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NASA NTRS1992

TRASYS form factor matrix normalization

A method has been developed for adjusting a TRASYS enclosure form factor matrix to unity. This approach is not limited to closed geometries, and in fact, it is primarily intended for use with open geometries. The purpose of this approach is to prevent optimistic form factors to space. In this method, nodal form factor sums are calculated within 0.05 of unity using TRASYS, although deviations as large as 0.10 may be acceptable, and then, a process is employed to distribute the difference amongst the nodes. A specific example has been analyzed with this method, and a comparison was performed with a standard approach for calculating radiation conductors. In this comparison, hot and cold case temperatures were determined. Exterior nodes exhibited temperature differences as large as 7 C and 3 C for the hot and cold cases, respectively when compared with the standard approach, while interior nodes demonstrated temperature differences from 0 C to 5 C. These results indicate that temperature predictions can be artificially biased if the form factor computation error is lumped into the individual form factors to space.

Tsuyuki, Glenn T.↗

NASA NTRS2001

Satellite Remote Sensing of Tropical Precipitation and Ice Clouds for GCM Verification

This project, supported by the NASA New Investigator Program, has primarily been funding a graduate student, Darren McKague. Since August 1999 Darren has been working part time at Raytheon, while continuing his PhD research. Darren is planning to finish his thesis work in May 2001, thus some of the work described here is ongoing. The proposed research was to use GOES visible and infrared imager data and SSM/I microwave data to obtain joint distributions of cirrus cloud ice mass and precipitation for a study region in the Eastern Tropical Pacific. These joint distributions of cirrus cloud and rainfall were to be compared to those from the CSU general circulation model to evaluate the cloud microphysical amd cumulus parameterizations in the GCM. Existing algorithms were to be used for the retrieval of cloud ice water path from GOES (Minnis) and rainfall from SSM/I (Wilheit). A theoretical study using radiative transfer models and realistic variations in cloud and precipitation profiles was to be used to estimate the retrieval errors. Due to the unavailability of the GOES satellite cloud retrieval algorithm from Dr. Minnis (a co-PI), there was a change in the approach and emphasis of the project. The new approach was to develop a completely new type of remote sensing algorithm - one to directly retrieve joint probability density functions (pdf's) of cloud properties from multi-dimensional histograms of satellite radiances. The usual approach is to retrieve individual pixels of variables (i.e. cloud optical depth), and then aggregate the information. Only statistical information is actually needed, however, and so a more direct method is desirable. We developed forward radiative transfer models for the SSM/I and GOES channels, originally for testing the retrieval algorithms. The visible and near infrared ice scattering information is obtained from geometric ray tracing of fractal ice crystals (Andreas Macke), while the mid-infrared and microwave scattering is computed with Mie scattering. The radiative transfer is performed with the Spherical Harmonic Discrete Ordinate Method (developed by the PI), and infrared molecular absorption is included with the correlated k-distribution method. The SHDOM radiances have been validated by comparison to version 2 of DISORT (the community "standard" discrete-ordinates radiative transfer model), however we use SHDOM since it is computationally more efficient.

Evans, K. Franklin↗

NASA NTRS1989

Radiation thermometry at NIST: An update of services and research activities

An overview of activities at the National Institute of Standards and Terminology (NIST) in radiation thermometry and related temperature scale research is presented. An expansion of calibration services for pyrometers will be described as well as efforts to develop calibration services for blackbody simulators. Research relevant to the realization of the new international temperature scale (ITS 90) will be discussed.

Hillard, G. Barry↗

NASA NTRS2014

Lightweight Damage Tolerant Radiators for In-Space Nuclear Electric Power and Propulsion

Nuclear electric propulsion (NEP) is a promising option for high-speed in-space travel due to the high energy density of nuclear power sources and efficient electric thrusters. Advanced power conversion technologies for converting thermal energy from the reactor to electrical energy at high operating temperatures would benefit from lightweight, high temperature radiator materials. Radiator performance dictates power output for nuclear electric propulsion systems. Pitch-based carbon fiber materials have the potential to offer significant improvements in operating temperature and mass. An effort at the NASA Marshall Space Flight Center to show that woven high thermal conductivity carbon fiber mats can be used to replace standard metal and composite radiator fins to dissipate waste heat from NEP systems is ongoing. The goals of this effort are to demonstrate a proof of concept, to show that a significant improvement of specific power (power/mass) can be achieved, and to develop a thermal model with predictive capabilities. A description of this effort is presented.

Craven, Paul↗

NASA NTRS2003

The Sombrero: An Edge-On Spiral: Halo Dynamics, A Massive Dark Object and XRBS

We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy and we discuss possible explanations for its very sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands. The X-ray investigation shows a hard (Gamma=1.89) and moderately absorbed ( N(sub H) = 1.8 x l0(exp 21)/sq cm) nuclear source of 1.5 x l0(exp 40) erg/s in the 2-10 keV band, surrounded by hot gas at a temperature of approx. 0.6 keV. The bolometric nuclear luminosity is at least approx. 200 times lower than expected if mass accreted on the supermassive black hole, that HST shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disc. The low luminosity, coupled to the observed absence of Fe-K emission in the nuclear spectrum, indicates that such a disc is not present. This nucleus differs from bright unobscured AGNs also for the lack of high flux variability and of prominent broad Halpha emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could be too radio bright, instead, for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion.

Mushotzky, Richard F.↗

NASA NTRS2004

The Sombrero: An Edge-On Spiral: Halo Dynamics, A Massive Dark Object, and XRBS

We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy, and we discuss possible explanations for its very Sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands. The X-ray investigation shows a hard (gamma=1.89) and moderately absorbed (NH=1.8x1021 per square centimeter) nuclear source of 1.5x1040 ergs per second in the 2-10 keV band, surrounded by hot gas at a temperature of approximately 0.6 keV. The bolometric nuclear luminosity is at least approximately 200 times lower than expected if mass accreted on the supermassive black hole, which Hubble Space Telescope shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disk. The low luminosity, coupled to the observed absence of Fe K emission in the nuclear spectrum, indicates that such a disk is not present. This nucleus also differs from bright unobscured active galactic nuclei in the lack of high flux variability and prominent broad H emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could instead be too radio bright for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion.

Fabbiano, G.↗

NASA NTRS2015

Lightweight Damage Tolerant, High-Temperature Radiators for Nuclear Power and Propulsion

NASA is increasingly emphasizing exploration to bodies beyond near-Earth orbit. New propulsion systems and new spacecraft are being built for these missions. As the target bodies get further out from Earth, high energy density systems, e.g., nuclear fusion, for propulsion and power will be advantageous. The mass and size of these systems, including supporting systems such as the heat exchange system, including thermal radiators, will need to be as small as possible. Conventional heat exchange systems are a significant portion of the total thermal management mass and size. Nuclear electric propulsion (NEP) is a promising option for high-speed, in-space travel due to the high energy density of nuclear fission power sources and efficient electric thrusters. Heat from the reactor is converted to power for use in propulsion or for system power. The heat not used in the power conversion is then radiated to space as shown in figure 1. Advanced power conversion technologies will require high operating temperatures and would benefit from lightweight radiator materials. Radiator performance dictates power output for nuclear electric propulsion systems. Pitch-based carbon fiber materials have the potential to offer significant improvements in operating temperature, thermal conductivity, and mass. These properties combine to allow significant decreases in the total mass of the radiators and significant increases in the operating temperature of the fins. A Center-funded project at NASA Marshall Space Flight Center has shown that high thermal conductivity, woven carbon fiber fins with no matrix material, can be used to dissipate waste heat from NEP systems and because of high specific power (kW/kg), will require less mass and possibly less total area than standard metal and composite radiator fins for radiating the same amount of heat. This project uses an innovative approach to reduce the mass and size required for the thermal radiators to the point that in-space NEP and power is enabled. High thermal conductivity carbon fibers are lightweight, damage tolerant, and can be heated to high temperature. Areal densities in the NASA set target range of 2 to 4 kg/m2 (for enabling NEP) are achieved and with specific powers (kW/kg) a factor of about 7 greater than conventional metal fins and about 1.5 greater than carbon composite fins. Figure 2 shows one fin under test. All tests were done under vacuum conditions.

Craven, Paul D.↗

NASA NTRS2014

Lightweight Radiator for in Space Nuclear Electric Propulsion

Nuclear electric propulsion (NEP) is a promising option for high‐speed in‐space travel due to the high energy density of nuclear fission power sources and efficient electric thrusters. Advanced power conversion technologies may require high operating temperatures and would benefit from lightweight radiator materials. Radiator performance dictates power output for nuclear electric propulsion systems. Game‐changing propulsion systems are often enabled by novel designs using advanced materials. Pitch‐based carbon fiber materials have the potential to offer significant improvements in operating temperature, thermal conductivity, and mass. These properties combine to allow advances in operational efficiency and high temperature feasibility. An effort at the NASA Marshall Space Flight Center to show that woven high thermal conductivity carbon fiber mats can be used to replace standard metal and composite radiator fins to dissipate waste heat from NEP systems is ongoing. The goals of this effort are to demonstrate a proof of concept, to show that a significant improvement of specific power (power/mass) can be achieved, and to develop a thermal model with predictive capabilities making use of constrained input parameter space. A description of this effort is presented.

Craven, Paul↗

NASA NTRS2000

Comparison of Columnar Water Vapor Measurements During The Fall 1997 ARM Intensive Observation Period: Optical Methods

In the fall of 1997 the Atmospheric Radiation Measurement (ARM program conducted an intensive Observation Period (IOP) to study water vapor at its Southern Great Plains (SGP) site. Among the large number of instruments, four sun-tracking radiometers were present to measure the columnar water vapor (CWV). All four solar radiometers retrieve CWV by measuring solar transmittance in the 0.94-micrometer water vapor absorption band. As one of the steps in the CWV retrievals the aerosol component is subtracted from the total transmittance, in the 0.94-micrometer band. The aerosol optical depth comparisons among the same four radiometers are presented elsewhere. We have used three different methods to retrieve CWV. Without attempting to standardize on the same radiative transfer model and its underlying water vapor spectroscopy we found the CWV to agree within 0.13 cm (rms) for CWV values ranging from 1 to 5 cm. Preliminary results obtained when using the same updated radiative transfer model with updated spectroscopy for all instruments will also be shown. Comparisons to the microwave radiometer results will be included in the comparisons.

Schmid, Beat↗

NASA NTRS2000

Comparison of Columnar Water Vapor Measurements During The Fall 1997 ARM Intensive Observation Period: Solar Transmittance Methods

In the fall of 1997, during an Intensive Observation Period (IOP), the Atmospheric Radiation Measurement (ARM) program conducted a study of water vapor abundance measurement at its Southern Great Plains (SGP) site. Among a large number of instruments, four sun-tracking radiometers were present to measure the columnar water vapor (CWV). All four solar radiometers retrieve CWV by measuring total solar transmittance in the 0.94-gm water vapor absorption band and subtracting contributions due to Rayleigh, ozone and aerosol transmittances. The aerosol optical depth comparisons among the same four radiometers has been presented elsewhere (Geophys. Res. Lett., 26, 17, 2725-2728, 1999). We have used three different methods to retrieve CWV. In a first round of comparison no attempt was made to standardize on the same radiative transfer model and its underlying water vapor spectroscopy. In the second round of comparison we used the same line-by-line code (which includes recently corrected H2O spectroscopy) to retrieve CAN from all four suntracking radiometers. This decreased the mean CWV by 8% or 13%. The spread of 8% in the solar radiometer results found when using the same model is an indication of the other-than-model uncertainties involved in determining CWV from solar transmittance measurements with current instrumentation.

Schmid, B.↗

NASA NTRS2010

ELDRS Characterization for a Very High Dose Mission

Evaluation of bipolar linear parts which may have Enhanced Low Dose Rate Sensitivity (ELDRS) is problematic for missions that have very high dose radiation requirements. The accepted standards for evaluating parts that display ELDRS require testing at a very low dose rate which could be prohibitively long for very high dose missions. In this work, a methodology for ELDRS characterization of bipolar parts for mission doses up to 1 Mrad(Si) is evaluated. The procedure employs an initial dose rate of 0.01 rad(Si)/s to a total dose of 50 krad(Si) and then changes to 0.04 rad(Si)/s to a total dose of 1 Mrad(Si). This procedure appears to work well. No change in rate of degradation with dose has been observed when the dose rate is changed from 0.01 to 0.04 rad(Si)/s. This is taken as an indication that the degradation due to the higher dose rate is equivalent to that at the lower dose rate at the higher dose levels, at least for the parts studied to date. In several cases, significant parameter degradation or functional failure not observed at HDR was observed at fairly high total doses (50 to 250 krad(Si)) at LDR. This behavior calls into question the use of dose rate trend data and enhancement factors to predict LDR performance.

Radiation Damage↗

NASA NTRS2011

Radiation Status of Sub-65 nm Electronics

Ultra-scaled complementary metal oxide semiconductor (CMOS) includes commercial foundry capabilities at and below the 65 nm technology node Radiation evaluations take place using standard products and test characterization vehicles (memories, logic/latch chains, etc.) NEPP focus is two-fold: (1) Conduct early radiation evaluations to ascertain viability for future NASA missions (i.e. leverage commercial technology development). (2) Uncover gaps in current testing methodologies and mechanism comprehension -- early risk mitigation.

Pellish, Jonathan A.↗

NASA NTRS1973

Use of 4-D atmospheric models in the simulation of radiometric measurements

Atmospheric moisture data contained in the Global 4-D Atmospheric Models developed in previous studies were analyzed to establish regional differences. The regional values of precipitable water along latitudinal belts were compared with values derived from the corresponding atmospheric models defined in the U.S Standard Atmosphere Supplement. The effects of the differences between the 4-D Models and the Standard Atmosphere Models on radiometric computations in the infrared window and water vapor absorption band regions were evaluated using a standard computation model of radiation transfer through a cloudless atmosphere. The significance of these differences in simulation is discussed.

Chang, D. T.↗

NASA NTRS1977

Application of statistical inversion to ground-based microwave remote sensing of temperature and water vapor profiles

Surface-based observations of downwelling microwave thermal emission are related to temperature and humidity profiles via a standard integral equation of radiative transfer. Both in clear and in cloudy atmospheres, statistical inversion techniques are used to retrieve profiles from a data vector of brightness observations and surface meteorological constraints. For the clear case, accuracy predictions and profile retrievals are illustrated for: (1) single frequency angular scanned data; (2) multi-frequency angular scanned data; and (3) multi-frequency zenith data. For the last case predicted and achieved accuracies were compared in a recently conducted radiometric experiment. Retrievals of cloud contaminated radiometric data are elaborated.

Westwater, E. R.↗

NASA NTRS1984

Development of a radiation-hard photomultiplier tube

In a radiation-hard photomultiplier tube (PMT) such as has been developed for stabilization of the Galileo spacecraft as it goes through the Jovian high energy radiation belts, the primary effects of high energy electron and proton radiation that must be resisted are the production of fluorescence and Cerenkov emission. The present PMT envelope is ceramic rather than glass, and employs a special, electron-focusing design which will collect, accelerate and amplify electrons only from desired photocathode areas. Tests in a Co-60 radiation facility have shown that the radiation-hard PMT produces less than 2.5 percent of the radiation noise of a standard PMT.

Birnbaum, M. M.↗

NASA NTRS1986

The effects of space radiation on a chemically modified graphite-epoxy composite material

The effects of the space environment on the engineering properties and chemistry of a chemically modified T300/934 graphite-epoxy composite system are characterized. The material was subjected to 1.0 x 10 to the 10th power rads of 1.0 MeV electron irradiation under vacuum to simulate 30 years in geosynchronous earth orbit. Monotonic tension tests were performed at room temperature (75 F/24 C) and elevated temperature (250 F/121 C) on 4-ply unidirectional laminates. From these tests, inplane engineering and strength properties (E sub 1, E sub 2, Nu sub 12, G sub 12, X sub T, Y sub T) were determined. Cyclic tests were also performed to characterize energy dissipation changes due to irradiation and elevated temperature. Large diameter graphite fibers were tested to determine the effects of radiation on their stiffness and strength. No significant changes were observed. Dynamic-mechanical analysis demonstrated that the glass transition temperature was reduced by 50 F(28 C) after irradiation. Thermomechanical analysis showed the occurrence of volatile products generated upon heating of the irradiated material. The chemical modification of the epoxy did not aid in producing a material which was more radiation resistant than the standard T300/934 graphite-epoxy system. Irradiation was found to cause crosslinking and chain scission in the polymer. The latter produced low molecular weight products which plasticize the material at elevated temperatures and cause apparent material stiffening at low stresses at room temperature.

Reed, S. M.↗

NASA NTRS1986

Radiation flux tables for ICRCCM using the GLA GCM radiation codes

Tabulated values of longwave and shortwave radiation fluxes and also cooling and heating rates in the atmosphere for standard atmospheric profiles are presented. The radiation codes used in the Goddard general circulation model were employed for the computations. These results were obtained for an international intercomparison projected called Intercomparison of Radiation Codes in Climate Models (ICRCCM).

HARSHVARDHAN↗

NASA NTRS1993

Oscillator strengths of the Si II 181 nanometer resonance multiplet

We report Si II experimental log (gf)-values of -2.38(4) for the 180.801 nm line, of -2.18(4) for the 181.693 nm line, and of -3.29(5) for the 181.745 nm line, where the number in parentheses is the uncertainty in the last digit. The overall uncertainties (about 10 percent) include the 1 sigma random uncertainty (about 6 percent) and an estimate of the systematic uncertainty. The oscillator strengths are determined by combining branching fractions and radiative lifetimes. The branching fractions are measured using standard spectroradiometry on an optically thin source; the radiative lifetimes are measured using time-resolved laser-induced fluorescence.

Bergeson, S. D.↗