Rotational shearing interferometers produce Fourier holograms of incoherent objects. Optical aberrations affect only the phase of the object Fourier transform. Phase errors can be corrected if interferograms are also taken either of a reference source with the same shear, or of the same source with a different shear.
A relatively low cost and low technical risk method has been developed for optically butting mosaic detector arrays to achieve very large focal plane assemblies. This method is being developed for infrared staring sensor applications which may require as many as 108 pixels in the focal plane. The method is applicable with virtually any detector array type or size. This patented technology (Patent No. 4634882, trade named RIMSTAR) is being applied to a proof of principle infrared staring sensor which is designated IRX.
Very tightly bound scatterers in a nearly perfect crystal interact strongly with neutrinos and antineutrinos. Total cross sections are orders greater than for scatterers which are not tightly bound. The theory is extended into the MeV region. Experiments are described.
This paper describes some of our recent progress in laboratory research aimed at fabricating multilayers for imaging applications at EUV wavelengths. Some of the equipment available in our laboratory for multilayer research is described and results of some calculations that aided in design of the multilayers and in the selection of materials are discussed. Finally, the results of multilayer deposition on both flat substrates and figured optical surfaces are presented and the visible light test of a system for investigating the normal incidence imaging properties of multilayer optics is described.
We are developing and testing a high resolution, large area gamma ray telescope to be employed in a space based environment. The new concept in the telescope design is the combination of liquid and gas drift chambers with scintillation detectors for imaging and tracking the secondary particles generated by the gammas with a very high angular and energy resolution. The telescope consists of a Liquid Argon Converter, an Argon-Methane Gas Drift Chamber, and a Liquid Argon Calorimeter, and has an event by event processing and decision making feature.
The separation of large projectiles with heavy materials from decoys can be accomplished by using penetrating radiation that results in a characteristic photon emission spectra. In order for such a scheme to be successful it is necessary to develop a capable sensor of the photon emission. The UT-Dallas and UCLA groups are developing such a sensor (called the High Resolution Gamma Ray Telescope, HRGRT). This sensor uses electron drift techniques in liquid Argon and Xenon and promises to give the best angular resolution for a Compton telescope. The appropriate penetrating radiation will consist of neutrons from a pulsed isotropic source.
Plasmas have recently received considerable attention as poten-tially efficient long wavelength (infrared to microwave) phase con-jugators using degenerate four-wave mixing (DFWM). However, the current theory assumes an idealized isotropic, homogeneous, quies-cent plasma. In order to ascertain the suitability of plasmas for phase conjugation applications, a more complete theory of DFWM in the presence of plasma density inhomogeneities, turbulence, etc. is necessary. This paper focuses on the effect of density inhomogeneities on the DFWM and phase conjugate properties of plasmas. In addition, a comparison is made between DFWM in collisionless and collisional plasmas.
The optimum choice of wavelength for a space-borne imaging radar may be near 1 millimeter. Design calculations of such a near-millimeter radar indicate transmitter peak power Z 26 kW is required for resolution of 1.3 m at a range of 3000 km. The feasibility of developing an amplifier in the optical klystron configuration to meet the source requirements for the radar has been investigated. It was found that a 3-cavity amplifier based on a small period wiggler magnet and incorporating a depressed collector might deliver the required power with efficiency of - 10% and require an electron beam energy of only 315 kV.
A high power submillimeter, harmonic gyrotron oscillator is currently being operated at M.I.T. To achieve second harmonic emission, three different cavity configurations were studied: a tapered cavity, a slotted cavity, and a cavity with an output iris. In the cavity containing an output iris, second harmonic emission was observed at 6 frequencies between 301 GHz to 373 GHz with output powers of 4-6 kW, and at 417 GHz with an output power of 15 kW. The latter measurement corresponds to a total efficiency of 6%. A variety of diagnostics capable of discriminating between fundamental and second harmonic emission were also investigated.
We have demonstrated a two-dimensional horn imaging array at 240 GHz. In this configuration, a dipole antenna is suspended in a horn on a thin silicon-oxynitride membrane. The horns are fabricated by anisotropic etching of silicon in an ethylene-diamine pyrocatechol solution. This results in pyramidal pits bounded by the <111> crystal planes, and where the sides make an angle of 54.7° with the <100> surface plane. The membrane is so thin compared to a wavelength, that the dipole is effectively radiating in free space bounded by conductive horn walls. The dipole antenna is integrated with a bismuth microbolometer detector on the membrane. We have measured the patterns of a single element in a 9 x 9 imaging array at 240 GHz, with a horn opening of 1.45 Å. The directivity of the antenna element found from the two-dimensional pattern measurements is 12.5 dB.
Recent coupled Josephson junction experiments in our laboratory have demonstrated that high critical current density tunnel junctions can serve as effective local oscillators at frequencies up to and in excess of the gap sum frequency of the junction, i.e. well above 1 Terahertz for a niobium or niobium compound tunnel junction. While the details of the behavior of such a THz. oscillator were found not to be in accord with the predictions of the accepted theory of the A.C. Josephson effect in the gap region significant radiation could be capacitively coupled from the oscillator junction to an adjacent junction, sufficient for SIS mixer experiments at Terahertz frequencies. Research efforts are now under way to further extend and ex-pand these studies. A high critical current density all NbN tunnel junction system is now under development for Terahertz applications and a new set of coupled Josephson oscillator - SIS detector experiments is being initiated using NbN tunnel junc-tions. In this paper we will review the original coupled junction high frequency experiments and report on the recent progress of the current NbN tunnel junction experiments.
Research is in progress to develop a local oscillator at 1 THz based on the Josephson effect in superconductors. In general, it is necessary to use arrays of Josephson junctions to obtain the required power, linewidth and impedance levels from such sources. The criteria for properly phase-locking the junctions in such an array, distributed over many wavelengths, are discussed along with the projected performance at 1 THz. A power level of 1μW at 300GHz has been demonstrated using these designs.
Analog superconductive components have been integrated to form a device capable of crosscorrelation between two wideband analog input signals. Arrays of Nb/Nb205/Pb tunnel junctions perform the mixing of delayed samples of two frequency-offset analog signals counterpropagating along a niobium tapped transmission line. The resultant mixer products from the junction array are integrated and stored in a high-Q (ft,' 600) lumped element L-C resonator tuned to the signal difference frequency. A superconductive tunnel junction imbedded in the resonator circuit is operated as a variable-threshold comparator to detect the time-integrated current stored in the resonator. We present results showing the correlation properties of such an individual correlator cell, together with the design and performance of a 4-junction-logic (4JL)-based digital address encoder to be used in a multichannel correlator device. We also discuss the important design issues as they relate to analog signal processing.
We have studied a variety of circuits for low-accuracy (4-bit) A/D converters; these have been of two types. The "periodic-threshold" type requires only one comparator for each bit. The "fully parallel" configuration requires one comparator for each digital level (15 for 4 bits). Results for both types of A/D converter will be sum-marized. The periodic-threshold type of circuit depends on the quan-tum interferometric property of Superconducting QUantum Interference Devices (SQUIDs). Periodic-threshold circuits using two junction SQUIDs are limited to 1-2 GHz of analog bandwidth. Two different circuits for the fully parallel type of A/D converter circuit have been studied. One employs latching comparators for each of the levels and the comparators drive a novel binary encoder. Simulations show that the analog bandwidth is at least 3.5 GHz. Another fully parallel A/D converter circuit uses the bistability of a single junction SQUID to achieve high sensitivity, and the rapid switching of such a SQUID to obtain the short aperture time needed for wideband A/D conversion. Simulations indicate an analog bandwidth of about 5 GHz. Our simulations of shift-register circuits show that a circuit with a three-phase clock and three AND gates per bit can operate at least to 25 GHz. A two-phase circuit, in which current is diverted to one or the other branch of a superconducting loop to indicate a "1" or "0," has been shown by simulation to operate correctly when clocked at 62.5 GHz.
An integrated circuit chip was designed for a Josephson based shift register, and chip processing was initiated. The circuit design simulates operation at 25 GHz in the SPICE program. The transmission lines used to distribute the three-phase clock were modeled with the SUPERCOMPACT program to provide balanced, in-phase circuit drive, up to 10 GHz. Integrated circuit processing procedures have been developed to permit reactive ion etching of all seven deposited layers. The 6.25 mm square chip featured a twelve-gate, four-stage shift register fabricated with Nb/A10x/Nb Josephson junctions of 2000 A/cm critical current density.
We study analytically the detection and tracking of rockets and payloads by microwave or millimeter-wave passive radio temperature measurements, using radio astronomical tech-niques. Such targets may be viewed from synchronous equatorial orbit against either the warm earth or cold sky as background. Their observability depends on the temperature difference between target and background. Simultaneous measurements of output power of multiple antenna beams are processed by spatial filters, to yield a reconstructed image. The image model consists of multiple point targets, viewed against a uniform background. We assume stationary targets in the present work. For background measurements of the warm earth without targets, or for multiple targets that are not well-resolved, Wiener spatial filters are appropriate. In contrast, the best linear processor for well-resolved targets uses matched spatial filters. We relate the system performance to the target, background, antenna, and receiver parameters, for these different types of linear processors.
Description of, and computer experiment results from, a boost-phase signature generation evaluation and requirement determination technique. This technique uses variable fidelity Lagrangian interpolation of computer generated plumes as input to the DYNAMORE (DYnamic MOment REgression) hardbody acquisition algorithm. Algorithm output accuracy is compared to truth data to determine the effects on hardbody acquisition of errors in plume generation and interpolation.
At high altitudes the thickness of a hypersonic shock wave in front of a hard body becomes sizable compared to the shock standoff distance. Peak radiation intensity occurs within the shock wave structure itself. In order to compute accurately hard body radiation in flows at high altitude with thick shock waves, the motion equations must be capable of yielding realistic profiles of temperature and density through the structure of a hypersonic shock wave. The conventional Navier-Stokes equations do not provide this fundamental capability, since they yield hypersonic shock thicknesses in nitrogen that are about a factor of four too thin. An investigation is made of some possible reasons for this failure of the Navier-Stokes equations. Models for bulk viscosity in a monatomic gas are explored which yield a realistic thickness for the shock-wave density profile, but not the temperature profile, and hence are not satisfactory. A tentative computational model for nitrogen is explored which yields considerably more realistic results than the Navier-Stokes equations. This model involves a nonlinear stress-strain tensor, nonlinear heat flux vector, and non-equilibrium rotational energy.
One-dimensional shock structure is investigated for various monatomic gases from Mach 1.4 to Mach 35. Kinetic theory solutions are computed using the Direct Simulation Monte Carlo method. Steady-state solutions of the Navier-Stokes equations and of a slightly truncated form of the Burnett equations, termed the "Burnett(-)" equations, are determined by relaxation to a steady state of the time-dependent continuum equations. Monte carlo results are in excellent agreement with published experimental data, and are used as bases of comparison for continuum solutions. For a Maxwellian gas the Burnett(-) equations are shown to produce far more accurate solutions of shock structure than the Navier-Stokes equations.
The flow about a body traveling at hypersonic speed is energetic enough to cause the atmospheric gases to chemically react and reach states in thermal nonequilibrium. The prediction of hypersonic flowfields requires a numerical method capable of solving the conservation equations of fluid flow, the chemical rate equations for specie formation and dissociation, and the transfer of energy relations between translational and vibrational temperature states. Because the number of equations to be solved is large, the numerical method should also be as efficient as possible. The proposed paper presents a fully implicit method that fully couples the solution of the fluid flow equations with the gas physics and chemistry relations. The method flux splits the inviscid flow terms, central differences the viscous terms,preserves element conservation in the strong chemistry source terms, and solves the resulting block matrix equation by Gauss Seidel line relaxation.
Superconducting 123 films can be fabricated using the green 211 phase as a substrate. The superconducting characteristics of these films are better than the characteristics found when other oxide compounds are used as substrates. Using high temperature processing, 211 phase oxide can be partially converted to 123 phase. Using the same process, a new high Tc copper oxide compound with non-rare earth elements was prepared. High temperature proceEsing presents an alternative synthetic route in the search for new high Tc superconductors.
A review is given of the general types of experiments that can be performed to distinguish between broad classes of theories of superconductivity. It is concluded that this distinction will be very difficult for all theories that are similar to Bardeen, Cooper and Schriefer (BCS) but with a different interaction causing the pairing. Theories that start from a correlated normal state should give significantly different results from experiments.
The EXAFS technique was used to investigate the structural parameters of the Y and Cu atoms in YBa2Cu307' An antisite disorder of 0.16±0.05 mole fraction was found between the Y and Cu2sites. It was also demonstrated that powder x-ray and neutron diffraction tech-niques are insensitive to this degree of disorder.
We demonstrate that granular films of Y-Ba-Cu-0 can detect broadband optical radiation, at temperatures up to 100 K. A lower bound of D* has been measured and found to be 108 cm-Hz172 /Watt, and a response time of 20 nanoseconds has been determined.
We describe here the deposition of superconducting and semiconducting thin films by the laser evaporation technique. The, characterization of this process, and possible optimization with regards to wavelength and pulse duration of the laser will be discussed. Results of laser interaction experiments will also be described.
We have measured the surface impedance of two different orientations of thin films of YBa2Cu307-8 (YBCO) at 100 and 150 GHz. The surface resistance drops rapidly below the transition temperature, but the loss at low temperatures is in disagreement with the convential BCS theory. The measured temperature, frequency, and orientation dependence are presented and discussed.
Currently, consolidation of oxide superconductor powders is done by sintering, which is not effective in the reduction of porosity. This work demonstrates the feasibility of hot isostatic pressing (HIP) to obtain fully dense bulk superconductor. Modeling of the HIP process was used to .predict HIP conditions for production of fully dense superconducting material. It is predicted that fully dense YBa2Cu307 can be obtained in reasonable times at temperatures down to around 650°C. The trade-offs between temperature, time, and pressure during HIP densification are analyzed as well as the effects of powder particle size and entrapped gas pressure. The HIP modeling has been verified experimentally under three conditions: 100 MPa HIP at 900°C for 2 hours (99.3% dense-full density), 100 MPa HIP at 750°C for 2 hours (96% dense), and sintering at 950°C for 16 hours (65% dense). A superconducting Meissner signal has been obtained on the fully dense specimen after a post-HIP heat treatment.
The means by which superconducting YBa2Cu307-x powders can be synthesized are summarized. Methods of forming powders into useful shapes are then described. Lastly, heat treatment schedules used for consolidation of the powders into dense, sound bodies with good superconducting properties are discussed.
American Superconductor was formed to fabricate commercial quality wires and bulk materials from the new high-Tc superconducting oxides. The company is using a proprietary metal precursor approach as its primary forming method. This paper will discuss the history of wire fabrication in classic superconducting materials and the status of alternative methods of fabrication for the new higher temperature materials. It will also focus on key problems to be solved in developing a truly useful bulk superconducting material such as critical current and strength.
A review of the basic technical issues relevant to the effective utilization of high temperature superconductivity in electronic device and component applications is presented. Many of the key issues pertain to material science; in this discourse, however, other more fundamental consequences of high temperature superconductivity are addressed, which inherently influence, and in fact, may limit the role of high temperature superconductivity in many conventional technology applications. BCS theory is used to estimate the characteristics of high temperature superconductivity and to provide insight into the use as well as the potential limitations of high temperature superconductivity in high-impact technology areas.
The existence of superconductors with TcOOK (which implies device operating temper-atures the order of Top ≈45K) opens up a variety of potential applications within the aerospace/defense industry. This is partly due to the existence of well developed cooler technologies to reach this temperature regime and partly due to the present operation of some specialized components at cryogenic temperatures. In particular, LWIR focal planes may operate at 10K with some of the signal processing electronics at an intermediate temperature of 40K. Addition of high Tc superconducting components in the latter system may be "free" in the sense of additional system complexity required. The established techniques for cooling in the 20K to 50K temperature regime are either open cycle, expendable material (stored gas with Joule-Thomson expansion, liquid cryogen or solid cryogen) or mechanical refrigerators (Stirling cycle, Brayton cycle or closed cycle Joule-Thomson). The high Tc materials may also contribute to the development of coolers through magnetically levitated bearings or providing the field for a stage of magnetic refrigeration. The discovery of materials with Tc, 90K has generated a veritable shopping list of applications. The superconductor properties which are of interest for applications are (1) zero resistance, (2) Meissner effect, (3) phase coherence and (4) existence of an energy gap. The zero resistance property is significant in the development of high field magnets requiring neglible power to maintain the field. In addition to the publicized applications to rail guns and electromagnetic launcher, we can think of space born magnets for charged particle shielding or whistler mode propagation through a plasma sheath. Conductor losses dominate attenuation and dispersion in microstrip transmission lines. While the surface impedance of a superconductor is non vanishing, significant improvements in signal transmission may be obtained. The Meissner effect may be utilized for some magnetic shielding applications but the penetration depth and high frequency effects will have to be considered. Phase coherence forms the basis for Josephson junction devices which, in turn are used for mixers, detectors and parametric amplifiers in the microwave/millimeter wave regime and for A/D converters, sampling and switching circuits and voltage standards in electronics. The energy gap has been the basis of optical and IR detection through modulation of the order parameter (or gap energy) by generation of quasi particles.