Focal plane arrays for submillimeter waves using two-dimensional electron gas elements: A grant under the Innovative Research Program

This final report describes a three-year research effort, aimed at developing new types of THz low noise receivers, based on bulk effect ('hot electron') nonlinearities in the Two-Dimensional Electron Gas (2DEG) Medium, and the inclusion of such receivers in focal plane arrays. 2DEG hot electron mixers have been demonstrated at 35 and 94 GHz with three orders of magnitude wider bandwidth than previous hot electron mixers, which use bulk InSb. The 2DEG mixers employ a new mode of operation, which was invented during this program. Only moderate cooling is required for this mode, to temperatures in the range 20-77 K. Based on the results of this research, it is now possible to design a hot electron mixer focal plane array for the THz range, which is anticipated to have a DSB receiver noise temperature of 500-1000K. In our work on this grant, we have found similar results the the Cronin group (resident at the University of Bath, UK). Neither group has so far demonstrated heterodyne detection in this mode, however. We discovered and explored some new effects in the magnetic field mode, and these are described in the report. In particular, detection of 94 GHz and 238 GHz, respectively, by a new effect, 'Shubnikov de Haas detection', was found to be considerably stronger in our materials than the cyclotron resonance detection. All experiments utilized devices with an active 2DEG region of size of the order of 10-40 micrometers long, and 20-200 micrometers wide, formed at the heterojunction between AlGaAs and GaAs. All device fabrication was performed in-house. The materials for the devices were also grown in-house, utilizing OMCVD (Organo Metallic Chemical Vapor Deposition). In the course of this grant, we developed new techniques for growing AlGaAs/GaAs with mobilities equalling the highest values published by any laboratory. We believe that the field of hot electron mixers and detectors will grow substantially in importance in the next few years, partly as a result of the opportunity given us through this grant, which represents the major effort in the US so far. We note, however, that parallel research on hot electron mixers in thin film superconductors in Russia, and recently in Sweden, have demonstrated mixing up to 1 THz, with the potential for low-noise receivers for frequencies up to many THz. The three groups recently assessed the relative adtantages of 2DEG and superconducting film mixers in a joint paper (Kollberg et al., 1992; see Appendix II)

Development of theoretical models of integrated millimeter wave antennas

Extensive radiation patterns for Linear Tapered Slot Antenna (LTSA) Single Elements are presented. The directivity of LTSA elements is predicted correctly by taking the cross polarized pattern into account. A moment method program predicts radiation patterns for air LTSAs with excellent agreement with experimental data. A moment method program was also developed for the task LTSA Array Modeling. Computations performed with this program are in excellent agreement with published results for dipole and monopole arrays, and with waveguide simulator experiments, for more complicated structures. Empirical modeling of LTSA arrays demonstrated that the maximum theoretical element gain can be obtained. Formulations were also developed for calculating the aperture efficiency of LTSA arrays used in reflector systems. It was shown that LTSA arrays used in multibeam systems have a considerable advantage in terms of higher packing density, compared with waveguide feeds. Conversion loss of 10 dB was demonstrated at 35 GHz

Investigation of a prototype industrial gas turbine combustor using alternative gaseous fuels

In this thesis, the effect of alternative gaseous fuels, with high hydrogen content and lower calorific value, on gas turbine combustion was investigated experimentally. The aim of the investigation was to find operational limitations for an experimental burner and to supply data for validation of computational fluid dynamics (CFD). Before examination of the actual burner, the laminar flame speed was measured for a range of gases. The measurement technique was based on Schlieren imaging which is a measure of the density gradient through a flame surface. A Bunsen type burner was used to measure the angle of a conical flame from which the laminar flame speed was calculated. In order to improve the comparability of these measurements with other measurement methods the laminar flame speed was corrected for the influence of stretch. The effect of stretch will increase or decrease the flame speed depending on the curvature of the flame and the physical properties of the gases involved in the combustion, e.g. the Lewis number and preferential diffusivity. The gas turbine burner examined was a downscaled version of the burner that is now found in the commercial gas turbine, SGT-750. The burner consists of three concentric sections. The central part is a precombustor called rich-pilot-lean (RPL). The purpose of the RPL is to supply heat and radicals to the other sections to stabilize combustion. The next section is the Pilot, which serves as an intermediate burner in which the equivalence ratio can be optimized to stabilize combustion and minimize NOX emissions. The outermost section is the Main. For the experimental burner approximately 79% of the mass flow passes through this section. All sections have their own swirlers that create recirculation zones for flame stabilization. The experimental work in this thesis includes measurements of the lean stability limit, emission optimization (primarily NOX), flame diagnostic through OH-Laser induced fluorescence (LIF) and particle image velocimetry (PIV). Tests were conducted at both atmospheric conditions with preheated air (650 K) and at elevated pressure up to 9 bar. Results from the experimental investigations were also used to validate CFD computations using reduced chemical kinetic schemes, and to validate reactor network calculations based on perfectly stirred reactors (PSR) and plug flow reactors (PFR). Lean stability limit experiments showed how the RPL equivalence ratio could be optimized to lower the lean blowout limit. Increasing the RPL equivalence ratio was shown to extend the lean blowout limit, up to a limit after which the RPL flame was quenched. Reactor network modelling showed that the stabilizing effect of the RPL was a combination of thermal energy and reactive radicals supplied to the flame zone. The important radicals were shown to be H, O and OH. The emission optimization measurements showed that lowering the equivalence ratio in both the RPL and the pilot minimized the NOX emissions. CFD simulation showed that the degree of mixing of both the RPL and the Pilot at point of ignition was not perfect. Imperfect mixing causes pockets of stoichiometric mixtures to react, which in turn create hot spots where thermal NOX can be formed. At rich RPL equivalence ratios, a flame could be visualized with OH-LIF after the RPL exit. This flame probably to some extent combusts closer to stoichiometry, which increases thermal NOX. These theories of how NOX is formed were confirmed by reactor network calculations

Samuel Hearne, The Denesuline, and The Beaver: Zoology and Its Effect in an Early Canadian Natural-Cultural Contact Zone

Recent scholarship on Samuel Hearne's A Journey to the Northern Ocean (1795) has highlighted how Hearne's journey of exploration functioned to demonstrate the Hudson's Bay Company's strategic geopolitical worth, obscure the violence of its colonialist enterprise, and generate images of an empty North conducive to colonial settlement. Drawing on such scholarship, this essay attempts to nuance statements regarding Hearne's complicity in “emptying” the North by showing how the Journey establishes images of the Canadian North as neither completely barren nor fertile enough for settlement. Applying a natural-cultural contact zone perspective on Hearne's old text, I argue that the anthropocentric bias of the Journey's reception has impeded the realization that Hearne's zoological descriptions and sometimes sophisticated ecological contemplations owe much to the Denesuline who guide his travels. In part through his “beaver science”, Hearne deliberately opposes prospects of further colonization based on ideas of systemic expansion of the fur trade detached from the realities of local environmental conditions. His concern regarding the anthropomorphism and uncritical use of cultural metaphors in the emerging science of zoology nevertheless causes Hearne's “beaver science” to consolidate the distinctly anthropocentric and objectifying qualities of natural science that ultimately facilitate the exploitative activities of the Hudson's Bay Company

Holger J. Schubert, ed.: Im Querschnitt. Volker Braun. Gedichte, Prosa, Stücke, Aufsätze

Halle/Leipzig: Mitteldeutscher Verlag, 1978. 345 p. 12,50 M

Reinhard Tgahrt and Ute Doster, eds.: Johannes Bobrowski oder Landschaft mit Leuten

Marbach am Neckar: Deutsche Schillergesellschaft, 1993 (Marbacher Kataloge, 46), 848 p., DM 40