36,201 research outputs found
Nuclear thermionic converter
Efficient nuclear reactor thermionic converter units are described which can be constructed at low cost and assembled in a reactor which requires a minimum of fuel. Each converter unit utilizes an emitter rod with a fluted exterior, several fuel passages located in the bulges that are formed in the rod between the flutes, and a collector receiving passage formed through the center of the rod. An array of rods is closely packed in an interfitting arrangement, with the bulges of the rods received in the recesses formed between the bulges of other rods, thereby closely packing the nuclear fuel. The rods are constructed of a mixture of tungsten and thorium oxide to provide high power output, high efficiency, high strength, and good machinability
Electro-optic bunch diagnostics on ALICE
An electro-optic longitudinal bunch profile monitor has been implemented on ALICE (Accelerators and Lasers in Combined Experiments) at the Daresbury Laboratories and will be used both to characterise the electron bunch and to provide a testbed for electro-optic techniques. The electro-optic station is located immediately after the bunch compressor, within the FEL cavity; its location allows nearby OTR, beam profile monitors and Coherent Synchrontron Radiation (CSR) diagnostics to be used for calibration and benchmarking. We discuss the implementation and the planned studies on electro-optic diagnostics using this diagnostic station
X-ray photoelectron spectroscopy measurement of valence-band offsets for Mg-based semiconductor compounds
We have used x-ray photoelectron spectroscopy to measure the valence-band offsets for the lattice matched MgSe/Cd0.54Zn0.46Se and MgTe/Cd0.88Zn0.12Te heterojunctions grown by molecular beam epitaxy. By measuring core level to valence-band maxima and core level to core level binding energy separations, we obtain values of 0.56+/-0.07 eV and 0.43+/-0.11 eV for the valence-band offsets of MgSe/Cd0.54Zn0.46Se and MgTe/Cd0.88Zn0.12Te, respectively. Both of these values deviate from the common anion rule, as may be expected given the unoccupied cation d orbitals in Mg. Application of our results to the design of current II-VI wide band-gap light emitters is discussed
Modification of Coulomb's law in closed spaces
We obtain a modified version of Coulomb's law in two- and three-dimensional
closed spaces. We demonstrate that in a closed space the total electric charge
must be zero. We also discuss the relation between total charge neutrality of a
isotropic and homogenous universe to whether or not its spatial sector is
closed.Comment: 11 pages, 3 figure
n-CdSe/p-ZnTe based wide band-gap light emitters: Numerical simulation and design
The only IIāVI/IIāVI wide bandāgap heterojunction to provide both good lattice match and pā and nātype dopability is CdSe/ZnTe. We have carried out numerical simulations of several light emitter designs incorporating CdSe, ZnTe, and Mg alloys. In the simulations, Poissonās equation is solved in conjunction with the hole and electron current and continuity equations. Radiative and nonradiative recombination in bulk material and at interfaces are included in the model. Simulation results show that an nāCdSe/pāZnTe heterostructure is unfavorable for efficient wide bandāgap light emission due to recombination in the CdSe and at the CdSe/ZnTe interface. An nāCdSe/Mg_(x)Cd_(1āx)Se/pāZnTe heterostructure significantly reduces interfacial recombination and facilitates electron injection into the pāZnTe layer. The addition of a Mg_(y)Zn_(1āy)Te electron confining layer further improves the efficiency of light emission. Finally, an nāCdSe/Mg_(x)Cd_(1āx)Se/Mg_(y)Zn_(1āy)Te/pāZnTe design allows tunability of the wavelength of light emission from green into the blue wavelength regime
Nonlinear acoustic and microwave absorption in glasses
A theory of weakly-nonlinear low-temperature relaxational absorption of
acoustic and electromagnetic waves in dielectric and metallic glasses is
developed. Basing upon the model of two-level tunneling systems we show that
the nonlinear contribution to the absorption can be anomalously large. This is
the case at low enough frequencies, where freqeuency times the minimal
relaxation time for the two-level system are much less than one. In dielectric
glasses, the lowest-order nonlinear contribution is proportional to the wave's
intensity. It is negative and exhibits anomalous frequency and temperature
dependencies. In metallic glasses, the nonlinear contribution is also negative,
and it is proportional to the square root of the wave's intensity and to the
frequency. Numerical estimates show that the predicted nonlinear contribution
can be measured experimentally
Upconversion of a relativistic Coulomb field terahertz pulse to the near infrared
We demonstrate the spectral upconversion of a unipolar subpicosecond terahertz (THz) pulse, where the THz pulse is the Coulomb field of a single relativistic electron bunch. The upconversion to the optical allows remotely located detection of long wavelength and nonpropagating components of the THz spectrum, as required for ultrafast electron bunch diagnostics. The upconversion of quasimonochromatic THz radiation has also been demonstrated, allowing the observation of distinct sum- and difference-frequency mixing components in the spectrum. Polarization dependence of first and second order sidebands at ĻoptĀ±ĻTHz, and ĻoptĀ±2ĻTHz, respectively, confirms the Ļ(2) frequency mixing mechanism
Preparing and probing atomic number states with an atom interferometer
We describe the controlled loading and measurement of number-squeezed states
and Poisson states of atoms in individual sites of a double well optical
lattice. These states are input to an atom interferometer that is realized by
symmetrically splitting individual lattice sites into double wells, allowing
atoms in individual sites to evolve independently. The two paths then
interfere, creating a matter-wave double-slit diffraction pattern. The time
evolution of the double-slit diffraction pattern is used to measure the number
statistics of the input state. The flexibility of our double well lattice
provides a means to detect the presence of empty lattice sites, an important
and so far unmeasured factor in determining the purity of a Mott state
Nuclear electric propulsion stage requirements and description
The application of a nuclear electric propulsion (NEP) stage in the exploration of near-earth, cometary, and planetary space was discussed. The NEP stage is powered by a liquid-metal-cooled, fast spectrum thermionic reactor capable of providing 120 kWe for 20,000 hours. This power is used to drive a number of mercury ion bombardment thrusters with specific impulse in the range of 4000-5000 seconds. The NEP description, characteristics, and functional requirements are discussed. These requirements are based on a set of five coordinate missions, which are described in detail. These five missions are a representative part of a larger set of missions used as a basic for an advanced propulsion comparison study. Additionally, the NEP stage development plan and test program is outlined and a schedule presented
Strongly inhibited transport of a 1D Bose gas in a lattice
We report the observation of strongly damped dipole oscillations of a quantum
degenerate 1D atomic Bose gas in a combined harmonic and optical lattice
potential. Damping is significant for very shallow axial lattices (0.25 photon
recoil energies), and increases dramatically with increasing lattice depth,
such that the gas becomes nearly immobile for times an order of magnitude
longer than the single-particle tunneling time. Surprisingly, we see no
broadening of the atomic quasimomentum distribution after damped motion. Recent
theoretical work suggests that quantum fluctuations can strongly damp dipole
oscillations of 1D atomic Bose gas, providing a possible explanation for our
observations.Comment: 5 pages, 4 figure
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