Optical engineering at Los Alamos, which began in 1943, has continued because scientific researchers usually want more resolving power than commercially available optical instruments provide. In addition, in-house engineering is often advantageous--when the technology for designing and making improved instrumentation is available locally--because of our remote location and the frequent need for accurate data. As a consequence, a number of improved research cameras and lens systems have been developed locally--especially for explosion and implosion photography, but even for oscilloscope photography. The development of high-speed cameras led to the ultimate in practical high-speed rotating mirrors and to the invention of a rapid, precise, and effective lens design procedure that has produced more than a hundred lens systems that give improved imaging in special conditions of use. Representative examples of this work will be described.
The laser programs at Los Alamos began in 1969 to investigate the feasibility of laser-induced fusion. However, within a year they had been expanded to include a number of other applications including laser isotope separation. These programs now compose a substantial part of the Laboratory's research programs.
The story of Visual Simulation as a useful tool in Training and Engineering Design is now over twenty years old. It has expanded to a multi-billion dollar industry and yet not very much is known about the details outside those directly involved in the field.
The alignment of tilted and decentered optics is ordinarily difficult, because such optics have neither simple alignment points amenable to ordinary boresignt methods, nor a simple alignment tneory. Several different alignment examples which provide insight into a practical universal approacn to all such systems are explored. The examples detailed are segments of tne Antares Laser Fusion Project's optical train.
Antares is a 24-beam, 40-TW carbon-dioxide laser fusion system currently nearing completion at the Los Alamos National Laboratory. The 24 beams will be focused onto a tiny target (typically 300-1000 pm in diameter) located approximately at the center of a 7.3-m diameter by 9.3-m long vacuum (10-6 torr) chamber. The design goal is to position the targets to within 10 μm of a selected nominal position, which may be anywhere within a fixed spherical region 1 cm in diameter. The Antares Reference Telescope System is intended to help achieve this goal for alignment and viewing of the various targets used in the laser system. The Antares Reference Telescope System consists of two similar electro-optical systems positioned in a near orthogonal manner in the target chamber area of the laser. Each of these consists of four subsystems:
1) a fixed 9X optical imaging subsystem which produces an image of the target at the vidicon;
2) a reticle projection subsystem which superimposes an image of the reticle pattern at the vidicon;
3) an adjustable front-lighting subsystem which illuminates tne target; and
4) an adjustable back-lighting subsystem which also can be used to illuminate the target. The various optical, mechanical, and vidicon design considerations and trade-offs are discussed. The final system chosen (which is being built) and its current status are described in detail.
The Antares Automatic Alignment System employs a specially designed telescope for alignment of its laser beamlines. There are two telescopes in the system, and since eacn telescope is a primary alignment reference, stringent boresight accuracy and stability over the focus range were required. Optical and mechanical designs, which meet this requirement as well as that of image quality over a wide wavelength band, are described. Special test techniques for initial assembly and alignment of the telescope are also presented. The telescope, which has a 180-mm aperture FK51-KZF2 type glass doublet objective, requires a boresight accuracy of 2.8 prad at two focal lengths, and object distances between 11 meters and infinity. Travel of a smaller secondary doublet provides focus from 11 m to infinity with approximately 7.8 m effective focal length. By flipping in a third doublet, the effective focal length is reduced to 2.5 m. Telescope alignment was accomplished by using a rotary air bearing to establish an axis in front of the system and placing the focus of a Laser Unequal Path Interferometer (LUPI) at the image plane.
The Antares high energy gas laser system uses electroplated copper mirrors to reflect the laser beam through the amplifiers and focus it on the target. These mirrors are either polished or single point diamond machined to meet rigid optical specifications. They are inspected within 24 hours after final machining using a laser interferometer. A routine reinspection of these mirrors after arrival at Los Alamos revealed that some had severe distortions far greater than allowed by the optical specifications. The worst examples exhibited a roughened surface, visible to the eye, similar to an "orange-peel" effect. Metallographic analysis of some of the mirrors showed a highly stressed layer within the electrodeposited copper. This layer had a fibrous or needle-like structure and had a micro-hardness of 170 to 180 DPH. Large, stress-free grains with a micro-hardness of 50 to 55 DPH were mixed in with the fibrous layer in some instances. It soon became evident that the hard, fibrous structure was in varying stages of recrystallization, and the recrystallization was taking place at room temperature. Further investigation revealed that the fibrous layer was the result of excessive levels of organic brightening agent in the electrolyte during the electroplating process. Enough stress was stored in the deposit due to the addition agent to provide an unstable microstructure, which subsequently recrystallized over a period of three to six months. In order to salvage the mirrors, a thermal treatment was developed to force recrystallization. Heating the mirrors to 140 for 90 minutes provides a stable microstructure which machines well. Although some of the grains are large, they have no apparent effect on machineability or reflectivity.
Many laser systems use several laser beams which may interact either coherently or incoherently. Most physical optics codes model the propagation of a single laser beam. Some codes can analyze the kinetics of multiple laser lines. A powerful computer code, called LOTS2, has been developed at the Los Alamos National Laboratory by UNM which allows the simultaneous treatment of different laser beam trains. These beams may have different wavelengths, apertures, divergences, substantially different Fresnel numbers, and need not be colinear. Multiphoton phenomena such as Raman processes and isotope separation may be studied as well as conventional single photon phenomena. LOTS2 has a virtual memory feature that allows computer arrays to be larger than the central memory of mainframe machines such as the CDC 7600 and Cray 1. The separability of the diffraction propagation mathematics has been exploited to allow cylindrical lenses and high aspect ratio beam cross sections to be modeled. Considerable effort has been devoted to making the code easy to use.
A unique high-resolution Fourier transform spectrometer is being designed for use at Los Alamos. The resolution will be between 0.001 and 0.002 cm-1 and the spectral coverage will be from the ultraviolet to the infrared (200 nm to 20 μm). The design is based on an enhanced version of the instrument at Kitt Peak National Observatory. Our FTS will have twice the resolution and several innovations which will improve the speed and ease of use. The step size is variable with a resolution of 0.3 nm; this will eliminate the problems with aliasing caused by a fixed free spectral range. In addition, the scanning will be continuous; a total high-resolution scan will take 4 minutes. To improve the signal to noise ratio, multiple scans can be co-added. With this instrument we will be able to measure line positions to 3 MHz (0.0001 cm-1) and intensities to better than 1% in selected cases. The design of this instrument is in the preliminary stages. The proposed performance specifications are given in Table I. Figure 1 is a system schematic which gives the major components of the system. There will be two Motorola 68000 microcomputers controlling the instrument, with one large host computer interfacing to the users. The cat's-eye reflectors replace the mirrors in a Michelson inteferometer and eliminate most of the problems with tilt in the mirrors because cat's-eye reflectors are insensitive to small changes in their orientation.
Action spectra for a number of light-mediated physiological processes, (e.g. germination, flowering, elongation) indicated that the effective wavelength for induction was between 600-700 nm and for supression was between 700-760 nm, with maxima at 660 nm and 730 nm respectively (see Smith 1975 for review). These studies predicted the existence of the photoreversible pigment phytochrome (P) existing in two forms, interconvertible by red and far-red light. The photo-equilibrium of the red absorbing (Pr) and far-red absorbing (Pfr) forms is determined by the proportions of red and far-red light available. Most of the infornation cooes from studies on dark grown plants using narrow band or uonochromatic light and until recently very little work has been done on the role of phytochrome in the natural environment. Because changes in the distribution of this physiologically active light in nature will result in an altered photo-equilibrium of the two forms of phytochrome, a new quantity c (zeta) was defined, as the ratio of the quantum flux at 660 ni to the quantum flux at 730 nm (Holmes and McCartney 1976, Monteith 1976). This relationship of zeta to the photochrome photoequilibrium (% Pfr) was determined for a series of natural and artificial light sources (Smith and Holmes 1977). owever, radiation of shorter wavelengths also has an infuence on plant development through its action on phytochrome (Parker et al 1946, Bertsch 1963). The absorption spectra of the two forms of phytochrome show, in addition to the vajor absorption bands in the red and far-red regions, minor bands in the blue and near uv (Hendricks 1962, Siegelman and Fuer 1964). Also photochrome does undergo light-induced absorbance changes 'in vitro' in the blue region of the spectrum (Everett and Briggs 1970). A more accurate estimate of photochrome photoequilibria would
Basic modifications of the Sacramento Peak Observatory Universal Birefringent Filter system and resultant major improvements in its performance are described. Spectral tuning is accomplished by rotating the nine birefringent elements that comprise the filter. For this, stepper motors are used under computer control, where one step is equivalent to a precision of 0.0025 Å (at 7000 A) for the narrowest bandpass element. The element angles are accurately set for an arbitrary wavelength by an algorithm derived from known tune solutions. Absolute wavelength calibration is provided by a He-Ne laser source, while an integrated monochromator can be used for the same purpose, though less accurately, throughout the 4100 A to 7000 A spectral range of the filter. The filter is maintained in a thermally stable environment controlled to a precision of 0.05° C; any incremental temperature change in the filter itself can be detected and its effects compensated by means of a laser calibration. In operation, the filter can typically be tuned to a selected wavelength in less than is with a precision of 1 mÅ.
We consider in this work the anisotropic microstructure contribution to the polarization effect in thin films deposited at high angles of vapor incidence. Previous reports that metal films mainly respond as polarizers under such conditions have been confirmed. Structural anisotropy in dielectric films, rather than affecting the extinction coefficient values differently along different directions, seems to induce significant asymmetry in the refractive index values that enables us to find a retarder-like behavior. Quantitative analysis of the effect under normal incidence light at 632.8 nm is presented for aluminum and zirconium oxide films, and its structural origin is discussed.
The gain region in a tunable dye laser cavity is typically quite small and, therefore, the beam emerging from this region has a small diameter. In order to make effective use of a wavelength selective element such as a Littrow mounted diffraction grating in such a cavity, a beam expander is required. The use of prisms in this beam expander requires a high angle of incidence at some interfaces. Anti-reflection coatings of TiO2 and Si02 were designed and fabricated on fused quartz (75°) and BK-7 glass (83.1°) substrates. Both coatings were designed to select p polarization in the dye cavity.
A new imaging block was developed to record a full 3600 image from the space around its optical axis without using any rotation. This imaging block is suitable for infrared imaging in the near-IR, and this capability can be extended to the middle or even far infra-red by choosing appropriate material the block is manufactured of. The advantage of this imaging block is its relative simplicity and low costs. It also permits panoramic reconstruction of the recorded image if the recording is reprojected through a similar optical block onto a cylindrical screen.
Large, inexpensive, lightweight mirrors needed for ground and space based astronomy can be made from borosilicate glass. The thermal gradients in glass that degrade the figure of thick borosilicate mirrors during use can be largely eliminated in a honeycomb structure, by internal ventilation (in air) or careful control of the radiation environment (in space). We anticipate that ground based telescopes with honeycomb mirrors will give better images than those with low expansion solid mirrors, because even slight temperature differences in the air near a slowly equilibrating solid mirror can significantly degrade the wavefront at a turbulent boundary. Two fabrication methods are under investigation, remelting glass into complex molds of vacuum formed ceramic fiber and fusion methods. This paper descibes the former method used to cast in one piece either waffleplate or full honeycomb sandwich blanks. Details are given on the construction of molds and the cycle for melting and annealing. The technique is an extension of the method used nearly 50 years ago for the Palomar 5.1m mirror. In October 1982 we cast a 1.1m square blank with a 2.4cm thick face and 1.6cm ribs, 15cm deep on 15cm squares. A 1.8m circular blank of full honeycomb construction is scheduled for production in April 1983. We anticipate casting blanks up to 8m in diameter. Densities of 200kg/m2, like that of the ST mirror, are typical of casting. A summary is given of our development of a new method for making honeycomb structure by fusion bonding.
The radiant transmittance of a tilted plano surface in a convergent conical beam of varying intensity depends upon the "local" angle of incidence and the projection of the plane-polarized flux onto the plane of incidence. For the surface at Brewster's angle and modest cone angles, the insertion loss for a plane-polarized beam can be as large as 10%, negating any advantage of the Brewster's angle window. Details of the calculations will be given and examples will be shown.
Optical fibers provide important advantages over coaxial cables for many data trans-mission applications. Some of these applications require that the fibers transmit data during a radiation pulse. Other applications utilize the fiber as a radition-to-light transducer. In either case, radiation-induced luminescence and absorption must be under-stood.
Advances in optical design have been closely related to advances in numerical computation. The microcomputer revolution now taking place promises to open the way for new and innovative optical design. This will take place as optical design becomes easier, more available, and far less costly, as the result of powerful "canned" design programs outfitted on low-cost personal computers. A user-friendly program is described which makes possible low cost "automatic" lens design. Solutions to some typical optical design problems are presented. Prospects for future capabilities are described.
Experiments with a tapered-wiggler free-electron laser have demonstrated extraction of about 3% of the energy from the electron beam and measured the corresponding optical emission. These results are in excellent agreement with theory and represent an order-of-magnitude improvement over all previous results.
The evolution of a long pulse (pulse length much greater than the slippage distance) in a tapered wiggler free electron laser oscillator is studied by numerical solution of the one dimensional theoretical model for a realistic set of magnet, electron beam, and optical resonator parameter values. Single pass gain curves are calculated for low and high light intensities. We find that an initial, low amplitude, incoherent pulse grows into a coherent pulse whose growth rate agrees with the calculated small signal gain curve. The transient evolution of coherent pulses is calculated for several different cavity length detunings, and a quasi-steady state desynchronism curve is obtained. Detailed pulse properties at two points of the desynchronism curve are given. The frequency changing behavior ("chirping") of the optical pulse during transient evolution is examined.
Rigorous numerical solutions of a stable resonator, free-electron laser are obtained using 3-D wave propagation algorithms in the presence of a radially and azimuthally varying gain. Assumptions of this time-independent formulation of the loaded-resonator cavity are discussed. Wave propagation in the cavity is performed by computing numerically the Fresnel-Kirchoff diffraction integral by the Gardner-Fresnel-Kirchoff algorithm. Results of steady-state numerical iterative solutions, in which both the gain and the optical fields achieve self-consistency throughout the resonator, are presented. These consist of: (1) mode pattern and (2) variations in gain with variations in the resonator parameters.
The Kroll-Morton-Rosenbluth description of FEL amplifiers is utilized to model gain in Rocketdyne's numerical wave-optics analysis of FEL resonators. The tapered-wiggler amplifier model includes the effects of off-axis variations in the initial electron-beam density and azimuthally dependent effects of betatron oscillations on electron orbits. Off-axis variations in the linearly polarized wiggler magnetic field are also included. When run in the amplifier mode it incorporates the standard analysis of Gaussian beam propagation. This numerical model is an elaboration of the one-dimensional computer program FELMOV written by R. K. Cooper of Los Alamos. Computations showing the importance of the above physical effects and sensitivity studies indicating the degree of refinement needed to adequately treat them will be presented.
A Cerenkov gas laser would consist of a suitable gas at near atmospheric pressure, a highly relativistic electron beam, and an optical cavity. The electron beam emits spontaneous Cerenkov radiation which is reflected on the beam at the Cerenkov angle by the cavity. This, in turn, stimulates further emission. In an idealized situation the predicted gain of such a system, when operated in the visible region of the spectrum, could be quite high (many percent/pass). Results of an idealized calculation and departures therefrom caused by finite beam emittance and energy spread, velocity space diffusion of the beam in the resonator, and constraints imposed by beam and cavity characteristics will be discussed.
Recent investigations of the multiphoton ionization (MPI) spectrum of gaseous ammonia have led to the discovery of a novel two-photon pumped molecular gas electronic transition laser. Resonant, two-photon electronic excitation of NH3 in the near uv (~305 nm) leads to the first observation of fluorescence from NH3 excited states (B and C'), and, at higher pressures, to lasing action (~570 nm) between numerous C' and A state vibronic levels. A frequency-doubled Nd:YAG pumped dye laser (a few mJ) is focused into a cell containing NH3 (or ND3). Stimulated emission is observed in the forward and backward direction at NH3 pressures greater than -200 torr, without external mirrors to provide feedback. Conversion efficiencies (output NH3 pulse energy/input pulse energy) as high as 2% have been observed.
The 342-nm molecular iodine and 1315-nm atomic iodine lasers have been optically pumped by intense light from exploding-metal-film and exploding-wire discharges. Brightness temperatures for the exploding-film discharges were -25,000 K and for the wire discharges were -30,000 K. For the 12 laser the 3.5-cm diameter by 40-cm long pumped volume lies adjacent to the wire or film of the same length. Pressures of 1-6 torr 12 and 1-3 atm SF; CF4, or Ar were used in the stainless-steel cell. Using 20-pF capacitance charged to 40 kV, a 0.25-mm tungsten wire, 3-torr 12, and a 2-atm SF6, an energy of 2 J was obtained from the laser in a pulse of 8-Ns duration. The specific output energy was 7 J/k. Substitution of a cylin-drical A2 film for the wire, under otherwise similar conditions, led to a X10 output energy reduction. With an active volume and wire of 70-cm length greater output energies and efficiencies were obtained with similar input energy. An output pulse of 12 J and 12-ps duration was measured for a specific output energy of 18 J/ℓ. A laser energy of 110 J in a 20-us-long pulse has been measured from atomic iodine using a wire discharge along the axis of a larger cell. The active volume available was 20 cm in diameter and 80 cm in length. Input energy was 32 kJ. In similar measurements using a cylindrical AR, film for discharge initiation, the measured output energy was 40 J.
A copper vapor laser was developed with a flat-plate Blumlein circuit that can generate a fast transverse discharge current pulse with a risetime of 9 ns. This design opens up a new possibility for the development of the high-power copper vapor laser.
Laser emission has been generated in 15ND3 by pumping at 860.4 cm-1. Strong laser action has been observed at 123 cm-1, 109 cm-1 and, with up to 10 millijoules extracted, at 628.1 cm -1. Spectroscopic analysis indicates that the pumping arises from, and the 628.1-cm-1 emission terminates on, the same rotational state. Analysis of the time histories of the three laser emissions as well as studies of laser output energies indicates that 15.9-micron output arises from a 4-wave type process. This system, in addition, clarifies the interpretation of earlier studies of CO2-laser-pumped ammonia lasers.
High-repetition-rate generation of up to 325 mJ of Raman amplified radiation near 615 cm-1 has been demonstrated in CO2-pumped para-H2 using a low-power, microwave-shifted CF4 laser as an input Stokes seed source. Experiments were limited to 200 Hz, but single-shot Schlieren measurements indicate that our flowing room-temperature 44-pass Raman con-verter should be capable of the design goal of 1 kHz. Strong conversion was achieved even with no flow at 100 Hz. Details of the overall system design, experimental parameters, and present system limitations are discussed.
InGaAsP/InP buried crescent lasers with separate optical confinement can be grown to emit at any wavelength from ~ 1 pm to 1.68 μm by adjusting only the quaternary compositions, with other growth or processing steps unchanged. The addition of the separate optical confine-ment waveguides on each side of the active layer yields (a) higher optical power (b) improved optical linearity (c) higher external quantum efficiency (d) improved beam quality and (e) higher device yield. The reproducibility of growth is further demonstrated by forming arrays of 5 lasers on 8 pm centers which operate in lowest order transverse mode up to 50 mW per facet and have thresholds as low as 60 mÅ with external quantum efficiencies 55%.
The beam alignment system for the 24-beam-sector Antares CO2 fusion laser automatically aligns more than 200 optical elements. A visible-wavelength alignment tecnnique is employed which uses a telescope/TV system to view point-light sources appropriately located down the beamline.. The centroids of the light spots are determined by a video tracker, which generates error signals used by the computer control system to move appropriate mirrors in a closedÃ¢â‚¬â€?loop system. Final touch-up alignment is accomplisned by projecting a CO2 alignment laser beam through the system and sensing its position at the target location. Tne techniques and control algorithms employed have resulted in alignment accuracies exceeding design requirements. By employing video processing to determine the centroids of diffraction images and by averaging over multiple TV frames, we achieve alignment accuracies better than 0.1 times system diffraction limits in the presence of air turbulence.
The Antares Driver Amplifier utilizes Cassegrain optics. It was observed that for an input of a single 1-ns pulse, multiple pulses were produced at the output. The characteristic time between these pulses was exactly the round-trip time of the two Cassegrain optics. The study to eliminate these extra pulses had two parts, one experimental and the other a computer modeling. Many ideas were tried during the experimental phase, and only one technique was found to significantly reduce the extra pulses; that was to properly mask the primary mirror. The computer modeling made use of a ray-tracing code to look for multiple passes due to scattering or diffraction of various optical surfaces, or misalignment of the optics themselves. The results showed that there were several such paths, but the most prominent one was scattering or diffraction of the primary mirror around the inner hole, in agreement with the experimental phase. However, some of the other paths are also of interest.
An optical profilometer that uses a Techmet LaserMike scanning, focused, laser-beam, optical micrometer is installed in a remote alpha-gamma containment cell at the Los Alamos Hot-Cell Facility.1 A hot-cell extension chamber provides the nominal 30-cm (12-in.) working distance required by the LaserMike and, at the same time, keeps the LaserMike components outside the high-radiation-containment environment. This system provides measurement accu-racy better than±5 pm (0.0002 in.) on diameters between 2 and 13 mm (0.08 and 0.5 in.) at a rate of 33 measurements per second. The Hot-Cell Facility also uses a Korad 20-J-output ruby pulsed laser to drill a hole in reactor fuel element cladding to sample fission gas. The laser is then used to reweld the hole so that the fuel element will not be contaminated and may be stored without an alpha-containment barrier. The wall thickness of the fuel elements sampled varies from 0.25 to 0.50 mm (0.010 to 0.020 in.).
Stimulated rotational Raman scattering in a 300-K multipass cell filled with para-H2 with a single-mode CO2-pumped laser is studied using a frequency-narrowed optical parametric oscillator (OPO) as a probe laser at the Stokes frequency for the So(0) transition. Amplification and pump depletion are examined as a function of incident pump energy. The pump depletion shows clear evidence of transient behavior. A theoretical treatment of transient stimulated Raman scattering, including effects of both pump depletion and medium saturation is presented. In a first approximation, diffraction effects are neglected, and only plane-wave interactions are considered. The theoretical results are compared to the experimental pulse shapes.
The techniques of backward stimulated Raman scattering (BSRS) and reflected broad-band coherent anti-Stokes Raman scattering (RBBCARS) have been used to measure vibrational frequency shifts in shock-compressed liquid benzene and mixtures of liquid benzene and liquid deuterated benzene. BSRS was used only for measurements in neat liquid benzene as it only allows observation of the highest gain vibrational transition. RBBCARS was used to simultaneously measure multiple vibrational modes of multiple species. Accompanying static high pressure Raman experiments in a heated diamond anvil cell were used to establish the phase of the shocked samples. These experiments demonstrate the capabilities of fast non-linear optical techniques in the study of material structure changes and chemical reactions induced by shock-compression.
We predict theoretically and verify experimentally that coherent Stokes (and anti-Stokes) Raman scattering is an efficient method for obtaining the Raman spectrum of a film, provided that the incident and scattered fields are guided by the film. Further calculations indicate that by using various combinations of guided wave modes it should be possible to locate impurities in a thin film.
Beam divergence measurements were performed on a high repetition rate alexandrite laser currently being developed for a Los Alamos National Laboratory photochemistry research program, and were found to be spherically correctable and approximately constant at 8 times diffraction limited over a large input power range. A 0.5 x 10 cm alexandrite rod was pumped in a double ellipse head at a constant 42 joules/pulse input energy at repetition rates of 75 to 200 Hz. Several resonators were employed over the input power range to compensate for the thermal lensing, which varied from 0.63 to 0.12 m. The divergence measurements were performed by splitting a fraction of the output beam, passing it through a long focal length lens, and measuring the transmission percentage through calibrated apertures at the focal plane. This measurement was performed for seven resonators and cross-checked by imaging the far field pattern through a TV camera system and observing the spot sizes. With a similar experimental setup, a Glan prism was placed extra-cavity to examine the magnitude of depolarization losses due to stress induced birefringence under pumped conditions. No measurable effect was found up to 1 kW input power.
For the first time, laser-induced thermal desorption is examined with secondary ion mass spectrometry (SIMS). The desorption of small molecules on a polycrystalline tungsten surface by high-intensity (up to 176 MW/cm2), 1064-nm pulses of 8-10 nsec duration is analyzed by monitoring relative changes in the SIMS intensities. Despite the very high surface temperatures (3000 K) obtained by the absorption of intense laser radiation, the observed changes are much smaller than predicted by the equilibrium model of thermal desorption.
As a result of recent work, gas phase metathetical reactions of formaldehyde represent a particularly well-studied group of chemical reactions'-8. A wide variety of studies have been conducted including real-time108-8 and relativerate kinetics9, product energy distributions2'10'11 and theoretical calculations12. The interest prompting these studies is two-fold. First, the removal of reactive species, such as atomic chlorine, from the stratosphere via reaction with formaldehyde could have a tremendous impact on the ecological effects of anthropogenically released substances. Secondly, these reactions are generally quite fast and are of considerable interest from a dynamic point of view.
It has been demonstrated in a laboratory experiment that a mode-locked laser can be used as a local oscillator (LO) in a heterodyne ranging system to sample or bandwidth com-press the received signal. The bandwidth compression depends on the difference in the intermode frequency between the LO laser and the transmitter laser. Bandwidth compression of over 500 times was demonstrated, i.e., a 1.5 GHz He-Ne laser transmitter signal was reduced to 3 MHz upon detection. Since the mixing occurs on the surface of the detector only a narrow bandpass receiver that passes the compressed bandwidth is necessary to obtain ranging equivalent to that obtained by a wide bandwidth receiver that passes the full transmitter bandwidth
In certain fusion experiments using CO2 lasers, like Helios, it is desired to produce a focal spot several times larger than the nominal focal spot, with a flat beam profile. The typical focal spot in Helios is roughly 70 μm and just defocussing the beam produces beam breakup, with several hot spots with roughly the original diameter, and a gaussian distribution. A number of schemes were tried to achieve a large spot with desired characteristics. These are described in the article. Axicons were found to produce spots with desired characteristics. Axicons are lenses or mirrors having a cone-shaped surface. The various schemes are described, as well as an. experiment in Helios which confirmed that axicons produced the spots with desirable characteristics. Helios is an 8-beam CO2 laser which produces 10 kJ at power in excess of 20 TW. It is currently being used for Laser Fusion studies at the Los Alamos National Laboratory.
A gas Cerenkov detector has been developed for measuring radiation from the 16.7-MeV gamma branch of the D-T reaction. This has useful applications as a diagnostic tool for weapons tests at the Nevada Test Site (NTS), as well as for evaluation of ICF targets and Tokomak plasmas. The Cerenkov process was chosen because of excellent time response. A gas radiator allows threshold control to eliminate low energy background, such as gamma radiation produced by a neutron capture or scattering. The detector consists of a thin aluminum converter to provide energetic pair and compton electrons, a deflecting magnet, a Cerenkov radiator, and an optical system for collection and detection of Cerenkov light. The radiator is a gas chamber filled with approximately one atmosphere of carbon dioxide. A photodiode is used for light detection. The electron beam from the DOE/EG&G electron linear accelerator at EG&G's Santa Barbara Operations has been used to measure the detector response as functions of electron energy and gas pressure. A Monte Carlo production-transport code is used to calculate geometrical properties of the pair and Compton electron distributions as they enter the Cerenkov radiator. Fluorescence, transition radiation, and other optical backgrounds produced by subthreshold electrons are being evaluated in order to optimize the detector design.
Glass, et al.,'1 discovered the bulk photovoltaic effect in doped lithium niobate crystals, and described the effect by the equation J = KaI, where J = current density, K = a constant whose value depends on the dopant and the light wavelength, a = absorp-tion coefficient, and I =.light intensity. In subsequent studies it was shown that for iron - doped crystals K varies significantly with the Fe2+ concentration. We propose to extend the description of the photovoltaic effect for iron by using the equation J = K1+ I, where K1 depends on light wavelength, but is independent of concentration, and aFe2+ (the absorption coefficient for the ionization of Fe2+) is proportional to Fe2-1- concentration. Experimental procedures will be described for making bulk photovoltaic measurements and using the extended equation for the characterization of commercial lithium niobate, where Fez+ is a trace contaminant, and a fe2+<< at wavelengths of 400 - 500 nm.
Multilayer optical absorber coatings may be applied to the faces of thermal detectors to enhance their sensitivity at wavelengths at which the detector2material exhibits low absorptivity. These coatings have extremely low mass (<100 pg/cm ) and the absorptance may be varied from 0 to near 100%. The design of these coatings is discussed, based on considerations derived from the detector application and experimental data is presented showing the effect on sensitivity.
Pulsed CO2 laser interaction with doped silicon can produce significant heating of the semiconductor. This heating is studied and two major applications are explored. Fast CO2 laser detection due to the rapid thermal generation of carriers is demonstrated.
Progress in the development of a high-speed microchannel plate (MCP) photomultiplier is reviewed. The device consists of a semitransparent photocathode and MCP with a tapered 50-ohm anode. Two prototypes have been built and evaluated. One has an accelerating screen between the MCP and anode. The other has no screen, but is otherwise identical. Time response data is presented for the two prototypes showing the effect of the screen.
Three kinds of calorimeters, differing in their absorption mechanisms, will be described. Included are surface absorbing calorimeters constructed of beryllium oxide, volume absorbing calorimeters of kapton and copper laminate, and box calorimeters of various shapes constructed of electroformed copper enclosures blackened by the commercial Ebanol* process. In all cases, devices are constructed so that the dominant heat loss mechanism is radiation. That is, they are carefully insulated from their environment. In some cases, drift produced by the background is subtracted by using the twin technique. The heat capacities of the calorimeters are known and the temperature rise is sensed using thermopiles. This gives the energy absorbed. Each device is also calibrated in such a way that it is traceable to the NBS Laser Measurements Standards.
The occurrence of isobaric (same mass) interferences presents a persistent problem to the precise experimental determination of isotope ratios.1 The problem is particularly acute when large isotope ratios must be measured, and in the determination of extremely small samples, where counting statistics will make subtraction of a background signal rather inaccurate. While there exist classical techniques for reducing background interferences, the application of resonance ionization mass spectrometry (RIMS) currently seems most likely to provide an optimal solution to this problem.2-8 We present here a preliminary report on the application of RIMS to the selective detection of technetium.
We have used fiber optic sensors routinely to measure multi-megampere currents. The sensors are low noise, absolutely calibrated, and electrically decoupled from the pulsed power source. Polarized light from a HeNe laser is guided past the current carrier by a single-mode, low-birefringence fiber. The magnetic field from the current causes a Faraday rotation of the light polarization which is detected by a polarization analyzer and photo-diode at the end of the fiber. We observe a rotation of about 250°/MA ± 5%, slightly less than the Verdet constant for non-birefringent silica glass. We find that highly birefrin-gent (polarization preserving) optical fibers do not work in this application. We are now trying to ruggedize the sensor for field use with high-explosive-driven current sources by using a diode laser and single mode fiber couplers to replace the laboratory system of lenses and spatial filters.
Two different approaches to spectral line emission profile measurement in the ZT-40M Reversed Field Pinch will be discussed. In the first case, VUV emissions (>> 50 nm) are dispersed by a 1-meter normal incidence spectrometer, then wavelength converted by sodium salycilate. The resultant visible image is magnified and dissected using a series of slits machined in a rotating drum which contains the detector. Resolutions of 0.06 nm and 0.1 ms are achieved. In the second method, visible and near UV light is dispersed in a 1-meter spectrometer which has an ITT Vidissector mounted at the exit plane. In this detector, photoelectrons generated at the photocathode are magnetically swept across a narrow slit, permitting scans over > 10 nm of the spectrum in < 0.1 ms. Spectral resolution of < 0.1 nm is also possible.
High explosives have been used to shock-heat rare gases to brightness temperatures up to 36 000 K, with large radiating areas. Temperatures were determined from radiometer signals at both 280 and 520 nm. Shock velocities up to 9 mm/ps were used in both plane and cylindrical geometries. Neon, argon, krypton, and xenon gases at atmospheric initial pressure were examined in plane shocks. Using argon, the effects of increased initial pressure were studied. For cylindrical shock expansion in argon, brightness temperatures were measured over a range of shock velocities from 3 to 9 mm/ps. Up to 4% of the explosive energy was emitted as radiation. The shock waves are found to be reasonable approximations to black-bodies.
Optical sources of several megawatts-cm"" 2 over large surface areas are available from
electrical discharges initiated by exploding films and wires immersed in a gas. Discharging
high current capacitor banks into the metal conductors causes them to melt, vaporize,
and then eventually ionize to a high-conductivity plasma of less than a milliohm/cm-length
resistance. As such over 1,000 MW of ohmic heating goes into the discharge, elevating its
blackbody temperature up to 35,000 K.
Quantities characterizing high temperature thermodynamic equilibria such as vapor pressures, enthalpies of formation and thermodynamic activity coefficients have traditionally been determined by classical techniques such as the Knudsen effusion method. Such classical techniques usually suffer from poor accuracy and from interference from other vapor species present with the vapor being measured. By comparing the classical methods with the rather infrequently used optical technique based on opacity measurements, we can demonstrate that this latter technique is largely unaffected by interfering vapor species and that it also possesses a spectroscopic multiplexing advantage that considerably enhances its accuracy over classical techniques. The attainment of these two advantages is illustrated with examples of atomic uranium opacity determinations in the pure uranium system and in the uranium nitride system. In both cases, all the opacities were obtained in the presence of interfering uranium oxide vapor and it was still possible to derive the desired thermodynamic data with excellent accuracy.
A detector system for single shot mesurement of electron temperature and density profiles in a magnetically confined plasma has been developed. Results have been obtained with this multipoint Thomson scattering diagnostic on 4-tpe high beta ,tdokam4, Torus II. Data has been obtained in the density range of ne ~2x1013 cm -I to cm-} with a peak electron temperature of Te fk, 80 eV. The detector used is a multianode microchannel plate(MCP) with a 10x10 array of anodes to collect the amplified current.
Normally when one considers the intensity in the far field and how it scales with wavelength, a very quick and superficially simple answer is reached. Namely, the answer is that far field intensity scales inversely as wavelength squared, i.e., I(λ1)/I(λ2) = (λ2/λ1)2. Unfortunately, this relationship is only true for what is called the "on-axis intensity" of the central diffraction lobe. In most cases the interest is not to deliver a central intensity to the far field, but to deliver a maximum amount of power within a defined spot size in the far field. This spot size is normally defined to be within the first zero of the Airy pattern, or the exp (-2) point on a Gaussian intensity distribution. The purpose of this article is to examine how the far field average irradiance rather than on-axis intensity scales with wavelength, and how simple aberrations affect such average irradiance.