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Results for “radiation efficiency”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Design of high-efficiency, radiation-hard, GaInP/GaAs solar cells

In recently years, Ga(0.5)In((0.5)P/GaAs cells have drawn increased attention both because of their high efficiencies and because they are well suited for space applications. They can be grown and processed as two-junction devices with roughly twice the voltage and half the current of GaAs cells. They have low temperature coefficients, and have good potential for radiation hardness. We have previously reported the effects of electron irradiation on test cells which were not optimally designed for space. From those results we estimated that an optimally designed cell could achieve 20 percent after irradiation with 10(exp 15) cm(exp -2) 1 MeV electrons. Modeling studies predicted that slightly higher efficiencies may be achievable. Record efficiencies for EOL performance of other types of cells are significantly lower. Even the best Si and InP cells have BOL efficiencies lower than the EOL efficiency we report here. Good GaAs cells have an EOL efficiency of 16 percent. The InP/Ga(0.5)In(0.5)As two-junction, two-terminal device has a BOL efficiency as high as 22.2 percent, but radiation results for these cells were limited. In this study we use the previous modeling and irradiation results to design a set of Ga(0.5)In(0.5)P/GaAs cells that will demonstrate the importance of the design parameters and result in high-efficiency devices. We report record AMO efficiencies: a BOL efficiency of 25.7 percent for a device optimized for BOL performance and two of different designs with EOL efficiencies of 19.6 percent (at 10(exp 15) cm(exp -2) 1MeV electrons). We vary the bottom-cell base doping and the top-cell thickness to show the effects of these two important design parameters. We get an unexpected result indicating that the dopant added to the bottom-cell base also increases the degradation of the top cell.

Kurtz, Sarah R.↗
NASA NTRS1993

Generation of 369.4 nm Radiation by Efficient Doubling of a Diode Laser

A resonant cavity second harmonic generation system has been developed to produce 369.4 nm radiation from a 738.8 nm diode laser with 10 mW nominal output power. This system utilizes a polarization technique to lock the cavity to the laser frequency. In this paper we report on an evaluation of the system using a Titanium:Sapphire laser as the input source, and preliminary results with a diode laser source. To our knowledge, this is the deepest uv light ever produced by frequency-doubling a diode laser.

Williams, A.↗
NASA NTRS1979

Fabrication of high efficiency and radiation resistant GaAs solar cells

Systematic improvements in fabrication yield were obtained by appropriate control of the liquid phase epitaxial growth process, contact fabrication and surface preparation. To improve radiation hardness, the junction depth was decreased while overcoming the penalty in decreased solar cell efficiency which tends to go hand-in-hand with the reduction of junction depth in (AlGa) As-GaAs solar cells. Cells were made with an AMO efficiency of 18% and a junction depth of 0.5 micrometers, as compared to junction depths on the order of 1.0 micrometers. With respect to the damage caused by proton irradiation, the nature of the observed damage was correlated to the energy and penetration depth of the damaging protons.

Kamath, S.↗
NASA NTRS1994

Energetic and radiative constraints on highly relativistic jets

We examine constraints on the energetics and radiative efficiencies of highly relativistic, synchrotron-emitting jets. If the observed intraday radio variability of compact radio sources is intrinsic and results from incoherent synchrotron radiation, then the associated jets must have bulk Lorentz factors in the range Gamma varies approximately 30 to 100, several times larger than the largest values inferred from superluminal expansion, and larger even than the values required to avoid the synchrotron self-Compton catastrophe. We show that such highly relativistic jets produce synchrotron radiation with extremely low radiative efficiency. As a result they must carry enormous kinetic energy fluxes, L(sub j) approximately greater than 10(exp 47)(Delta Omega/0.1 sr), where Delta Omega is the solid angle subtended by the jet, in order to produce 'apparent' synchrotron brightness temperatures approximately greater than 10(exp 16) K. Energy losses by such jets should be strongly dominated by Compton scattering of diffuse ambient radiation, and they should produce large X-ray and gamma-ray fluxes.

Begelman, Mitchell C.↗
NASA NTRS2004

Printed Multi-Turn Loop Antenna for RF Bio-Telemetry

In this paper, a novel printed multi-turn loop antenna for contact-less powering and RF telemetry from implantable bio- MEMS sensors at a design frequency of 300 MHz is demonstrated. In addition, computed values of input reactance, radiation resistance, skin effect resistance, and radiation efficiency for the printed multi-turn loop antenna are presented. The computed input reactance is compared with the measured values and shown to be in fair agreement. The computed radiation efficiency at the design frequency is about 24 percent.

Simons, Rainee N.↗
NASA NTRS1990

Study of a cesium plasma as a selective emitter for thermophotovoltaic applications

This experimental study evaluates the potential of a cesium plasma as an emitter for a thermophotovoltaic (TPV) energy conversion system. A cesium plasma, as a result of the ground-state transitions of its single outer-shell electron, produces large amounts of radiation in the 850-890-nm wavelength region. This would provide excellent coupling to silicon, gallium arsenide, and indium phosphide photovoltaic cells. Measurements of the radiative efficiency, the sum of the power at the 852 and 894 nm wavelengths relative to the total emitted power, were made and correlated to the plasma operating variables. It was determined that, for atomic density in the range (3-6) x 10 exp 21/cu cm and electron temperature in the range 2000-3000 K, radiative efficiencies in excess of 70 percent are attainable. This would indicate that a cesium plasma with its selective emission characteristics and low electron operating temperatures of 2000-3000 K would be an excellent candidate as an emitter in a TPV system.

Lowe, R.↗
NASA NTRS1977

Solar Cell High Efficiency and Radiation Damage

Silicon solar cell analysis and fundamental measurements, silicon cell technology, gallium arsenide research and technology, and radiation effects on silicon and gallium arsenide cells, are reported.

Source record↗
NASA NTRS1993

Generation of 369.4 nm Radiation by Efficient Doubling of a Diode Laser

The trapped ytterbium ion frequency standard under development at JPL is designed to supply ultra-stable reference signals to be used in spacecraft tests of general relativity and other science experiments. Only diode lasers meet the requirements of low power comsumption and small size for use aboard spacecraft, but diode lasers are not currently available at 369.4 nm wavelength. It is therefore necessary to employ frequency doubling methods to reach this frequency with diode lasers. Here we present the characteristics of the resonant cavity doubling system we have designed and preliminary UV conversion results.

ultra-stable↗
NASA NTRS1994

High sensitivity dynamic spectral search for flare star radio

We observed ten well-known flare stars with the Arcibo radio telescope at 1.4 GHz and 5 GHz, using a special observing technique to discriminate between real flares and radio freqeuncy interference. With a high sensitivity of 5.5 K/Jy at 1.4 GHz when averaged over a 50 MHz band, we are able to recognize flux enhancements as weak as approximately 6 mJy above the sky background variations. In about 85 hours of observation, about a dozen bursts were detected, only from AD Leo. All had flux densities lower than 70 mJy, which probably explains their lack of fine structures (except for the strongest one), such as were reported in the literature for stronger flares. Half of the bursts that we recorded are 100% circularly polarized, and half are not circularly polarized. Our results are a first attempt of reliable statistics on dMe flare rates at 1.4 GHz. The high brightness temperatures we infer for the observed bursts are interpreted in terms of coherent emission processes, either the cyclotron maser instability or plasma radiation. Efficiencies are comparable to those of solar or planetary radio emissions in the case of the cyclotron maser, and higher than the solar efficiency in the case of plasma radiation, with the caveat that there are great uncertainties in the coronal model and the source size.

Abada-Simon, M.↗
NASA NTRS2010

Profiling Transboundary Aerosols over Taiwan and Assessing Their Radiative Effects

A synergistic process was developed to study the vertical distributions of aerosol optical properties and their effects on solar heating using data retrieved from ground-based radiation measurements and radiative transfer simulations. Continuous MPLNET and AERONET observations were made at a rural site in northern Taiwan from 2005 to 2007. The aerosol vertical extinction profiles retrieved from ground-based lidar measurements were categorized into near-surface, mixed, and two-layer transport types, representing 76% of all cases. Fine-mode (Angstrom exponent, alpha, approx.1.4) and moderate-absorbing aerosols (columnar single-scattering albedo approx.0.93, asymmetry factor approx.0.73 at 440 nm wavelength) dominated in this region. The column-integrated aerosol optical thickness at 500 nm (tau(sub 500nm)) ranges from 0.1 to 0.6 for the near-surface transport type, but can be doubled in the presence of upper-layer aerosol transport. We utilize aerosol radiative efficiency (ARE; the impact on solar radiation per unit change of tau(sub 500nm)) to quantify the radiative effects due to different vertical distributions of aerosols. Our results show that the ARE at the top-of-atmosphere (-23 W/ sq m) is weakly sensitive to aerosol vertical distributions confined in the lower troposphere. On the other hand, values of the ARE at the surface are -44.3, -40.6 and -39.7 W/sq m 38 for near-surface, mixed, and two-layer transport types, respectively. Further analyses show that the impact of aerosols on the vertical profile of solar heating is larger for the near-surface transport type than that of two-layer transport type. The impacts of aerosol on the surface radiation and the solar heating profiles have implications for the stability and convection in the lower troposphere.

Wang, Sheng-Hsiang↗
NASA NTRS2017

Optimization of Debris Shields on the NISAR Mission’s L-Band Radar Instrument

The NASA-ISRO Synthetic Aperture Radar (NISAR) space mission is a collaboration between NASA and the Indian Space Research Organization (ISRO), launching in the 2020s to a polar orbit of 747km altitude. The mission will provide spatial and temporal measurements of land surface changes (e.g. ice sheets, vegetation, earthquakes). Many of the SAR electronics boxes are mounted on the exterior of the structure. Their singlewall box lids efficiently radiate heat for thermal control, but are not very efficient debris shields. The initial design showed an unacceptably high impact risk as estimated with NASA’s ORDEM3 debris model and Bumper impact analysis tool. Each box has a different role in instrument functionality, and this was captured in a reliability model used to optimize the distribution of shield mass among the boxes: total added mass was minimized while maintaining a threshold of functionality and survival probability that was acceptable to the project.

Chinn, James Z.↗
NASA NTRS1994

Spectral properties of blast-wave models of gamma-ray burst sources

We calculate the spectrum of blast-wave models of gamma-ray burst sources, for various assumptions about the magnetic field density and the relativistic particle acceleration efficiency. For a range of physically plausible models we find that the radiation efficiency is high and leads to nonthermal spectra with breaks at various energies comparable to those observed in the gamma-ray range. Radiation is also predicted at other wavebands, in particular at X-ray, optical/UV, and GeV/TeV energies. We discuss the spectra as a function of duration for three basic types of models, and for cosmological, halo, and galactic disk distances. We also evaluate the gamma-ray fluences and the spectral characteristics for a range of external densities. Impulsive burst models at cosmological distances can satisfy the conventional X-ray paucity constraint S(sub x)/S(sub gamma)less than a few percent over a wide range of durations, but galactic models can do so only for bursts shorter than a few seconds, unless additional assumptions are made. The emissivity is generally larger for bursts in a denser external environment, with the efficiency increasing up to the point where all the energy input is radiated away.

Meszaros, P.↗
NASA NTRS2012

Toward a New Paradigm for the Unification of Radio Loud AGN and its Connection to Accretion

We recently argued [21J that the collective properties. of radio loud active galactic nuclei point to the existence of two families of sources, one of powerful sources with single velocity jets and one of weaker jets with significant velocity gradients in the radiating plasma. These families also correspond to different accretion modes and therefore different thermal and emission line intrinsic properties: powerful sources have radiatively efficient accretion disks, while in weak sources accretion must be radiatively inefficient. Here, after we briefly review of our recent work, we present the following findings that support our unification scheme: (i) along the broken sequence of aligned objects, the jet kinetic power increases. (ii) in the powerful branch of the sequence of aligned objects the fraction of BLLs decreases with increasing jet power. (iii) for powerful sources, the fraction of BLLs increases for more un-aligned objects, as measured by the core to extended radio emission. Our results are also compatible with the possibility that a given accretion power produces jets of comparable kinetic power.

Georganpoulos, Markos↗
NASA NTRS1994

Heat dissipation in water-cooled reflectors

The energy balance of a lamp varies with the thermal and optical characteristics of the reflector. The photosynthetic radiation efficiency of lamps, defined as input power divided by photosynthetically active radiation (PAR, 400-700 nm) emitted from the lamp ranges between 0.17 and 0.26. The rest of the energy input is wasted as longwave (3000 nm and over) and non-PAR shortwave radiation (from 700 nm to 3000 nm), convective, and conductive heat from the lamp, reflector, and ballast, and simply for increasing the cooling load. Furthermore, some portion of the PAR is uselessly absorbed by the inner walls, shelves, vessels, etc. and some portion of the PAR received by the plantlets is converted into sensible and latent heat. More than 98% of the energy input is probably converted into heat, with only less than 2% of the energy input being converted into chemical energy as carbohydrates by photosynthesis. Therefore, it is essential to reduce the generation of heat in the culture room in order to reduce the cooling load. Through use of a water-cooled reflector, the generation of convective and conductive heat and longwave radiation from the reflector can be reduced, without reduction of PAR.

Kozai, Toyoki↗
NASA NTRS1994

The role of radiative acceleration in outflows from broad absorption line QSOs. 1: Comparison with O star winds

We investigate the role of radiation pressure in accelerating the broad absorption line (BAL) outflows in QSOs by comparing their properties with those of radiatively driven O star winds. We find that, owing to their lower column densities and higher velocity spreads, BAL outflows have only a few tens of strong resonance lines that are dynamically important, as compared with 10(exp 3) - 10(exp 4) lines in O star winds. We show that the combined radiative force (the 'force multiplier') declines more rapidly as a function of column density for BAL outflows than for O star winds. This is mainly attributed to the absence of lines from excited states in the BAL region. The absorbing gas in BAL outflows must have a small filling factor in order for radiative acceleration to be important dynamically. This allows the absorbing material to remain at a high enough density to maintain the ion species necessary for efficient radiative acceleration as well (as those responsible for the observed absorption), without the average flow density becoming so large that the absorbing matter cannot be accelerated by an increment larger than its own sound speed. The latter condition is necessary if the outflow is to tap a large portion of the incident photon momentum. Once a small filling factor is assumed, radiative acceleration can be more efficient in BAL outflows than in O stars. We show that terminal velocities of a few times 10(exp 4) km/sec can be expected, provided that the absorbing matter does not have to drag with it a much heavier substrate.

Arav, Nahum↗
NASA NTRS1987

High temperature radiator materials for applications in the low Earth orbital environment

Radiators must be constructed of materials which have high emittance in order to efficiently radiate heat from high temperature space power systems. In addition, if these radiators are to be used for applications in the low Earth orbital environment, they must not be detrimentally affected by exposure to atomic oxygen. Four materials selected as candidate radiator materials (304 stainless steel, copper, titanium-6% aluminum-4% vanadium (Ti-6%Al-4%V), and niobium-1% zirconium (Nb-1%Zr)) were surface modified by acid etching, heat treating, abrading, sputter texturing, electrochemical etching, and combinations of the above in order to improve their emittance. Combination treatment techniques with heat treating as the second treatment provided about a factor of two improvement in emittance for 304 stainless steel, Ti-6%Al-4%V, and Nb-1%Zr. A factor of three improvement in emittance occurred for discharge chamber sputter textured copper. Exposure to atomic oxygen in an RF plasma asher did not significantly change the emittance of those samples that had been heat treated as part of their texturing process. An evaluation of oxygen penetration is needed to understand how oxidation affects the mechanical properties of these materials when heat treated.

Rutledge, Sharon K.↗