Broadband, unpolarized repumping and clearout light sources for Sr+^+ single-ion clocks

Future transportable optical clocks require compact and reliable light sources. Here, broadband, unpolarized repumper and state clearout sources for Sr+ single-ion optical clocks are reported. These turn-key devices require no frequency stabilization nor external modulators. They are fiber based, inexpensive, and compact. Key characteristics for clock operation are presented, including optical spectra, induced light shifts and required extinction ratios. Tests with an operating single-ion standard show a clearout efficiency of 100%. Compared to a laser-based repumper, the achievable fluorescence rates for ion detection are a few tens of per cent lower. The resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the ion system. Similar repumper light sources could be made for Ca+ (866 nm) and Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure

Influence of wind on crop canopy reflectance measurements

Remote sensing techniques of measuring red and far-red crop canopy reflectance are frequently used to estimate crop canopy characteristics. The variability introduced in reflectance data from nonvegetative factors such as wind decreases the usefulness of the techniques. The objective of this study was to quantify and minimize the variability from wind on spectral reflectances. Red and far-red reflectances were acquired above wheat, barley, and alfalfa canopies throughout days of changing wind conditions. Periods of 312 s with little changes in irradiance values were used for the analysis. Wind had negligible effect on reflectances of a short canopy such as cut alfalfa, while it had a significant effect on reflectances from canopies with a higher vertical structure, particularly during gusty conditions. Within the windy and calm periods, extreme values of spectral reflectance differed by 60% and 12%, respectively, in the red, and by 40% and 8% in the far-red for the barley canopy. For the compact and dense canopy structure of alfalfa, these differences reached a maximum of 10% under windy conditions in both spectral regions. The plant canopy architecture, the wind conditions, and the spectral regions all affected the magnitude of the influence of wind on crop canopy spectral reflectances. The mean reflectance of a canopy overestimated the true reflectance by 2–4% while the use of the median reduced this overestimation. Sampling requirements for this sensor are evaluated, and the possibility of decreasing either the sampling rate or the sampling period is discussed

Sun-angle effects on the red and near infrared reflectances of five different crop canopies

The objective of this paper is to study relationships between dai/y variations in sun angles and red and near infrared reflectances measured throughout a growing season over different types of crop canopies. Five architecturally different crop canopies were planted: wheat (NNW-SSE rows 0.18 m and 0.35 m apart), barley (NNW-SSE rows 0.18 m apart), corn (NNWSSE rows 0.46 m apart), and sunflower (N-S rows 0.92 m apart). Each canopy was planted at three different densifies in order to add intraspecies variability. Spectral reflectances were measured with a sensor in the nadir position located on a ground platform. Changes in sun angles usually affected the red more than near infrared reflectances. Within each crop, the canopies with the lowest LAI a/ways showed the larges! variations in spectral reflectances. But among the crops, the importance of changing sun angles on crop canopy reflectances varied with the architecture of the canopy. Red reflectances collected over corn and sunflower at LAI of 6 were insensitive to changes in sun angles, while those collected above sunflower showed significant differences when LAI was around 4. The higher proportion of horizontal foliar area compared to the vertical one in corn canopies could account for this difference between the two crops, since a canopy with horizontal foliar distribution is a nearer approximation of a lambertian surface. The differences in row width and orientation could also explain some of these differences. Reflectances of wheat canopies planted with rows 0.35 m apart showed larger red reflectance variations within a day than those planted 0.17 m apart. The maximum red reflectances always appeared when the sun was in fine with row direction. The fact that most rapid variations in red reflectances were occurring around this particular time of the day also confirmed that there is a strong interaction between row direction and width with the position of the sun in the sky. The magnitude of the effect of the variations of sun angles on crop canopy spectral reflectances was shown to be dependent on the species, planting patterns, wavelength band, and sky conditions. Le présent article décrit une étude des relations entre les variations quotidiennes de l'angle du soleil ainsi que des réflectances dans le rouge et le proche infrarouge mesurées tout au long d'une saison de récolte sur différents types de couverts végétaux. Cinq couverts végétaux de structure différente ont été semés: blé (rangs NNO-SSE à intervalles de 0,18 et 0,35 mètres), orge (rangs NNO-SSE à intervalles de 0,18 mètre), maïs (rangs NNO-SSE à intervalles de 0,46 mètre) et tournesol (rangs N-S à intervalles de 0,92 mètre). Chaque couvert végétal a été semé à trois densités différentes de manière à pouvoir mesurer la variabilité au sein de chaque espèce. Les réflectances spectrales ont été mesurées à la position nadir à l'aide d'un capteur fixé à une plate-forme au sol. En général, les variations de l'angle du soleil touchaient davantage les réflectances dans le rouge que les réflectances dans le proche infrarouge. Pour chaque culture, les couverts ayant l'indice de surface foliaire (ISF) le moins élevé ont systématiquement donné la plus grande variation au niveau des réflectances spectrales. Par contre, entre les diverses cultures, la variation de l'angle du soleil sur les réflectances des couverts végétaux était fonction de la structure du couvert. Les réflectances rouges recueillies au-dessus du maïs et du tournesol avec un ISF de 6 étaient insensibles aux variations de l'angle du soleil, tandis que celles mesurées au-dessus des tournesols variaient substantiellement lorsque l'ISF était aux environs de 4. Cette différence entre les deux cultures peut s'expliquer par la présence d'une plus grande distribution foliaire horizontale comparativement à la surface verticale offerte par le couvert de maïs, puisqu'un couvert ayant une distribution foliaire horizontale se rapproche davantage d'une surface lambertienne. La différence dans la largeur et l'orientation des rangs pourrait aussi expliquer certaines de ces différences. Les couverts de blé semés en rangs à intervalles de 0,35 mètre ont donné des variations de réflectances dans le rouge plus grandes dans une journée que ceux semés à intervalles de 0,17 mètre. Les réflectances maximales dans le rouge se produisaient toujours lorsque le soleil était en ligne avec les rangs. Le fait que la plupart des variations rapides des réflectances dans le rouge se produisaient à ce moment particulier de la journée a aussi confirmé qu'il existe une forte corrélation entre la direction et la largeur des rangs et la position du soleil dans le ciel. Cette étude a démontré que l'ampleur de l'incidence des variations de l'angle du soleil sur les réflectances spectrales des couverts végétaux est fonction de l'espèce, du mode de semence, de la longueur d'ondes de la bande et des conditions du ciel

Lithium-ion battery aging experiments at subzero temperatures and model development for capacity fade estimation

Lithium-ion (Li-ion) batteries widely used in electric vehicles (EVs) and hybrid EVs (HEVs) are insufficient for vehicle use after they have degraded to 70% to 80% of their original capacity. Battery lifespan is a large consideration when designing battery packs for EVs/HEVs. Aging mechanisms, such as metal dissolution, growth of the passivated surface film layer on the electrodes, and loss of both recyclable lithium ions, affect the longevity of the Li-ion battery at higherature operations. Even vehicle maneuvers at low temperatures (T<0°C)contribute to battery lifetime degradation, owing to the anode electrode vulnerability to other degradation mechanisms such as lithium plating. Nowadays, only a few battery thermal management schemes have properly considered lowerature degradation. This is due to the lack of studies on aging of Li-ion batteries at sub-zero temperature. This paper investigates how load cycle and calendar life properties affect the lifetime and aging processes of Li-ion cells at low temperatures. Accelerated aging tests were used to determine the effect of the ambient temperature on the performance of three 100-Ah LiFeMnP04 Li-ion cells. Two of them were aged through a normalized driving cycle at two temperature tests (-20°C and 25°C). The calendar test was carried out on one single battery at -20 °C and mid-range of state of charge (50%). Their capacities were continuously measured every two or three days. An aging model is developed and added to a preliminary single-cell electrothermal model to establish, in future works, a thermal strategy capable of predicting how the cell ages. This aging model was then validated by comparing its predictions with the aging data obtained from a cycling test at 0 °C. © 1967-2012 IEEE

Variations of crop canopy spectral reflectance measurements under changing sky conditions

The possibility of using reflectance data collected under both cloudy and sunny conditions is studied. Extrenle values of red and far-red reflectances measured directly over bare soil and crop canopies during intermittently cloudy conditions can differ by more than 100 percent and 60 percent, respectively. Eliminating the irradiance variations by using only data collected under sunny conditions reduced such variation to within 1 percent for bare soil and to within 8 percent and 20 percent for crop canopy red and far-red reflectances, respectively. In general, the reflectances measured under cloudy conditions with relatively constant irradiance values are constant and approximately 10 percent larger than the ones measured at similar sun angles during sunny conditions. This result is independent of the species and the spectral regions under study

Field spectrometer to measure percent ground coverage and leaf area index of agriculture crops

Measurement of percent ground cover (PGC) and leaf area index (LAI) are required for crop modelling, yield estimation and for ground truth data in remote sensing studies. An instrument which traverses on a track above a crop and continuously measures the ratio of incident and reflected radiation at various wavelengths was developed and tested. Spectral irradiance and reflectance measurements were made at four wavelengths (647.8, 675.5, 739.9, 790.4 nm) at eight stages of growth of wheat and barley for three crop densities. Data provided information on the relations between spectral properties at selected wavelength and leaf area expansion at different growth stages. The spectral data were highly correlated with leaf area index measurements

Perceptions of HIV Virologic Control Strategies among Younger and Older Age Groups of People Living with HIV in the United States: A Cross-Sectional Survey

Two HIV virologic control advances are in various stages of development, including long-acting antiretroviral therapy (ART) formulations and strategies aimed at sustained ART-free HIV control. Perceptions of risks and benefits toward HIV virologic control strategies may be different based on an individual's age due to differing experiences of the impacts of the domestic HIV epidemic, altruistic attitudes toward research participation, and general levels of engagement in health care. We examined preferences of HIV virologic control strategies by age groups. In 2018, we conducted a nationwide, online cross-sectional survey to examine differences in HIV virologic control strategies among a sample of people living with HIV who were = 50 years of age. From a total of 281 participants, 3 findings were noteworthy: (1) Participants = 50 years; (2) participants >= 50 years of age were more motivated by altruistic notions compared with those = 50 years. Our analysis provides a deeper understanding of differences in perceptions among various age groups regarding desirable future ART characteristics, and motivations and barriers to participating in HIV cure-related strategies. Our findings can help inform community engagement and education, and assist researchers in tailoring study design and recruitment efforts to major age groups

Experiments of Interfacial Roughening in Hele-Shaw Flows with Weak Quenched Disorder

We have studied the kinetic roughening of an oil--air interface in a forced imbibition experiment in a horizontal Hele--Shaw cell with quenched disorder. Different disorder configurations, characterized by their persistence length in the direction of growth, have been explored by varying the average interface velocity v and the gap spacing b. Through the analysis of the rms width as a function of time, we have measured a growth exponent beta ~= 0.5 that is almost independent of the experimental parameters. The analysis of the roughness exponent alpha through the power spectrum have shown different behaviors at short (alpha_1) and long (alpha_2) length scales, separated by a crossover wavenumber q_c. The values of the measured roughness exponents depend on experimental parameters, but at large velocities we obtain alpha_1 ~= 1.3 independently of the disorder configuration. The dependence of the crossover wavenumber with the experimental parameters has also been investigated, measuring q_c ~ v^{0.47} for the shortest persistence length, in agreement with theoretical predictions.Comment: 20 pages, 22 figure

Field spectrometer to measure percent ground coverage and leaf area index of agriculture crops [remote sensing]

Measurement of percent ground cover (PGC) and leaf area index (LAI) are required for crop modelling, yield estimati-on and for ground truth data in remote sensing studies. An instrument which traverses on a track above a crop and continuously measures the ratio of incident and reflected radiation at various wavelengths was developed and tested. Spectral irradiance and reflectance measurements were made at four wavelengths (647.8 , 675.5 , 739.9 , 790.4 nm) at eight stages of growth of wheat and barley for three crop densities. Data provided information on the relations between spectral properties at selected wavelength and leaf area expansion at different growth stages. The spectral data were highly correlated with leaf area index measurements

Variabilité introduite dans des mesures de réflectances spectrales de couverts végétaux agricoles par les facteurs état du ciel et vent

Parmi les facteurs introduisant de la variabilité indésirable dans les mesures de réflectance spectrale prises au-dessus de couverts végétaux, ceux de type radiatif ont été moins étudiés. A l'aide d'expériences isolant certains de ces facteurs et de méthodes empiriques d'analyses, nos résultats montrent que la présence de radiation diffuse non-directionnelle dans la radiation solaire incidente n'introduit pas beaucoup de variabilité indésirable dans les mesures de réflectance spectrale. Ils montrent aussi que la présence de nuages vis-à-vis le disque solaire entraîne généralement une sous-estimation des réflectances mesurées sous ces conditions par rapport aux conditions de ciel serein. Il semble toutefois possible de corriger cette erreur et ainsi d'utiliser indifféremment l'une ou l'autre de ces deux conditions radiatives pour relier les mesures de réflectances spectrales prises au niveau du sol à des paramètres du couvert végétal. Le vent est un autre facteur qui introduit passablement de variabilité indésirable dans ces mesures. Si la structure du couvert, la région spectrale et la vitesse du vent font varier l'importance de ce facteur, il n'en demeure pas moins que ces fluctuations continuelles dans le temps peuvent conduire à mesurer des réflectances qui ne sont pas représentatives de la réflectance réelle du couvert végétal au moment de la mesure. Many factors including the radiative ones introduce indesirable variability in measurements of spectral reflectance. Based upon experimental studies related to one specific factor at a time and a1so on empirica1 method of analysis, our results show that non-directional diffuse radiation from incident solar radiation introduce little indesirable variability in spectral reflectance measurements. They also show that clouds hiding the solar disc 1ead usually to an underestimation of reflectances measured under these conditions compared to those measured under clear sky. However, it seems easy to account for this difference. It then implies that the measurement of the spectral reflectances at the ground level under either one of these radiative conditions will give values which will be possible to relate to plant canopy structure