto a segmented photomultiplier tube provided spatially resolved measurements. A simple model that relates the temperature to the fluorescence lifetime has been developed.

Characterization of sub-10-μm 30-ps flash duration point sources for X radiography

Flash x radiography can provide important information about high‐density, high‐opacity plasmas. Experiments at KMS investigated the characteristics of point x‐ray sources produced when coated fibers, wires, and mounted particles were irradiated with high intensity (I>1016 W/cm2) green laser light. The target materials Bi, CsI, Au, and Ag were evaluated in the areas of (1) the spatial and temporal resolutions that can be achieved, (2) the effect of source size and laser pulse duration on x‐ray signal strength, and (3) the practical considerations associated with using the technique to actually obtain radiographs of accelerated polyvinyl alcohol shells.

Noninvasive ultrasonic examination technology in support of counter-terrorism and drug interdiction activities: the acoustic inspection device (AID)

The Pacific Northwest National Laboratory (PNNL) has developed a portable, battery-operated, handheld ultrasonic device that provides non-invasive container interrogation and material identification capabilities. The technique governing how the acoustic inspection device (AID) functions, involves measurements of ultrasonic pulses (0.1 to 5 MHz) that are launched into a container or material. The return echoes from these pulses are analyzed in terms of time-of-flight and frequency content to extract physical property measurements (the acoustic velocity and attenuation coefficient) of the material under test. The AID performs an automated analysis of the return echoes to identify the material, and detect contraband in the form of submerged packages and concealed compartments in liquid filled containers and solid-form commodities. An inspector can quickly interrogate outwardly innocuous commodity items such as shipping barrels, tanker trucks, and metal ingots. The AID can interrogate container sizes ranging from approximately 6 inches in diameter to over 96 inches in diameter and allows the inspector to sort liquid and material types into groups of like and unlike; a powerful method for discovering corrupted materials or miss-marked containers co-mingled in large shipments. This manuscript describes the functionality, capabilities and measurement methodology of the technology as it relates to homeland security applications.

Non-invasive ultrasonic instrument for counter-terrorism and drug interdiction operations - the acoustic inspection device (AID)

The Pacific Northwest National Laboratory (PNNL) has developed a portable, battery-operated, handheld ultrasonic device that provides non-invasive container interrogation and material identification capabilities. The technique governing how the acoustic inspection device (AID) functions, involves measurements of ultrasonic pulses (0.1 to 5 MHz) that are launched into a container or material. The return echoes from these pulses are analyzed in terms of time-of-flight and frequency content to extract physical property measurements (the acoustic velocity and attenuation coefficient) of the material under test. The AID performs an automated analysis of the return echoes to identify the material, and detect contraband in the form of submerged packages and concealed compartments in liquid filled containers and solid-form commodities. This device utilizes a database consisting of material property measurements acquired from an automated, ultrasonic fluid characterization system called the Velocity-Attenuation Measurement System (VAMS). Both prototype technologies are discussed here. This manuscript describes the functionality, capabilities and measurement methodology of the technology as it relates to the material property measurements and homeland security applications.

Stress measurements in glass by use of double thermal gratings

We developed a nondestructive and noncontact method for measuring stress at the midplane of tempered glass plates that uses Bragg scattering from a pair of thermal gratings. These gratings are formed by 1064-nm beams from a seeded Nd:YAG laser, and we measure the polarization state of light from a 532-nm beam that scatters from both thermal gratings. The change in polarization of the doubly scattered light with separation between the two gratings allows measurement of the in-plane stress. A model of the Bragg scattering efficiency, experimental investigations of the scattered beams, and stress measurements are reported.

Hydrodynamic Aspects of X-Ray Laser Targets

X-ray gain experiments at λ ~ 200A in laser-induced exploding selenium foil targets were successfully demonstrated at LLNL’s Novette laser facility in 1984. Recent experiments at KMS were designed to examine parameter variations of target thickness, laser duration and intensity, and one-sided vs two-sided irradiation, as a means of improving the performance of the gain experiments. Preliminary experiments were also performed on double-foil targets in attempts to prolong the duration and spatial extent of the x-ray lasing conditions in the space between the two foils. Four-frame holographic interferometry was used in determining the time dependence of the evolving density profiles obtained by Abel inversion of the interferometric fringe field and x-ray crystal spectroscopy was applied to monitor the x-ray emission.

Image Enhancement Tools For Tracing Fringe Patterns In Holographic Interferograms Acquired During Laser Fusion Experiments

Pulsed holographic interferometry is essentially the only direct method for determining electron density profiles in inertial fusion plasmas. Consequently, it is a very important diagnostic tool in laser fusion experimentation. The tracing of fringe patterns in the reconstructed holograms is required to determine their precise number and location for subsequent Abel inversion. This is a very labor-intensive task, for which computer assistance has long been sought. In the KMS Fusion multiframe optical probing system, a sequence of four time resolved image frames is produced at rates equivalent to over 5 billion/sec. The increased number of images thus generated has spurred the development of improved methods for handling data. A plan has evolved for providing scientists with interactive adaptive image enhancement tools to assist in locating the fringes. The feasibility of applying digital techniques to aid in the analysis of holographic interferograms has been demonstrated by others. However, only limited success has been achieved in tracing highly dense fringes in the presence of noise. Traditional noise reduction methods tend to fail in the case of high density fringes, where the spatial frequency of the noise is close to that of the pattern to be discerned. Other problems are introduced by uneven lighting conditions, competing fringe patterns (due to aberrations in optical components or other attenuators in the optical path), and bonafide discontinuities in the fringes. Newly developed digital enhancement tools apply tailorable neighborhood operators to individual pixels as directed by a cursor that may be manipulated via joystick or keyboard control. Operations may be performed on a sectional blow-up while viewing both the full image and the enlarged section. In this manner, global information can be utilized to aid in the local enhancement operations, and vice versa. This paper constitutes a progress-to-date report on work that is continuing for the U.S. Department of Energy.