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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

A passively tunable acoustic metamaterial lens for selective ultrasonic excitation

In this paper, we present an approach to ultrasonic beam-forming and beam-steering in structures based on the concept of embedded acoustic metamaterial lenses. The lens design exploits the principle of acoustic drop-channel that enables the dynamic coupling of multiple ultrasonic waveguides at selected frequencies. In contrast with currently available technology, the embedded lens allows exploiting the host structure as a key component of the transducer system therefore enabling directional excitation by means of a single ultrasonic transducer. The design and the performance of the lens are numerically investigated by using Plane Wave Expansion and Finite Difference Time Domain techniques applied to bulk structures. Then, the design is experimentally validated on a thin aluminum plate waveguide where the lens is implemented by through-holes. The dynamic response of the embedded lens is estimated by reconstructing, via Laser Vibrometry, the velocity field induced by a single source located at the center of the lens.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Compact electro-absorption modulator integrated with vertical-cavity surface-emitting laser for highly efficient millimeter-wave modulation

We demonstrate a compact electro-absorption slow-light modulator laterally-integrated with an 850 nm vertical-cavity surface-emitting laser (VCSEL), which enables highly efficient millimeter-wave modulation. We found a strong leaky travelling wave in the lateral direction between the two cavities via widening the waveguide width with a taper shape. The small signal response of the fabricated device shows a large enhancement of over 55 dB in the modulation amplitude at frequencies beyond 35 GHz; thanks to the photon-photon resonance. A large group index of over 150 in a Bragg reflector waveguide enables the resonance at millimeter wave frequencies for 25 μm long compact modulator. Based on the modeling, we expect a resonant modulation at a higher frequency of 70 GHz. The resonant modulation in a compact slow-light modulator plays a significant key role for high efficient narrow-band modulation in the millimeter wave range far beyond the intrinsic modulation bandwidth of VCSELs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Competition between modes with different axial structures in gyrotrons

This study was motivated by some experiments in which it was found that during the voltage rise, instead of expected excitation of a high-frequency parasitic mode, the excitation of a lower-frequency parasitic mode takes place in a certain range of voltages. For explaining this fact, the dependence of start currents of possible competing modes on the beam voltage was carried out in the cold-cavity approximation and by using the self-consistent approach. It was found that in the case of cavities, which consist of the combination of a section of constant radius waveguide and a slightly uptapered waveguide, these two approaches yield completely different results. Thus, experimentally observed excitation of the low-frequency parasitic mode can be explained by the self-consistent modification of the axial profile of the excited field, which has strong influence on the diffractive quality factor of competing modes. This modification is especially pronounced in the case of excitation of modes with many axial variations which can be excited in the region of beam interaction with the backward-wave component of such modes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

1.9 W continuous-wave single transverse mode emission from 1060 nm edge-emitting lasers with vertically extended lasing area

High-brightness edge-emitting semiconductor lasers having a vertically extended waveguide structure emitting in the 1060 nm range are investigated. Ridge waveguide (RW) lasers with 9 μm stripe width and 2.64 mm cavity length yield highest to date single transverse mode output power for RW lasers in the 1060 nm range. The lasers provide 1.9 W single transverse mode optical power under continuous-wave (cw) operation with narrow beam divergences of 9° in lateral and 14° (full width at half maximum) in vertical direction. The beam quality factor M{sup 2} is less than 1.9 up to 1.9 W optical power. A maximum brightness of 72 MWcm{sup −2}sr{sup −1} is obtained. 100 μm wide and 3 mm long unpassivated broad area lasers provide more than 9 W optical power in cw operation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

An X-band overmoded relativistic klystron

An X-band overmoded relativistic klystron is proposed, the operation mode of which is the TM{sub 02} mode. The drift tube could not cut off the TM{sub 01} mode; isolating the buncher cavity from the input cavity is achieved by introducing a sectional RF lossy material. Microwaves are extracted from the modulated electron beam using a cylindrical waveguide, rather than a coaxial waveguide; thereby, the output structure is significantly simplified. Particle-in-cell simulations show that microwaves with power of 1.28 GW and frequency of 9.30 GHz can be obtained, corresponding to an efficiency of 32% and relative bandwidth of about 8%.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Mechanical 144 GHz beam steering with all-metallic epsilon-near-zero lens antenna

An all-metallic steerable beam antenna composed of an ε-near-zero (ENZ) metamaterial lens is experimentally demonstrated at 144 GHz (λ{sub 0} = 2.083 mm). The ENZ lens is realized by an array of narrow hollow rectangular waveguides working just near and above the cut-off of the TE{sub 10} mode. The lens focal arc on the xz-plane is initially estimated analytically as well as numerically and compared with experimental results demonstrating good agreement. Next, a flange-ended WR-6.5 waveguide is placed along the lens focal arc to evaluate the ENZ-lens antenna steerability. A gain scan loss below 3 dB is achieved for angles up to ±15°.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

High-power narrow-vertical-divergence photonic band crystal laser diodes with optimized epitaxial structure

900 nm longitudinal photonic band crystal (PBC) laser diodes with optimized epitaxial structure are fabricated. With a same calculated fundamental-mode divergence, stronger mode discrimination is achieved by a quasi-periodic index modulation in the PBC waveguide than a periodic one. Experiments show that the introduction of over 5.5 μm-thick PBC waveguide contributes to only 10% increment of the internal loss for the laser diodes. For broad area PBC lasers, output powers of 5.75 W under continuous wave test and over 10 W under quasi-continuous wave test are reported. The vertical divergence angles are 10.5° at full width at half maximum and 21.3° with 95% power content, in conformity with the simulated angles. Such device shows a prospect for high-power narrow-vertical-divergence laser emission from single diode laser and laser bar.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Low temperature p-type doping of (Al)GaN layers using ammonia molecular beam epitaxy for InGaN laser diodes

We demonstrate state-of-the-art p-type (Al)GaN layers deposited at low temperature (740 °C) by ammonia molecular beam epitaxy (NH{sub 3}-MBE) to be used as top cladding of laser diodes (LDs) with the aim of further reducing the thermal budget on the InGaN quantum well active region. Typical p-type GaN resistivities and contact resistances are 0.4 Ω cm and 5 × 10{sup −4} Ω cm{sup 2}, respectively. As a test bed, we fabricated a hybrid laser structure emitting at 400 nm combining n-type AlGaN cladding and InGaN active region grown by metal-organic vapor phase epitaxy, with the p-doped waveguide and cladding layers grown by NH{sub 3}-MBE. Single-mode ridge-waveguide LD exhibits a threshold voltage as low as 4.3 V for an 800 × 2 μm{sup 2} ridge dimension and a threshold current density of ∼5 kA cm{sup −2} in continuous wave operation. The series resistance of the device is 6 Ω and the resistivity is 1.5 Ω cm, confirming thereby the excellent electrical properties of p-type Al{sub 0.06}Ga{sub 0.94}N:Mg despite the low growth temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A spin-wave logic gate based on a width-modulated dynamic magnonic crystal

An electric current controlled spin-wave logic gate based on a width-modulated dynamic magnonic crystal is realized. The device utilizes a spin-wave waveguide fabricated from a single-crystal Yttrium Iron Garnet film and two conducting wires attached to the film surface. Application of electric currents to the wires provides a means for dynamic control of the effective geometry of waveguide and results in a suppression of the magnonic band gap. The performance of the magnonic crystal as an AND logic gate is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Progress towards developing neutron tolerant magnetostrictive and piezoelectric transducers

Current generation light water reactors (LWRs), sodium cooled fast reactors (SFRs), small modular reactors (SMRs), and next generation nuclear plants (NGNPs) produce harsh environments in and near the reactor core that can severely tax material performance and limit component operational life. To address this issue, several Department of Energy Office of Nuclear Energy (DOE-NE) research programs are evaluating the long duration irradiation performance of fuel and structural materials used in existing and new reactors. In order to maximize the amount of information obtained from Material Testing Reactor (MTR) irradiations, DOE is also funding development of enhanced instrumentation that will be able to obtain in-situ, real-time data on key material characteristics and properties, with unprecedented accuracy and resolution. Such data are required to validate new multi-scale, multi-physics modeling tools under development as part of a science-based, engineering driven approach to reactor development. It is not feasible to obtain high resolution/microscale data with the current state of instrumentation technology. However, ultrasound-based sensors offer the ability to obtain such data if it is demonstrated that these sensors and their associated transducers are resistant to high neutron flux, high gamma radiation, and high temperature. To address this need, the Advanced Test Reactor National Scientific User Facility (ATR-NSUF) is funding an irradiation, led by PSU, at the Massachusetts Institute of Technology Research Reactor to test the survivability of ultrasound transducers. As part of this effort, PSU and collaborators have designed, fabricated, and provided piezoelectric and magnetostrictive transducers that are optimized to perform in harsh, high flux, environments. Four piezoelectric transducers were fabricated with either aluminum nitride, zinc oxide, or bismuth titanate as the active element that were coupled to either Kovar or aluminum waveguides and two magnetostrictive transducers were fabricated with Remendur or Galfenol as the active elements. Pulse-echo ultrasonic measurements of these transducers are made in-situ. This paper will present an overview of the test design including selection criteria for candidate materials and optimization of test assembly parameters, data obtained from both out-of-pile and in-pile testing at elevated temperatures, and an assessment based on initial data of the expected performance of ultrasonic devices in irradiation conditions.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2015

High-power blue laser diodes with indium tin oxide cladding on semipolar (202{sup ¯}1{sup ¯}) GaN substrates

We demonstrate a high power blue laser diode (LD) using indium tin oxide as a cladding layer on semipolar oriented GaN. These devices show peak output powers and external quantum efficiencies comparable to state-of-the-art commercial c-plane devices. Ridge waveguide LDs were fabricated on (202{sup ¯}1{sup ¯}) oriented GaN substrates using InGaN waveguiding layers and GaN cladding layers. At a lasing wavelength of 451 nm at room temperature, an output power of 2.52 W and an external quantum efficiency of 39% were measured from a single facet under a pulsed injection current of 2.34 A. The measured differential quantum efficiency was 50%.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The influence of ions and the induced secondary emission on the nanosecond high-gradient microwave breakdown at metal surface

The mechanism of ultrafast breakdown at metal/vacuum interface in the high-power microwave waveguides is studied. In order to realize the nanosecond discharge, the required ambient gas pressure above the metal surface is approximately calculated as high as several Torr due to the low ionization-rate for high-energy electrons and short pulse. The local high pressure may come from the evaporated microscopic protrusions due to Joule heating and gas desorption. Besides, ions accelerated by the ambient space charge field could obtain sufficient high energy to collide and sputter the metal atoms to increase the ambient pressure. The positive feedbacks during the rapid discharge are studied by particle-in-cell simulation. The relatively high-energy ions could generate secondary electrons. It is shown that, as the positive feedback, the secondary electrons induce the gas desorption and stronger ionization, resulting in ion and electron density increasing as well as sheath field further increasing. As a result, more higher-energy ions bombard metal surface, leading to higher secondary electron yield and higher density plasma generated to cut off the microwave transmission finally. These nonlinear courses realize the ultrafast discharge in waveguides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Realization of non-linear coherent states by photonic lattices

In this paper, first, by introducing Holstein-Primakoff representation of α-deformed algebra, we achieve the associated non-linear coherent states, including su(2) and su(1, 1) coherent states. Second, by using waveguide lattices with specific coupling coefficients between neighbouring channels, we generate these non-linear coherent states. In the case of positive values of α, we indicate that the Hilbert size space is finite; therefore, we construct this coherent state with finite channels of waveguide lattices. Finally, we study the field distribution behaviours of these coherent states, by using Mandel Q parameter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Self-consistent evolution of plasma discharge and electromagnetic fields in a microwave pulse compressor

Nanosecond-scale evolution of plasma and RF electromagnetic fields during the release of energy from a microwave pulse compressor with a plasma interference switch was investigated numerically using the code MAGIC. The plasma was simulated in the scope of the gas conductivity model in MAGIC. The compressor embodied an S-band cavity and H-plane waveguide tee with a shorted side arm filled with pressurized gas. In a simplified approach, the gas discharge was initiated by setting an external ionization rate in a layer crossing the side arm waveguide in the location of the electric field antinode. It was found that with increasing ionization rate, the microwave energy absorbed by the plasma in the first few nanoseconds increases, but the absorption for the whole duration of energy release, on the contrary, decreases. In a hybrid approach modeling laser ignition of the discharge, seed electrons were set around the electric field antinode. In this case, the plasma extends along the field forming a filament and the plasma density increases up to the level at which the electric field within the plasma decreases due to the skin effect. Then, the avalanche rate decreases but the density still rises until the microwave energy release begins and the electric field becomes insufficient to support the avalanche process. The extraction of the microwave pulse limits its own power by terminating the rise of the plasma density and filament length. For efficient extraction, a sufficiently long filament of dense plasma must have sufficient time to be formed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Demonstration of a directional sonic prism in two dimensions using an air-acoustic leaky wave antenna

Analysis and experimental demonstration of a two-dimensional acoustic leaky wave antenna is presented for use in air. The antenna is comprised of a two-dimensional waveguide patterned with radiating acoustic shunts. When excited using a single acoustic source within the waveguide, the antenna acts as a sonic prism that exhibits frequency steering. This design allows for control of acoustic steering angle using only a single source transducer and a patterned aperture. Aperture design was determined using transmission line analysis and finite element methods. The designed antenna was fabricated and the steering angle measured. The performance of the measured aperture was within 9% of predicted angle magnitudes over all examined frequencies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A broadband gyrotron backward-wave oscillator with tapered interaction structure and magnetic field

The gyro-monotron and gyrotron backward-wave oscillator (gyro-BWO) are the two oscillator versions of gyrotrons. While serving different functions, they are also radically different in the RF field formation mechanisms. The gyro-monotron RF field profile is essentially fixed by the resonant interaction structure, while the gyro-BWO possesses an extra degree of freedom in that the axial RF field profile is self-determined by the beam-wave interaction in a waveguide structure. The present study examines ways to utilize the latter feature for bandwidth broadening with a tapered magnetic field, while also employing a tapered waveguide to enhance the interaction efficiency. We begin with a mode competition analysis, which suggests the theoretical feasibility of broadband frequency tuning in single-mode operation. It is then shown in theory that, by controlling the RF field profile with an up- or down-tapered magnetic field, the gyro-BWO is capable of efficient operation with a much improved tunable bandwidth.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Effect of the electric field pattern on the generation of fast electrons in front of lower hybrid launchers

The effect of the detailed waveguide spectrum on the electron acceleration has been studied for the 3.7 GHz LHCD launchers in Tore Supra, i.e. the ITER-like passive-active multijunction (PAM) launcher and the fully-active-multijunction (FAM) launcher, using test electron modelling technique. The detailed launched antenna wave spectrum is used as input to the code that computes the dynamics of the electrons in the electric field. Comparison with the LHCD launchers in EAST, operating at 2.45 GHz and 4.6 GHz, has also been made. The simulations show that the PAM-design generates lower flux of fast electrons than FAM-launchers, this could be the consequence of the wider waveguide of PAM-launcher (14.65 mm for Tore-Supra) than FAM-launcher (8 mm for Tore-Supra)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Study on a discal TEM-TE{sub 11} mode converter loaded high-efficiency magnetically insulated transmission line oscillator

An integrative high power microwave device is proposed, which consists of a high-efficiency L-band Magnetically Insulated Transmission Line Oscillator (MILO) and a discal TEM–TE{sub 11} mode converter. The MILO with a shrunken load composed of a step-like cathode and a ladder-like beam collector can generate a 1.58 GHz, 5.7 GW microwave with the efficiency of 20.8% at the diode voltage of 560 kV in simulation. The discal converter utilizes a pair of sectorial two-double radial waveguides and a pair of sectorial cross section waveguides to adjust the phase-difference and realizes the mode conversion in a length of less than half wavelength at 1.58 GHz. In the preliminary experiment, the proposed device generates over 2 GW, 1.575 GHz microwave with the pulse duration of over 50 ns in a 420 kV diode voltage; the corresponding efficiency is 14.9%; the radiation pattern is the ideal TE{sub 11} mode.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗