Engineering Papers⌕ Search

Engineering topics

Lee, H. S.

Publications and source records attributed to Lee, H. S..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Recovery of Electron/Proton Radiation-Induced Defects in n+p AlInGaP Solar Cell by Minority-Carrier Injection Annealing

A high efficient In0.48Ga0.52P/In0.01Ga0.99As/Ge triple junction solar cell has been developed for application in space and terrestrial concentrator PV system [1-3]. Recently, a high conversion efficiency of 31.5% (AM1.5G) has been obtained in InGaP/(In)GaAs/Ge triple junction solar cell, and as a new top cell material of triple junction cells, (Al)InGaP [1] has been proposed to improve the open-circuit voltage (Voc) because it shows a higher Voc of 1.5V while maintaining the same short-circuit current (ISC) as a conventional InGaP top cell under AM1.5G conditions as seen in figure 1 (a). Moreover, the spectral response of 1.96eV AlInGaP cell with a thickness of 2.5..m shows a higher response in the long wavelength region, compared with that of 1.87eV InGaP cell with 0.6..m thickness, as shown in figure 1 (b). Its development will realize next generation multijunction (MJ) solar cells such as a lattice mismatched AlInGaP/InGaAs/Ge 3-junction and lattice matched AlInGaP/GaAs/InGaAsN/Ge 4-junction solar cells. Figure 2 shows the super high-efficiency MJ solar cell structures and wide band spectral response by MJ solar cells under AM1.5G conditions. For realizing high efficient MJ space solar cells, the higher radiation-resistance under the electron or proton irradiation is required. The irradiation studies for a conventional top cell InGaP have been widely done [4-6], but little irradiation work has been performed on AlInGaP solar cells. Recently, we made the first reports of 1 MeV electron or 30 keV proton irradiation effects on AlInGaP solar cells, and evaluated the defects generated by the irradiation [7,8]. The present study describes the recovery of 1 MeV electron / 30 keV proton irradiation-induced defects in n+p- AlInGaP solar cells by minority-carrier injection enhanced annealing or isochronal annealing. The origins of irradiation-induced defects observed by deep level transient spectroscopy (DLTS) measurements are discussed.

Lee, H. S.↗
NASA NTRS1996

Laboratory Experiments on the Reactions of PAH Cations with Molecules and Atoms of Interstellar Interest

The C10H8(+) cation and its dehydrogenated derivatives, C10H7(+) and C10H6(+), have been studied using a selected ion flow tube (SIFT). Reactions with molecules and atoms of interstellar interest show that C10H8(+) reacts with N md O to give neutral products HCN and CO, respectively. C10H6(+) and C10H6(+) are moderately reactive and reactions proceed through association with molecules. The implications of these results for the depletion of C10H(n)(+) in the interstellar medium are briefly discussed.

LePage, V.↗
NASA NTRS1994

Transient nucleate pool boiling in microgravity: Some initial results

Variable gravity provides an opportunity to test the understanding of phenomena which are considered to depend on buoyancy, such as nucleate pool boiling. The active fundamental research in nucleate boiling has sought to determine the mechanisms or physical processes responsible for its high effectiveness, manifested by the high heat flux levels possible with relatively low temperature differences. Earlier research on nucleate pool boiling at high gravity levels under steady conditions demonstrated quantitatively that the heat transfer is degraded as the buoyancy normal to the heater surfaced increases. Correspondingly, it was later shown, qualitatively for short periods of time only, that nucleate boiling heat transfer is enhanced as the buoyancy normal to the heater surface is reduced. It can be deduced that nucleate pool boiling can be sustained as a quasi-steady process provided that some means is available to remove the vapor generated from the immediate vicinity of the heater surface. One of the objectives of the research, the initial results of which are presented here, is to quantify the heat transfer associated with boiling in microgravity. Some quantitative results of nucleate pool boiling in high quality microgravity (a/g approximately 10(exp -5)) of 5s duration, obtained in an evacuated drop tower, are presented here. These experiments were conducted as precursors of longer term space experiments. A transient heating technique is used, in which the heater surface is a transparent gold film sputtered on a qua rtz substrate, simultaneously providing the mean surface temperature from resistance thermometry and viewing of the boiling process both from beneath and across the surface. The measurement of the transient mean heater surface temperature permits the computation, by numerical means, of the transient mean heat transfer coefficient. The preliminary data obtained demonstrates that a quasi-steady boiling process can occur in microgravity if the bulk liquid subcooling is sufficiently high and if the imposed heat flux is sufficiently low. This is attributed to suface tension effects at the liquid-vapor-solid junction causing rewetting to take place, sustaining the nucleate boiling. Otherwise, dryout at the heater surface will occur, as observed.

Merte, Herman, Jr.↗
NASA NTRS1991

The inflatable sphere - A technique for the accurate measurement of middle atmosphere temperatures

The present study illustrates the inflatable sphere's capability to produce accurate temperatures up to 85 km and higher, given that the necessary reduction initialization conditions are met. At heights below 60 km, comparison of sphere temperatures with in situ thermistor measurements obtained close in space and time shows good agreement. Comparison with OH-radical rotational temperatures also confirms excellent agreement at 86 km. It is concluded that the sphere technique is an independent and highly accurate source of temperature measurement, is unique in being the only low-cost source of in situ measurement of temperature throughout the mesosphere and lower thermosphere, and qualifies as an intrinsic method to establish the accuracy of other atmosphere measurement systems.

Schmidlin, F. J.↗
NASA NTRS1991

Nongray radiative gas analyses using the S-N discrete ordinates method

The S-N discrete ordinates method is applied to analyze radiative heat transfer in nongray gases. Spectral correlation between the terms in the equation of transfer is considered for black or nearly nonreflecting walls. Formulations to apply the S-N method using a narrow-band or the exponential wide-band model are presented. The net radiative wall heat fluxes and the radiative source distributions are obtained for uniform, parabolic, and boundary layer-type temperature profiles, as well as for a parabolic concentration profile. The narrow- and wide-band nongray solutions are compared with gray-band approximations using the same band models. The computational speed of the gray-band approximation is obtained at the expense of accuracy in the internal fluxes and radiative source distributions. The wall radiative flux predictions by the gray-band approximation are satisfactory.

Kim, T. K.↗
NASA NTRS1991

Comparisons of satellite retrieved layer temperature and correlative radiosonde data

A quantitative analysis of satellite temperature accuracy is performed using an extensive correlative data set for selected radiosonde stations along eastern North America from midlatitudes through the tropics. The satellite-retrieved layer temperature agrees with the radiosonde temperature within +/- 1.2 K globally, although variations are observed with layers as well as radiosonde station locations. The satellite measurement show lowest sensitivity and accuracy for the Layer 7 temperature in general.

Lee, H. S.↗
NASA NTRS1991

Theoretical and experimental analysis of injection seeding a Q-switched alexandrite laser

Injection seeding is a method for achieving linewidths of less than 500 MHz in the output of broadband, tunable, solid state lasers. Dye lasers, CW and pulsed diode lasers, and other solid state lasers have been used as injection seeders. By optimizing the fundamental laser parameters of pump energy, Q-switched pulse build-up time, injection seed power and mode matching, one can achieve significant improvements in the spectral purity of the Q-switched output. These parameters are incorporated into a simple model for analyzing spectral purity and pulse build-up processes in a Q-switched, injection-seeded laser. Experiments to optimize the relevant parameters of an alexandrite laser show good agreement.

Prasad, C. R.↗
NASA NTRS1991

Narrowband alexandrite laser injection seeded with frequency dithered diode laser

Narrowband radiation is produced from a pulsed alexandrite laser when injection seeded with the output of a low power, tunable, continuous wave single mode diode laser. Injection seeded power oscillators are easier to frequency stabilize than etalon narrowed lasers, are more efficient and less prone to optical damage. AlGaAs diode lasers are available with wavelengths from 760 to 770 nm in the oxygen A band that can be used for differential absorption lidar remote sensing of atmospheric pressure and temperature. Diodes with room temperature output at 740 nm may be cooled sufficiently to emit in the water vapor absorption band at 720-730 nm for humidity remote sensing. The diode laser linewidth of 200 MHz is sufficient to seed 2 or 3 longitudinal modes of the multi-transverse mode alexandrite laser, giving the pulsed laser a bandwidth of 0.007 to 0.014/cm.

Schwemmer, Geary↗
NASA NTRS1989

Evidence for accurate temperatures from the inflatable falling sphere

The experiments performed with the inflatable falling sphere technique, for middle atmosphere studies, are reported. It is shown to be a potentially high accurate and independent source of temperature measurement and an intrinsic method for establishing accuracy of other atmospheric measurement techniques. Theoretical derivation, simulations, and actual measurements show that the sphere's temperature data are accurate. It is demonstrated that retrieved temperatures from falling spheres are not significantly affected by linear bias in density caused by uncertainties in sphere mass, volume, or cross sectional area. Case studies illustrate the sphere's capability to produce accurate temperatures. Comparisons with Datasonde temperature measurements obtained close in time and space are in agreement below 60 km.

Schmidlin, F. J.↗
NASA NTRS1989

Application of an optimal filter for inflatable sphere data processing

The improvement in the sphere data processing, concerning the signal to noise ratio, is discussed. Frequency analysis of the radar data is effectuated. It reveals a specific frequency component in the radar angle error, which may originate from the tracking radar mechanism itself. An optimal (Wiener) filter is applied to the radar data in order to suppress the systematic angular error components selectively. Using this technique, a significant improvement in the signal to noise ratio is achieved. The resolution of sphere measurements, previously limited by the length of the polynomial filter in the sphere data processing algorithm, is improved.

Lee, H. S.↗
NASA NTRS1987

ROCOZ-A (improved rocket launched ozone sensor) for middle atmosphere ozone measurements

An improved interference filter based ultraviolet photometer (ROCOZ-A) for measuring stratospheric ozone is discussed. The payload is launched aboard a Super-Loki to a typical apogee of 70 km. The instrument measures the solar ultraviolet irradiance as it descends on a parachute. The total cumulative ozone is then calculated based on the Beer-Lambert law. The cumulative ozone precision measured in this way is 2.0% to 2.5% over an altitude range of 20 and 55 km. Results of the intercomparison with the SBUV overpass data and ROCOZ-A data are also discussed.

Lee, H. S.↗
NASA NTRS1986

An improved rocket ozonesonde (ROCOZ-A). II - Preparation of stratospheric ozone profiles

The ROCOZ-A radiometer measures ozone by long pathlength photometry in the stratosphere and lower mesosphere. After a rocket launch to an apogee of 70 km, the instrument measures the solar ultraviolet irradiance over its four filter wavelengths as it descends on a parachute. The fundamental values from ROCOZ-A are ozone overburdens versus radar altitude from 53 to 20 km. The slope of these values gives ozone number density. At one standard deviation the repeatability of the ozone overburden measurements averages 2.4 percent. For ozone number density the repeatability averages 3.2 percent with a significant increase at altitudes below the ozone number density maximum. The accuracy limits for overburden and number density are estimated at 5-7 percent. With auxiliary measurements of pressure and temperature, ozone results are also produced in terms of ozone mixing ratio, albeit with a slight broadening of the estimated accuracy limits. The vertical response of ROCOZ-A ozone measurements (full width at half maximum) is 4 km. The assembly of ROCOZ-A profiles can be used to compare with measurements from each of the current NASA and NOAA satellite ozone instruments. In addition, the repeatability of ROCOZ-A allows the use of this instrument as a transfer standard between satellite instruments with different fundamental ozone measurements.

Barnes, Robert A.↗
NASA NTRS1986

Mesoscale ocean eddy measurements by multibeam altimetry

A multibeam microwave radar altimeter concept is numerically simulated to evaluate its capability to remotely sense mesoscale oceanographic features and eddies in particular. The study tests the sensitivity of the sensor to variations of systematic and environmental parameters, including sensor attitude angle, sensor position, and system errors. A novel concept of computing eddy vorticity from the multibeam data is explored. Application of this concept to the detection of simulated ocean eddies in the presence of tracker noise data gives excellent results; the technique is shown to be simple and accurate. The minimum size of the eddy detectable by the multibeam altimeter is presented for a given performance characteristic of the radar.

Lee, H. S.↗
NASA NTRS1985

Improved rocket ozonesonde (ROCOZ-A). I - Demonstration of precision

Measurements of the daytime ozone distribution in the stratosphere have been made with an improved rocket ozonesonde (ROCOZ-A). Vertical cumulative ozone as a function of geometric altitude is the basic information content of these measurements. The instrument-to-instrument repeatability of the ozonesonde was determined by two series of four soundings each. At one standard deviation the instrument repeatability averages from 2.0 to 2.5 percent over the entire altitude range of the instrument. The worst measured repeatability is 3.7 percent at 55 km for one of the flight series.

Holland, A. C.↗