Andrew W. Obst

A High‐Speed Four‐Channel Infrared Pyrometer

A high‐speed, four‐wavelength pyrometer has been developed for dynamic temperature measurements on samples that are heated by shock compression. The pyrometer uses a pair of off‐axis parabolic mirrors to collect radiance emitted from a target of 1 mm in diameter. A single optical fiber delivers the collected radiant flux to the detector housing. Three dichroic beam splitters are used to spectrally split the light into four beams that are then focused onto an equal number of LN2‐cooled InSb photodetectors. Broad bandwidth interference filters that are nominally centered at 1.8, 2.4, 3.4, and 5.0 μm define the wavelength ranges of the four channels. The blackbody‐temperature threshold of the pyrometer is at about 400 K. The signals are recorded at intervals as short as 20 ns using a four‐channel digital oscilloscope. Procedures for calibration and temperature measurements are described.

Ellipsometry in the Study of Dynamic Material Properties

Measurements of the time-dependent absolute temperature of surfaces shocked using high explosives (HE) provide valuable constraints on the equations-of-state (EOS) of materials and on the state of ejecta from those surfaces. In support of these dynamic surface temperature measurements, techniques for measuring the dynamic surface emissivity of shocked metals in the near infrared (IR) are being developed. These consist of time-dependent laser ellipsometric measurements, using several approaches. A discussion of these ellipsometric techniques is included here. Ellipsometry permits an accurate determination of the dynamic emissivity at a given wavelength, and may also provide a signature of melt in shocked metals.

Post‐Shock Temperature and Free Surface Velocity Measurements of Basalt

Basalt is the most common rock type on planetary surfaces. Post‐shock temperature and particle velocity measurements constrain the equation of state of basalt and provide fundamental information about the outcome of planetary impact events. A high‐speed, infrared, four‐wavelength pyrometer, developed at Los Alamos National Laboratory (LANL), is used with customized front end optics at the Harvard Shock Compression Laboratory for concurrent observations of particle velocity and free surface thermal emission. In an experiment on Columbia River basalt released from a peak shock pressure of 28.9±0.2 GPa, the apparent post‐shock temperature is wavelength dependent. The 3.5 and 4.8‐μm channels record apparent temperatures between 605 and 630 K, using an emissivity range of 0.7–1.0. The 1.8 and 2.3‐μm channels record apparent temperatures of ∼700 K and ∼800 K, respectively. The pyrometry data are well fit by a two component temperature distribution: (1) a predominantly 565–610 K free surface, in good agreement w...

X-ray emission from colliding laser plasmas

Colliding Au, CD, and Ti-CR plasmas have been generated by illuminating two opposing foils each with an approximately 100J, 0.5 nsec, 2(omega) Nd-glass laser beam from the Trident laser facility at Los Alamos. The plasmas are being used to study plasma interactions which span the parameter regime from interpenetrating to collisional stagnation. X-ray emission during the laser target interaction and the subsequent collision is used to diagnose the initial plasma conditions and the colliding plasma properties. X-ray instrumentation consists of a 100 ps gated x-ray pinhole imager, a time-integrated bremsstrahlung x-ray spectrograph and a gated x-ray spectrograph used to record isoelectronic spectra from the Ti-Cr plasmas. The imager has obtained multiframe images of the collision and therefore, a measure of the stagnation length which is a function of the ion charge state and density and a strong function of the electon temperature. Other isntrumentation includes a Thomson scattering spectrometer with probe beam, neutron detectors used to monitor the CE coated foil collisions, and an ion spectrometer. We will describe the current status of the experiments and current results with emphasis on the x-ray emission diagnostics. We will also briefly describe the modeling using Lasnex and ISIS, a particle-in-cell code with massless fluid electronics and inter-particle (classical) collisions.

A compact permanent magnet proton spectrometer

Abstract A compact spectrometer using samarium-cobalt permanent magnet material has been developed for observing 8–22 MeV protons. The spectrometer consists of a single quadrupole element followed by a dipole element. The device has an acceptance of approximately 3 msr and an instrumental resolution (fwhm) of approximately 80 keV at 10 MeV and 150 keV at 18 MeV.

Improving the time response of a gamma/neutron liquid detector

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.