David B. Holtkamp

Accuracy limits and window corrections for photon Doppler velocimetry

Symmetric, plate-impact experiments were performed to validate photon Doppler velocimetry (PDV) with established shock wave diagnostics. Impact velocity measurements using shorting pins demonstrated that the velocity accuracy of PDV can be 0.1% or better. Shock velocities and refractive indices were also measured with PDV (at 1550 nm) and velocity interferometer system for any reflector (VISAR) (at 532 nm) to obtain window corrections for single crystal LiF (100), c-cut sapphire, and z-cut quartz. Time-dependent, free-surface velocity histories for shocked LiF(100) provide a direct comparison between PDV and VISAR diagnostics and illustrate the benefits and shortcomings of the new diagnostic. Further implications of these results are presented.

An Emittance Scanner for Intense Low-Energy Ion Beams

An emittance scanner has been developed for use with low-energy H- ion beams to satisfy the following requirements: (1) angular resolution of ± 1/2 mrad, (2) small errors from beam space charge, and (3) compact and simple design. The scanner consists of a 10-cm-long analyzer containing two slits and a pair of electric deflection plates driven by a ±500-V linear ramp generator. As the analyzer is mechanically driven across the beam, the front slit passes a thin ribbon of beam through the plates. The ion transit time is short compared with the ramp speed; therefore, the initial angle of the ions that pass through the rear slit is proportional to the instantaneous ramp voltage. The current through the rear slit then is proportional to the phase-space density d2i/dxdx. The data are computer-analyzed to give, for example, rms emittance and phase-space density contours. Comparison of measured data with those calculated from a prepared (collimated) phase space is in good agreement.

Development of a scanning, solar-blind, water Raman lidar

The need for an instrument capable of measuring water-vapor fluxes over mixed canopy and large areas has long been recognized. Such a device would greatly enhance the study of evapotranspiration processes and has great practical value for water management. To address this problem, a scanning water Raman lidar has been designed and constructed. Analytical methods have also been developed to take advantage of the type of information that this lidar can generate. The lidar is able to measure the absolute water content and calculate the evaporative flux quickly over relatively large areas. This capability provides new opportunities for the study of microscale atmospheric processes. The variogram data indicate that the spatial sampling size must be of the order of 10 m if fluxes and scalars are to be properly represented. Examples of data are presented.

Spatial variability of water vapor turbulent transfer within the boundary layer

Although the physics of evaporation within the inner region of the boundary layer is believed to be well understood, observations of mass-energy exchange processes have been hindered by the limitations of point sensors. A combination of point sensors and active remote sensing, namely, water-Raman Lidar measurements, offers new opportunities to study relatively large areas at temporal and spatial scales previously unattainable. Results from experiments over uniform canopies both confirm some traditional theories and challenge some of the underlying assumptions concerning the homogeneity of the surface-atmosphere interface and the use of point sensors to characterize large areas.

Survey of Optical Velocimetry Experiments - Applications of PDV, A Heterodyne Velocimeter

Optical velocimetry has been an important experimental diagnostic for many experiments. Recent improvements to heterodyne techniques have resulted in compact, inexpensive and high performance velocimetry measurement systems. We report on developments and improvements in this area and illustrate the performance of Photon Doppler Velocimetry (PDV) by showing several experimental examples.

Derivation of water vapor fluxes from Lidar measurements

Two techniques are described by which the flux of water vapor can be derived from concentration measurements made by a Raman-Lidar. Monin-Obukhov similarity theory and dissipation techniques are used as the basis for these methods. The resulting fluxes are compared to fluxes from standard point instruments. The techniques described are appropriate for measuring the flux of any scalar quantity using Lidar measurements in the inner region of the boundary layer.

MEASUREMENT OF THE SHOCK-HEATED MELT CURVE OF LEAD USING PYROMETRY AND REFLECTOMETRY

Data on the high-pressure melting temperatures of metals is of great interest in several fields of physics including geophysics. Measuring melt curves is difficult but can be performed in static experiments (with laser-heated diamond-anvil cells, for instance) or dynamically (i.e., using shock experiments). However, at the present time, both experimental and theoretical results for the melt curve of lead are at too much variance to be considered definitive. As a result, we decided to perform a series of shock experiments designed to provide a measurement of the melt curve of lead up to about 50GPa in pressure. At the same time, we developed and fielded a reflectivity diagnostic, and conducted measurements on tin as well. The results show that the melt curve of lead is somewhat higher than the one previously obtained with static compression and heating techniques.

Measurement of proton induced radiation damage to CMOS transistors and p-i-n diodes

As part of a program to develop a silicon tracking device for the Superconducting Super Collider (SSC), radiation-hard CMOS transistors and p-i-n diodes have been exposed to the 800-MeV LAMPF (Los Alamos Meson Physics Facility) proton beam. The fluences accumulated in one week corresponded to the expected radiation levels of about ten SSC years. The leakage current constants for p-i-n diodes and threshold voltage shifts for CMOS transistors are determined under different biasing conditions. The results are presented and examined in detail. >

Using multispectral imaging to measure temperature profiles and emissivity of large thermionic dispenser cathodes

The authors measured temperature, temperature uniformity, and emissivity of two large thermionic tungsten cathodes. This was necessary because their operating life is strongly affected by nonuniformities in temperature. The cathodes were self-heated 6.5 and 8-in. disks operating in a vacuum at about 1l00 °C. We measured the temperature of the cathodes by a combination of multispectral imaging using a conventional CCD camera and spot pyrometry using three spot pyrometers. We took images at the blue end of the spectrum (0.4 μm) to determine temperature accurate to 3% K. Spot pyrometers at the red end of the spectrum (to 1.8 μm) yielded emissivity with an uncertainty of 15%. We emphasized using data in the blue to determine the temperature and data in the red to determine emissivity. Our analysis displays data so that assumptions in determining temperature and emissivity are transparent. Because we obtained a linear relationship between camera counts and integrated exitance of a blackbody, we concluded that ...

Release path temperatures of shock-compressed tin from dynamic reflectance and radiance measurements

Dynamic reflectance and radiance measurements were conducted for tin samples shock compressed to 35 GPa and released to 15 GPa using high explosives. We determined the reflectance of the tin samples glued to lithium fluoride windows using an integrating sphere with an internal xenon flashlamp as an illumination source. The dynamic reflectance (R) was determined at near normal incidence in four spectral bands with coverage in visible and near-infrared spectra. Uncertainties in R/R0 are <2%, and uncertainties in absolute reflectance are <5%. In complementary experiments, thermal radiance from the tin/glue/lithium fluoride interface was recorded with similar shock stress and spectral coverage as the reflectance measurements. The two sets of experiments were combined to obtain the temperature history of the tin surface with an uncertainty of <2%. The stress at the interface was determined from photonic Doppler velocimetry and combined with the temperatures to obtain temperature-stress release paths for tin. W...