17,126 research outputs found

    Green-Schwarz Formulation of Self-Dual Superstring

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    The self-dual superstring has been described previously in a Neveu-Schwarz-Ramond formulation with local N=2 or 4 world-sheet supersymmetry. We present a Green-Schwarz-type formulation, with manifest spacetime supersymmetry.Comment: 11 pg., (uuencoded dvi file) ITP-SB-92-5

    Confocal Ellipsoidal Reflector System for a Mechanically Scanned Active Terahertz Imager

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    We present the design of a reflector system that can rapidly scan and refocus a terahertz beam for high-resolution standoff imaging applications. The proposed optical system utilizes a confocal Gregorian geometry with a small mechanical rotating mirror and an axial displacement of the feed. For operation at submillimeter wavelengths and standoff ranges of many meters, the imaging targets are electrically very close to the antenna aperture. Therefore the main reflector surface must be an ellipse, instead of a parabola, in order to achieve the best imaging performance. Here we demonstrate how a simple design equivalence can be used to generalize the design of a Gregorian reflector system based on a paraboloidal main reflector to one with an ellipsoidal main reflector. The system parameters are determined by minimizing the optical path length error, and the results are validated with numerical simulations from the commercial antenna software package GRASP. The system is able to scan the beam over 0.5 m in cross-range at a 25 m standoff range with less than 1% increase of the half-power beam-width

    Terahertz heterodyne imager for biomedical applications

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    Terahertz heterodyne imaging is an established technique that offers the potential for extremely large dynamic range and high signal-to-noise ratio while maintaining fast data acquisition, stable magnitude and phase measurements, reasonable frequency flexibility and mm level penetration in tissue and other materials. The authors have set up an imaging system based around a custom fabricated 2.5 THz planar Schottky diode mixer pair and two optically pumped far IR lasers. One laser is used for the signal beam and supplies as much as 70mW at 2.5 THz. The other laser acts as a local oscillator (LO) source for the two mixers. Line pairs very close to each other (CH3OH and CH2F2) are chosen to provide a workable intermediate frequency output (IF=24 GHz). Broader RF bandwidth is possible with tunable signal sources and wider IF band amplifiers. A novel frequency stabilization scheme has been implemented to track and calibrate the laser power (magnitude and phase) over a sample run. The system uses the second THz mixer, a low frequency (GHz) reference oscillator and a lock-in amplifier to monitor and normalize the two lasers (LO and Signal). Stability of ~0.1 dB and <5 degrees have been achieved with a dynamic range of more than 90dB. The present system scans the sample through the focused beam and measures transmission or reflection at a fixed RF frequency. Applications to date include establishing contrast mechanisms in a range of test and biological materials. The measurement system is described and the merits and demerits discussed. Early results on a variety of samples are presented as well as plans to enhance the performance in the near future

    Time-Delay Multiplexing of Two Beams in a Terahertz Imaging Radar

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    We demonstrate a time-delay multiplexing technique that doubles the frame rate of a 660–690-GHz imaging radar with minimal additional instrument complexity. This is done by simultaneously projecting two offset, orthogonally polarized radar beams generated and detected by a common source and receiver. Beam splitting and polarization rotation is accomplished with a custom designed waveguide hybrid coupler and twist. A relative time lag of approximately 2 ns between the beams’ waveforms is introduced using a quasi-optical delay line, followed by spatial recombination using a selectively reflective wire grid. This delay is much longer than the approximately 20-ps time-of-flight resolution of the 30-GHz bandwidth radar, permitting the two beams’ reflected signals from a compact target to be easily distinguished in digital post-processing of the single receiver channel

    A planar quasi-optical SIS receiver for array applications

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    A planar, quasi-optical SIS receiver operating at 230 GHz is described. The receiver consists of a 2 x 5 array of half wave dipole antennas with ten niobium-aluminum oxide-niobium SIS junctions on a quartz dielectric-filled parabola. The 1.4 GHz intermediate frequency is coupled from the mixer via coplanar strip transmission lines and 4:1 balun transformers. The receiver is operated at 4.2 K in a liquid helium immersion cryostat. We report accurate measurements of the performance of single receiver elements. A mixer noise temperature of 89 K DSB, receiver noise temperature of 156 K DSB, and conversion loss of 3 dB into a matched load have been obtained

    The N=2(4) string is self-dual N=4 Yang-Mills

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    N=2 string amplitudes, when required to have the Lorentz covariance of the equivalent N=4 string, describe a self-dual form of N=4 super Yang-Mills in 2+2 dimensions. Spin-independent couplings and the ghost nature of SO(2,2) spacetime make it a topological-like theory with vanishing loop corrections.Comment: 7 pg., ITP-SB-92-24 (uuencoded dvi file; otherwise same as original

    Random lattice superstrings

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    We propose some new simplifying ingredients for Feynman diagrams that seem necessary for random lattice formulations of superstrings. In particular, half the fermionic variables appear only in particle loops (similarly to loop momenta), reducing the supersymmetry of the constituents of the Type IIB superstring to N=1, as expected from their interpretation in the 1/N expansion as super Yang-Mills.Comment: Section 5 which describes contributions of the string measure adde

    Self-Dual N=8 Supergravity as Closed N=2(4) Strings

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    As open N=2 or 4 strings describe self-dual N=4 super Yang-Mills in 2+2 dimensions, the corresponding closed (heterotic) strings describe self-dual ungauged (gauged) N=8 supergravity. These theories are conveniently formulated in a chiral superspace with general supercoordinate and local OSp(8|2) gauge invariances. The super-light-cone and covariant-component actions are analyzed. Because only half the Lorentz group is gauged, the gravity field equation is just the vanishing of the torsion.Comment: 17 pg., (uuencoded dvi file; revision: forgot 1 stupid term in the last equation) ITP-SB-92-3
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