Research in LMSS propagation

The Virginia Tech Satellite Communications Group has participated in the Land Mobile Satellite System (LMSS) program through JPL sponsorship since 1985. Involvement has mainly been in modeling and simulation of propagation characteristics and effects. Models developed to predict cummulative fade distributions for fading LMSS signals include LMSSMOD and the Simple Models which approximate LMSSMOD. Models to predict the mean and standard deviation of signal attenuation through roadside vegetation, namely the Average Path Model, were developed. In the area of simulation, efforts have centered around the development of a software simulator that uses data bases derived from experimental data to generate simulated data with arbitrary statistical behavior. This work has progressed to the development of an integrated analysis and simulation package, LIPS. The basic theory and results for the models and simulator have been previously documented in reports and papers. All LMSS activities are summarized and details of this year's efforts are given

Mobile satellite propagation measurements and modeling: A review of results for systems engineers

An overview of Mobile Satellite System (MSS) propagation measurements and modeling is given, including a summary of results. The simple models presented should be of some use to systems engineers. A complete summary of propagation experiments with literature references is included

Charge and fluence lifetime measurements of a dc high voltage GaAs photogun at high average current

GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic center of the photocathode, (b) limiting the photocathode active area to eliminate photoemission from regions of the photocathode that do not support efficient beam delivery, (c) using a large drive laser beam to distribute ion damage over a larger area, and (d) by applying a relatively low bias voltage to the anode to repel ions created within the downstream beam line. A combination of these techniques provided the best total charge extracted lifetimes in excess of 1000 C at dc beam currents up to 9.5 mA, using green light illumination of bulk GaAs inside a 100 kV photogun

PEPPo: Using a Polarized Electron Beam to Produce Polarized Positrons

An experiment demonstrating a new method for producing polarized positrons has been performed at the CEBAF accelerator at Jefferson Laboratory. The PEPPo (Polarized Electrons for Polarized Positrons) concept relies on the production of polarized e+/e− pairs originating from the bremsstrahlung radiation of a longitudinally polarized electron beam interacting within a 1.0 mm tungsten pair-production target. This paper describes preliminary results of measurements using an 8.2 MeV/c electron beam with polarization 84% to generate positrons in the range of 3.1 to 6.2 MeV/c with polarization as high as ∼80%

Photoinjector improvements at CEBAF in support of parity violation experiments

Three photoinjector modifications were undertaken at CEBAF to help ensure successful completion of the PREx and Qweak parity violation experiments: the development of a pockels cell high voltage switch that provides stable voltages at 960 Hz helicity flip rate with 60 μs rise/fall time, the installation of a two-Wien-filter spin flipper for slow spin reversal, and the installation of a new photogun with inverted insulator geometry that operates at higher bias voltage

Thermal Emittance and Lifetime of Alkali-Antimonide Photocathodes Grown On GaAs and Molybdenum Substrates Evaluated in a -300 kV dc Photogun

CsxKySb photocathodes grown on GaAs and molybdenum substrates were evaluated using a –300 kV dc high voltage photogun and diagnostic beam line. Photocathodes grown on GaAs substrates, with varying antimony layer thickness (estimated range from \u3c 20 nm to \u3e 1 um), yielded similar thermal emittance per rms laser spot size values (~0.4 mm mrad / mm) but very different operating lifetime. Similar thermal emittance was obtained for a photocathode grown on a molybdenum substrate but with markedly improved lifetime. For this photocathode, no decay in quantum efficiency was measured at 4.5 mA average current and with peak current 0.55 A at the photocathode

Strange Quark Contributions to Parity-Violating Asymmetries in the Backward Angle G0 Electron Scattering Experiment

We have measured parity-violating asymmetries in elastic electron-proton and quasi-elastic electron-deuteron scattering at Q^2 = 0.22 and 0.63 GeV^2. They are sensitive to strange quark contributions to currents in the nucleon, and to the nucleon axial current. The results indicate strange quark contributions of < 10% of the charge and magnetic nucleon form factors at these four-momentum transfers. We also present the first measurement of anapole moment effects in the axial current at these four-momentum transfers.Comment: 5 pages, 2 figures, changed references, typo, and conten

Phase dynamics of inductively coupled intrinsic Josephson junctions and terahertz electromagnetic radiation

The Josephson effects associated with quantum tunneling of Cooper pairs manifest as nonlinear relations between the superconductivity phase difference and the bias current and voltage. Many novel phenomena appear, such as Shapiro steps in dc cuurent-voltage (IV) characteristics of a Josephson junction under microwave shining, which can be used as a voltage standard. Inversely, the Josephson effects provide a unique way to generate high-frequency electromagnetic (EM) radiation by dc bias voltage. The discovery of cuprate high-Tc superconductors accelerated the effort to develop novel source of EM waves based on a stack of atomically dense-packed intrinsic Josephson junctions (IJJs), since the large superconductivity gap covers the whole terahertz frequency band. Very recently, strong and coherent terahertz radiations have been successfully generated from a mesa structure of $\rm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ single crystal which works both as the source of energy gain and as the cavity for resonance. It is then found theoretically that, due to huge inductive coupling of IJJs produced by the nanometer junction separation and the large London penetration depth of order of $\rm{\mu m}$ of the material, a novel dynamic state is stabilized in the coupled sine-Gordon system, in which $\pm \pi$ kinks in phase differences are developed responding to the standing wave of Josephson plasma and are stacked alternatively in the c-axis. This novel solution of the inductively coupled sine-Gordon equations captures the important features of experimental observations. The theory predicts an optimal radiation power larger than the one available to date by orders of magnitude, and thus suggests the technological relevance of the phenomena.Comment: review article (69 pages, 30 figures

Transverse Beam Spin Asymmetries at Backward Angles in Elastic Electron-Proton and Quasi-elastic Electron-Deuteron Scattering

We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasi-elastic scattering on the deuteron, at backward angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63 GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (piN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasi-static deuterium approximation, and is also in agreement with theory