Showing papers on "Power density published in 1995"

Efficiency of a Joule-Brayton engine at maximum power density

Abstract: A new kind of power analysis is conducted on a reversible Joule-Brayton cycle. Although many performance analyses have been carried out resulting in famous efficiencies (Carnot, Curzon-Ahlborn), most do not consider the sizes of the engines. In the studies of Curzon and Ahlborn and others, researchers utilized the thermal efficiency at maximum power as an efficiency standard for practical heat engines. In this paper, instead of just maximizing power for certain cycle parameters, the power density defined as the ratio of power to the maximum specific volume in the cycle, is maximised. Therefore the effects of the engine sizes were included in the analysis. The result showed a new type of efficiency at the maximum power density which is always greater than that at the maximum power (Curzon-Ahlborn efficiency). Evaluations show that design parameters at the maximum power density lead to smaller and more efficient Joule-Brayton engines.

Figure-8 undulator as an insertion device with linear polarization and low on-axis power density

Abstract: A new type undulator for linearly polarized radiation, figure-8 undulator, is proposed. It has the advantage that the on-axis power density is much lower than that of an ordinary planar undulator. Electrons inside the proposed device move along the trajectory which looks like a figure-8 when projected on the plane perpendicular to the beam axis. The spectral performance and the power density are calculated and compared to those of an ordinary planar undulator.

Laser Ablation Mass Removal versus Incident Power Density during Solid Sampling for Inductively Coupled Plasma Atomic Emission Spectroscopy

Abstract: For laser ablation solid sampling, the quantity of material ablated (removed) influences the sensitivity of chemical analysis. The mass removal rate depends strongly on the laser power density, which is the main controllable parameter for a given material and wavelength parameter using laser solid sampling for inductively coupled plasma atomic emission spectroscopy (ICP-AES). For a wide variety of materials, a decrease in the rate of change, or roll-off, in mass removed is observed with increasing incident laser power density. The roll-off results from a change in the efficiency of material removed by the laser beam, primarily due to shielding of the target from the incident laser energy by a laser-vapor plume interaction. Several analytical technologies were employed to study the quantity of mass removed versus laser power density. Data for mass ablation behavior versus laser power density are reported using ICP-AES, atomic emission from a laser-induced plasma near the sample surface, acoustic stress power in the target, and measurements of crater volumes. This research demonstrates that the change in ICP-AES intensity with laser power density is due to changes in the mass removal. The roll-off in mass ablation is not due to a change in particle size distribution of the ablated species, fractionation of the sample, or a change in transport efficiency to the ICP torch. Accurate tracking of the ICP-AES with the laser ablation process justifies the use of internal and external standardization.

Correlation of spectral emission intensity in the inductively coupled plasma and laser-induced plasma during laser ablation of solid samples

Abstract: Spectral atomic emission intensity from laser-induced plasmas (LIPs) exhibits excellent correlation with atomic emission intensity in the inductively coupled plasma (ICP) for a wide variety of materials and laser powers. Laser ablation sampling with introduction into an lCP for chemical analysis has, among other factors, a strong nonlinear dependence on laser energy, spot size, and material composition. The LIP emission also has a similar nonlinear dependence and is shown to correspond with the ICP behavior. The correlation is demonstrated for several homogeneous metallic and oxide materials during laser ablation sampling over a range of power densities and incident laser beam spot sizes. The correlation is best for higher melting temperature materials and moderate laser power density. The LIP and ICP emission intensities both show similar dependence for mass ablation rate versus power density and laser beam spot size. A normalized ICP/area over LIP emission ratio shows that a functional relationship can be found for changes in ICP intensity with changes in laser power density. The correlation shows that the ICP intensity accurately reflects changes in the laser ablation process and that the LIP may possibly be used for internal monitoring during laser sampling with the ICP. 32 refs., 10 figs.,more » 1 tab.« less

Comparison of SiC, GaAs, and Si RF MESFET power densities

Abstract: The maximum power density of Si, GaAs, and 4H-SiC MESFET's was modeled using material parameters, a planar MESFET cross section, and a piecewise linear MESFET drain characteristic. The maximum power density for the Si, GaAs, and 4H-SiC was calculated to be 0.45 W/mm, 0.78 W/mm, and 17.37 W/mm at drain voltages of 8.4 V, 8.3 V, and 105 V, respectively. Modeling power density as a function of drain voltage showed that, for low voltage applications, the GaAs MESFET has the highest power density because of its high electron mobility and very low channel resistance (R/sub on/). For high voltage applications, the 4H-SiC MESFET has the highest absolute power density because of the higher breakdown voltage of this material. Experiment data agree qualitatively with the modeled results. >

Semi-empirical high-field conduction model for polyethylene and implications thereof

Abstract: A semi-empirical model for high-field conduction in polyethylene is developed on the basis of data in the literature for ac conductivity in the range of 30 to 60 kV/mm and 'charge injection' from a needle in the range of 160 kV/mm. The model is used to compute high-field conduction-related phenomena by solving Poisson's equation with field-dependent conductivity and time-dependent applied voltage for a highly inhomogeneous field geometry. Parameters computed include field, current density, charge density, power density, and force density. Temperature rise in the dielectric is estimated based on the computed power density. >

Advanced electron cyclotron heating systems for next-step fusion experiments

Abstract: Electron cyclotron heating (ECH) is one of the major candidates for heating and current drive on ITER (170 GHz) and W7-X (140 GHz). ECH is extremely attractive from physics and reactor engineering points of view, offering start-up assist, efficient and localized power deposition, simple and compact launching structures with high injected power density, and a simple interface with shielding and blanket. High unit power (1 MW or greater) and high efficiency (35% or greater) single-mode continuous-wave (CW) gyrotron oscillators are under development in order to reduce significantly the systems costs by reducing the size of the auxiliary support systems. 140 GHz gyrotrons with 0.55 MW output power in the Gaussian free-space TEM 0,0 mode with a pulse length τ up to 3.0 s and efficiency η of 42% are already commercially available (Gycom). Improved internal quasi-optical mode transducers generate the TEM 0,0 output mode with efficiencies of 90–95% and separate the electron beam from the r.f. beam, thus allowing the use of large CW relevant depressed collectors for energy recovery. Tube efficiencies around 50% have been already achieved at JAERI and KfK. Face cooled double-disk sapphire windows, cryogenically edge-cooled single-disk sapphire windows (liquid-nitrogen, liquid-neon or liquid-helium cooling), distributed windows (metal or ceramics) as well as diamond and silicon windows are under investigation in order to solve the window problem. Long-distance high-power millimeter wave transmission from the source to the plasma device with very low ohmic losses and high mode purity can be accomplished by (1) a closed, highly overmoded, circumferentially corrugated or dielectrically lined, tubular HE 1,1 -hybrid mode waveguide and (2) open quasi-optical TEM 0,0 transmission through a gaussian beam waveguide using focusing reflectors as phase-correcting elements. This paper reports the state of the art in gyrotron, window, transmission line and antenna development and discusses possibilities of ECH systems cost reduction.

MR gradient coil heat dissipation

Abstract: The temperature responses of five different gradient coil designs were modeled using simplified engineering equations and measured. The model predicts that the coil temperature approaches a maximum as an inverse exponential, where the maximum temperature is governed by two parameters: a local power density and a cooling term. The power density term is a function of position and is highest where the current paths have minimum widths and are closely packed. The cooling parameter consists of convective, conductive, and radiative components which can be controlled by (1) providing forced cooling, (2) having a coil former with high thermal conductivity and thin walls, and (3) varying the emissivity of the coil surfaces. For a given gradient strength, the average temperature rise is minimized by designing a coil with a small radius and thick copper. The model predicted the local temperature rise, which is also dependent on the current density, to within 5 degrees C of measured values.

Thermal analysis of closely-packed two-dimensional etched-well surface-emitting laser arrays

Abstract: A self-consistent thermal-electrical model of etched-well surface-emitting GaAs-AlGaAs lasers is applied to study thermal properties of two-dimensional (2-D) arrays. Temperature profiles across small- and moderate-size arrays are generated. Thermal problems associated with integration of vertical-cavity surface-emitting lasers (VCSEL's) into closely packed 2-D arrays are addressed. In particular, thermal crosstalk between elements of the array is investigated. The results indicate severe thermal crosstalk, even at relatively low pumping currents, with strong individual-element temperature-profile nonuniformities. The asymmetry of individual waveguides caused by skewed temperature distributions can be exploited for controlling supermode structure in phase-locked arrays. Optimization of 2-D etched-well laser array designs is presented, aimed at maximizing the optical output power density from the array. We find that the highest output power density is strongly increasing for arrays of small diameter emitters, and that the optimal center-to-center spacing between emitters is equal to /spl sim/2.5-3 times their diameter. >

A High Energy and Power Novel Aluminum/Nickel Battery

Abstract: Recent advancements in aluminum materials and aluminum electrochemistry open a current density compatibility domain in which aluminum anodes and nickel oxide cathodes can simultaneously discharge at high faradaic efficiency. The resultant overlapping comparability region permits demonstration of an aluminum/nickel battery capable of unusually high specific power density and high specific energy density: Al+3NiOOH+NaOH+3H{sub 2}O {yields} NaAl(OH){sub 4}+3Ni(OH){sub 2}. The operational cell maintains a steady-state discharge voltage in excess of 2.3 V. Aluminum/nickel battery discharges are presented which yield a power density of 915 W/kg, and a specific energy density of up to 140 Wh/kg.

Performance of a liquid‐nitrogen‐cooled, thin silicon crystal monochromator on a high‐power, focused wiggler synchrotron beam

Abstract: An experiment was performed on beamline BL 3 at the European Synchrotron Radiation Facility to test the diffraction performance of a novel internally liquid‐nitrogen‐cooled, thin silicon crystal monochromator exposed to high heat loads. The beam parameters were chosen to closely match the conditions expected, in terms of absorbed power and beam profile, at the Advanced Photon Source (APS) for the closed gap undulator at 7 GeV and 100 mA. The cooled crystal was oriented at 11.4° in the symmetric Bragg geometry to diffract 30 keV x‐rays from the Si(333) planes. The source was a 44‐pole wiggler with the insertion device gap set at 25.0 mm corresponding to a deflection parameter, K, of 4.2. A tunable toroidal mirror was used to focus the wiggler beam onto the crystal. Double‐crystal rocking curves were measured at several power values using different attenuators. The maximum total power absorbed by the 0.6‐mm‐thick crystal was 154 W at a storage ring current of 136 mA. The peak power density at normal inciden...

Optically pumped intersubband lasers based on quantum wells

Abstract: A far infrared (FIR) laser based on intersubband transitions in quantum wells is proposed where a pumping laser is used to create population inversion in the structure. The goal is to develop a structure which operates essentially as a 4-level laser, to minimize bottlenecking of the lower laser state. Multiple quantum wells can be used in the active laser of these structures to enhance the laser gain and the minimum required reflectivity in the cavity structure. The possibility of using both conduction and valence band quantum-well structures are investigated. Our study shows that, due to high intersubband scattering rates in the valence band structure, the creation of population inversion is more difficult and requires a high pumping power density while in the conduction band structure, population inversion can be achieved by a moderate pumping power density. The maximum population inversion in the conduction band structure is estimated to be 2.1/spl times/10/sup 11/ cm/sup 2/, which requires a pumping power density 2 kW cm/sup -2/ for a single quantum well. The threshold power as well as the minimum required reflectivity of the cavity structure for the conduction band scheme are estimated for different well numbers. >

1 W Ku-band AlGaAs/GaAs power HBT's with 72% peak power-added efficiency

Abstract: High power and high-efficiency multi-finger heterojunction bipolar transistors (HBT's) have been successfully realized at Ku-band by using an optimum emitter ballasting resistor and a plated heat sink (PHS) structure. Output power of 1 W with power-added efficiency (PAE) of 72% at 12 GHz has been achieved from a 10-finger HBT with the total emitter size of 300 /spl mu/m/sup 2/. 72% PAE with the output power density of 5.0 W/mm is the best performance ever reported for solid-state power devices with output powers more than 1 W at Ku-band. >

Apparatus and method for measuring the power density of a laser beam with a liquid crystal

Abstract: Meters detect power parameter information about laser beams using liquid crystals to propagate the beam therethrough with photodiode detectors. The power parameters include intensity in watts per square meter, beam waist size, and the location of the focal spot based in light induced orientational phenomena in the liquid crystal. The detectors can can count the number of interference fringe rings produced by a laser beam passing through the liquid crystal(LC). Alternatively, the time between the occurrence of each interference fringe ring can be measured to determine the power parameters. A preferred embodiment has a standard liquid crystal oriented at approximately 45 degrees to the axis of an incoming laser beam to be measured. The beam passing through the LC can be centered through a pinhole on a planar plate causing the interference fringe rings to appear on the surface of the plate. An alternative embodiment measures intensity based on determining the voltage necessary to produce fringe ring patterns. Corresponding the fringe ring patterns to intensity values can be done manually upon visual observation and calculation or automatically by computer.

Cavity-enhaced and quasi-phase-matched optical frequency doublers in surface-emitting geometry

Abstract: We show that we can efficiently achieve surface-emitting second-harmonic generation in vertical and horizontal cavities. The fundamental beam is coupled into the waveguide, which consists of III–V or II–VI semiconductor multilayers or asymmetric quantum-well domain structures. The generated second-harmonic radiation propagates along the growth direction of these layers (which is normal to the propagation direction of the fundamental beam). The quasi-phase matching is achieved when second-order susceptibility is modulated along the growth direction in these structures. By the proper design of these structures, the frequency doublers based on these structures together can cover the spectrum of 0.8–2.0 μm. If the pump power density is sufficiently large, the conversion efficiency approaches saturation. The saturation power per unit waveguide width is between ~0.9 and ~435 mW/μm. At such a power density, 72% conversion efficiency can be achieved. In addition, the proposed frequency doublers are, in principle, broadband.

Factors in inductive storage system design

Abstract: The practical implementation of an inductive pulsed power supply depends on its size and EMI with neighboring environment. Parametric trade-off is made for the prime power battery and the coil. It is shown that the minimal volume of the battery-coil system with a Brooks coil is achieved at a ratio of their volumes 3:2, respectively. For such systems, a jellyroll coil is preferable to a pancake one in view of a higher transfer efficiency. At very high battery power density, a pancake coil may be a right choice due to a lower mechanical stress. The fringe field of cylindrical and quasitoroidal coils was calculated using dipole approximation. Measurements of the discharge efficiency and fringe field are in a good agreement with the theoretical analysis.

Power required to maintain an electron in a discharge: Its use as a reference parameter in magnetized high frequency plasmas

Abstract: Earlier works on discharges sustained by electromagnetic surface waves in absence of a magnetic field have revealed the central role played by the power balance per electron. This balance relation stated that, provided energy transport is negligible, the power θL lost by the electron on the average in collisions with heavy particles is exactly compensated under steady‐state conditions by the power θA taken by the electron on the average from the high frequency (hf) field, their common value being the parameter θ. Then, because θL is to a first approximation the same in all hf discharges under given discharge conditions and power density, a simple discharge model valid for all hf plasmas was used. The present article is an extension of this approach to hf magnetized plasmas, using surface‐wave plasma columns placed in an axially directed static magnetic field as a means of investigation. We observe that θ decreases monotonously when increasing the magnetic field intensity B0, showing no extremum at or close to the electron cyclotron resonance frequency match over the gas pressure range (5–100 mTorr) investigated. We show that θ is controlled either by classical ambipolar diffusion or anomalous diffusion, the actual diffusion regime depending on whether the novel parameter B0p (p is gas pressure) is small or large. Our measured θ values are further used to estimate the average electron density in helicon sources given the power density, showing fair agreement with the reported values.

Excitation power dependence of photoluminescence enhancement from passivated GaAs

Abstract: Photoluminescence (PL) efficiency plots versus laser excitation power are presented for bare and Na2S treated n‐GaAs. The plots demonstrate an increase in PL quantum yield with increasing excitation power density for both samples. The PL enhancement observed in the Na2S treated sample is presented and is shown to depend strongly on excitation power density. Application of the deadlayer model to the analysis of low power PL efficiency from bare GaAs indicates that surface electron‐hole recombination is significantly slower than surface minority carrier trapping.

High PAE pseudomorphic InGaAs/AlGaAs HEMT X-band high power amplifiers

Abstract: A systematic experiment was designed and implemented to optimize the 0.25 /spl mu/m gate InGaAs/AlGaAs pseudomorphic HEMT for the fabrication of high power X-band monolithic amplifiers. The material structure and gate recess process were engineered such that the device breakdown voltage was optimized without sacrificing gain and efficiency. A two-stage high power X-band monolithic amplifier based on the optimized device has been developed. When the amplifier was operated at V/sub ds/=10 V, an output power of 9 W was achieved across the 7 to 10 GHz frequency range. A peak saturated output power of 10 W, corresponding to a power density of 1 W/mm, occurred at 8.5 GHz. When biased at 7 V, the amplifier generated a peak power of 6.7 W with an associated power added efficiency of 40% at 8.5 GHz.

Laser power dependence of particulate formation on pulse laser deposited films

Abstract: The density and the size of particulates in films laser‐deposited at room temperature using various target materials were observed to depend strongly on the target material and the laser power density. However, loose universal relations between the deposition rates and the particulate density as well as the particulate size were found, where the latter corresponds approximately to the ratio of the laser power density to the ablation threshold. Furthermore, particulates consisting of only some of the target elements such as CuOx were found. Additionally, an acceptably high deposition rate was obtained by using halide and sulfide targets. These materials offer a possibility of deposition using a low power laser.

Basic Properties of Fusion Edge Plasmas and Role of Atomic and Molecular Processes

Abstract: The self-sustained thermonuclear burn of a deuterium-tritium (D-T) plasma in a fusion reactor relies on maintaining certain stringent conditions imposed on the plasma parameters (temperature and density), plasma energy confinement time, and power generating and loss processes. The plasma burn condition requires that the power density of fusion-born alpha particles exceeds the sum of the densities of all radiative and thermal power losses. While the thermal power losses are determined by collective plasma transport phenomena, the radiation losses (such as bremsstrahlung and line radiation) are determined by collisional and radiative atomic processes. Nonhydrogenic species (impurities) present in the plasma may substantially increase the radiation losses and, above certain critical amounts (e.g., 1% of plasma density for Fe and 0.1% of plasma density for W), may extinguish the burn. The fusion-born alpha particles carry about one-fifth of the total fusion power generated by D-T thermonuclear reactions, and only a small part of it is used to sustain the thermonuclear burn or is lost by bremsstrahlung radiation. The remaining power, as well as the alpha particles themselves, have to be removed from the burning reactor zone in order to avoid plasma overheating and poisoning. The accumulation of alpha particles in the reacting zone (He ash) dilutes the thermonuclear fuel, degrades the burning conditions, and, above certain levels, may extinguish the plasma burn. Under steady-state conditions, the alpha particles have to be removed from the reactor at a rate equal to that of their production.

High efficiency operation of 6-H SiC MESFETs at 6 GHz

Abstract: Summary form only given. SiC MESFETs are very promising candidates for RF power amplification, due to their unique combination of high saturation velocity, high breakdown strength, and high thermal conductivity. In the present work, we demonstrate for the first time high efficiency RF power operation at 6 GHz. We have obtained power output of 35 W, with 45.5% power added efficiency at 6 GHz from a 6-H SiC MESFET operating at a drain bias of 40 V. The gate length and width were 0.5 /spl mu/m and 2 mm respectively. The corresponding power density is 1.75 W/mm and is more than a factor of 3 higher than that obtained normally in GaAs. To our knowledge, these results represent the highest power output, efficiency, and operating frequency reported to date in SiC. The power MESFETs were fabricated on high resistivity SiC substrates grown at Westinghouse. Sintered Ni ohmic contacts, mesa isolation, and channel recessing using RIE were used in device fabrication. Air-bridge source interconnects were used for large periphery devices. The fabrication and characterization of these SiC power MESFETs are presented.

No-field, low power FeMn deposition giving high exchange films

Abstract: A method of producing a magnetoresistive read transducer with improved longitudinal bias due to high exchange coupling is disclosed A layer of antiferromagnetic material is sputtered deposited onto a layer of ferromagnetic material in the absence of a magnetic field and at a power density below 07 W/cm2 The layers of ferromagnetic material and antiferromagnetic material are annealed at a low temperature of between 200° C and 250° C for between 6 and 26 hours

A novel uninterruptible power supply system with sinusoidal input-output characteristics and simplified control circuit

Abstract: In this paper a novel UPS topology is presented and analysed. The proposed UPS consists of three power stages. The first stage is a switch-mode rectifier which utilises a buck-boost power converter for active power correction. The second stage is a high frequency inverter that provides ohmic isolation and voltage matching. The third stage is a low frequency sinusoidal PWM inverter. Moreover, the proposed UPS exhibits sinusoidal input/output characteristics, high power density and a simple control circuit.

Optical properties of reactively sputtered GeO2 in the vacuum ultraviolet region

Abstract: Optical properties of sputtered GeO 2 films submitted to various heat treatments are investigated with measurements in the vacuum ultraviolet region by means of the multi-angle reflectivity method. A Kramers-Kronig analysis of the near-normal-incidence reflectivity is compared with previous results on glassy and polycrystalline GeO 2 . The films were sputtered from a germanium target (99.999%) in an argon + oxygen mixture at a total pressure of 0.6 Pa and oxygen pressure of 0.24 Pa, with a target power density of 1.6 W/cm 2 . The deposition rate was about 1.5 nm/min. The film density has been evaluated by X-ray total reflection measurements and used for the Penn gap model calculations. Using the density correlation in the f -sum rule, the Penn gap energies for hexagonal and glassy GeO 2 have been obtained and are compared with the average interband transition energy in GeO 2 spectra.

A comparative optical analysis of laser side-firing devices: a guide to treatment

Abstract: OBJECTIVE: To compare the optical characteristics of five different laser side-firing fibres used to perform deep laser coagulation of the prostate. MATERIALS AND METHODS: The intensity profile, angle of exit and beam divergence from the launch fibre were measured underwater (to simulate endoscopic conditions) for each fibre using both the Helium Neon(633 nm) and Nd:YAG(1064 nm) lasers. RESULTS: The intensity profiles and spot sizes varied among fibres but broadly fell within two groups characterized by large and small footprints (low and high power density). There was a maximum 5.5 fold difference in average power densities between the fibres. CONCLUSIONS: Not all side-firing devices are the same. Variations in the power density profiles among the fibres suggest that the choice of fibre is of critical importance in determining the type of laser tissue interaction (i.e. coagulation or vaporization) that will follow the use of that fibre.