Ion correlations due to high-frequency electric field and their effect on the nonlinear plasma conductivity

The influence of a strong, high‐frequency electric field on the ion‐ion correlations in a fully ionized plasma is investigated in the limit of infinite ion mass, starting with the Bogoliubov‐Born‐Green‐Kirkwood‐Yvon hierarchy of equations; a significant departure from the thermal correlations is found. It is shown that the above effect may substantially modify earlier results on the nonlinear high‐frequency plasma conductivity.

Effect of magnetic field on current colection on bare tether in LEO

An eiectrodynamic Tether is a long thin conductive string deployed from a spacecraft. A part of the ED tether near one end, which is rendered positive by the Electromotive force (EMF)along the tether, collects electrons from the ambient plasma. In the frame of reference moving with theter, ions flow toward the tether, get deflected near the tether by its high positive potential and create a wake. Due to the asymmetry of plasma distribution and the weak but significant Geomagnetic field, the conventional probe theory becomes almost inapplicable. Computational work for the prediction of current collection is thus necessiated.. In this paper, we analyze effects of magnetic field on velocity distribution funtion at a point that is far from the tether, and discuss a new way to treat electrons at computational boundary. Three cases with different magnetic field are simulated and compiled so as to provide a part of the pre-flight prediction of the space experiment by NASA ProSEDS, which is planned September 2002.

New methods to reduce leakage errors in planar near-field measurements

This paper describes two methods to cancel the effect of two kinds of leakage signals which may be presented when an antenna is measured in a planar near-field range. One method tries to reduce leakage bias errors from the receiver¿s quadrature detector and it is based on estimating the bias constant added to every near-field data sample. Then, that constant is subtracted from the data, removing its undesired effect on the far-field pattern. The estimation is performed by back-propagating the field from the scan plane to the antenna under test plane (AUT) and averaging all the data located outside the AUT aperture. The second method is able to cancel the effect of the leakage from faulty transmission lines, connectors or rotary joints. The basis of this method is also a reconstruction process to determine the field distribution on the AUT plane. Once this distribution is known, a spatial filtering is applied to cancel the contribution due to those faulty elements. After that, a near-field-to-far-field transformation is applied, obtaining a new radiation pattern where the leakage effects have disappeared. To verify the effectiveness of both methods, several examples are presented.

Residual effect of organic zinc complexes in acid and calcareous soils on navy bean (phaseolus vulgaris l.) and flax (linum usitatissimum l.) crops under greenhouse conditions. Influence of the moisture conditions = Efecto residual de complejos orgánicos de zinc en suelos ácido y calizo en cultivos de judía (phaseolus vulgaris l.) y lino (linum usitatissimum l.) Realizados en condiciones de invernadero. Influencia de las diferentes condiciones de humedad

El Zn es un elemento esencial para el crecimiento saludable y reproducción de plantas, animales y humanos. La deficiencia de Zn es una de las carencias de micronutrientes más extendidas en muchos cultivos, afectando a grandes extensiones de suelos en diferentes áreas agrícolas. La biofortificación agronómica de diferentes cultivos, incrementando la concentración de micronutriente Zn en la planta, es un medio para evitar la deficiencia de Zn en animales y humanos. Tradicionalmente se han utilizado fertilizantes de Zn inorgánicos, como el ZnSO4, aunque en los últimos años se están utilizado complejos de Zn como fuentes de este micronutriente, obteniéndose altas concentraciones de Zn soluble y disponible en el suelo. Sin embargo, el envejecimiento de la fuente en el suelo puede causar cambios importantes en su disponibilidad para las plantas. Cuando se añaden al suelo fuentes de Zn inorgánicas, las formas de Zn más solubles pierden actividad y extractabilidad con el paso del tiempo, transformándose a formas más estables y menos biodisponibles. En esta tesis se estudia el efecto residual de diferentes complejos de Zn de origen natural y sintético, aplicados en cultivos previos de judía y lino, bajo dos condiciones de riego distintas (por encima y por debajo de la capacidad de campo, respectivamente) y en dos suelos diferentes (ácido y calizo). Los fertilizantes fueron aplicados al cultivo previo en tres dosis diferentes (0, 5 y 10 mg Zn kg-1 suelo). El Zn fácilmente lixiviable se estimó con la extracción con BaCl2 0,1M. Bajo condiciones de humedad por encima de la capacidad de campo se obtuvieron mayores porcentajes de Zn lixiviado en el suelo calizo que en el suelo ácido. En el caso del cultivo de judía realizado en condiciones de humedad por encima de la capacidad de campo se compararon las cantidades extraídas con el Zn lixiviado real. El análisis de correlación entre el Zn fácilmente lixiviable y el estimado sólo fue válido para complejos con alta movilidad y para cada suelo por separado. Bajo condiciones de humedad por debajo de la capacidad de campo, la concentración de Zn biodisponible fácilmente lixiviable presentó correlaciones positivas y altamente significativas con la concentración de Zn disponible en el suelo. El Zn disponible se estimó con varios métodos de extracción empleados habitualmente: DTPA-TEA, DTPA-AB, Mehlich-3 y LMWOAs. Estas concentraciones fueron mayores en el suelo ácido que en el calizo. Los diferentes métodos utilizados para estimar el Zn disponible presentaron correlaciones positivas y altamente significativas entre sí. La distribución del Zn en las distintas fracciones del suelo fue estimada con diferentes extracciones secuenciales. Las extracciones secuenciales mostraron un descenso entre los dos cultivos (el anterior y el actual) en la fracción de Zn más lábil y un aumento en la concentración de Zn asociado a fracciones menos lábiles, como carbonatos, óxidos y materia orgánica. Se obtuvieron correlaciones positivas y altamente significativas entre las concentraciones de Zn asociado a las fracciones más lábiles (WSEX y WS+EXC, experimento de la judía y lino, respectivamente) y las concentraciones de Zn disponible, estimadas por los diferentes métodos. Con respecto a la planta se determinaron el rendimiento en materia seca y la concentración de Zn en planta. Se observó un aumento del rendimiento y concentraciones con el efecto residual de la dosis mayores (10 mg Zn kg-1) con respecto a la dosis inferior (5 mg Zn 12 kg-1) y de ésta con respecto a la dosis 0 (control). El incremento de la concentración de Zn en todos los tratamientos fertilizantes, respecto al control, fue mayor en el suelo ácido que en el calizo. Las concentraciones de Zn en planta indicaron que, en el suelo calizo, serían convenientes nuevas aplicaciones de Zn en posteriores cultivos para mantener unas adecuadas concentraciones en planta. Las mayores concentraciones de Zn en la planta de judía, cultivada bajo condiciones de humedad por encima de la capacidad de campo, se obtuvieron en el suelo ácido con el efecto residual del Zn-HEDTA a la dosis de 10 mg Zn kg-1 (280,87 mg Zn kg-1) y en el suelo calizo con el efecto residual del Zn-DTPA-HEDTA-EDTA a la dosis de 10 mg Zn kg-1 (49,89 mg Zn kg-1). En el cultivo de lino, cultivado bajo condiciones de humedad por debajo de la capacidad de campo, las mayores concentraciones de Zn en planta ese obtuvieron con el efecto residual del Zn-AML a la dosis de 10 mg Zn kg-1 (224,75 mg Zn kg-1) y en el suelo calizo con el efecto residual del Zn-EDTA a la dosis de 10 mg Zn kg-1 (99,83 mg Zn kg-1). El Zn tomado por la planta fue determinado como combinación del rendimiento y de la concentración en planta. Bajo condiciones de humedad por encima de capacidad de campo, con lixiviación, el Zn tomado por la judía disminuyó en el cultivo actual con respecto al cultivo anterior. Sin embargo, en el cultivo de lino, bajo condiciones de humedad por debajo de la capacidad de campo, se obtuvieron cantidades de Zn tomado superiores en el cultivo actual con respecto al anterior. Esta tendencia también se observó, en ambos casos, con el porcentaje de Zn usado por la planta. Summary Zinc is essential for healthy growth and reproduction of plants, animals and humans. Zinc deficiency is one of the most widespread micronutrient deficiency in different crops, and affect different agricultural areas. Agronomic biofortification of crops produced by an increased of Zn in plant, is one way to avoid Zn deficiency in animals and humans Sources with inorganic Zn, such as ZnSO4, have been used traditionally. Although, in recent years, Zn complexes are used as sources of this micronutrient, the provide high concentrations of soluble and available Zn in soil. However, the aging of the source in the soil could cause significant changes in their availability to plants. When an inorganic source of Zn is added to soil, Zn forms more soluble and extractability lose activity over time, transforming into forms more stable and less bioavailable. This study examines the residual effect of different natural and synthetic Zn complexes on navy bean and flax crops, under two different moisture conditions (above and below field capacity, respectively) and in two different soils (acid and calcareous). Fertilizers were applied to the previous crop in three different doses (0, 5 y 10 mg Zn kg-1 soil). The easily leachable Zn was estimated by extraction with 0.1 M BaCl2. Under conditions of moisture above field capacity, the percentage of leachable Zn in the calcareous soil was higher than in acid soil. In the case of navy bean experiment, performed in moisture conditions of above field capacity, amounts extracted of easily leachable Zn were compared with the real leachable Zn. Correlation analysis between the leachable Zn and the estimate was only valid for complex with high mobility and for each soil separately. Under moisture conditions below field capacity, the concentration of bioavailable easily leachable Zn showed highly significant positive correlations with the concentration of available soil Zn. The available Zn was estimated with several commonly used extraction methods: DTPA-TEA, AB-DTPA, Mehlich-3 and LMWOAs. These concentrations were higher in acidic soil than in the calcareous. The different methods used to estimate the available Zn showed highly significant positive correlations with each other. The distribution of Zn in the different fractions of soil was estimated with different sequential extractions. The sequential extractions showed a decrease between the two crops (the previous and current) at the most labile Zn fraction and an increase in the concentration of Zn associated with the less labile fractions, such as carbonates, oxides and organic matter. A positive and highly significant correlation was obtained between the concentrations of Zn associated with more labile fractions (WSEX and WS + EXC, navy bean and flax experiments, respectively) and available Zn concentrations determined by the different methods. Dry matter yield and Zn concentration in plants were determined in plant. Yield and Zn concentration in plant were higher with the residual concentrations of the higher dose applied (10 mg Zn kg-1) than with the lower dose (5 mg Zn kg-1), also these parameters showed higher values with application of this dose than with not Zn application. The increase of Zn concentration in plant with Zn treatments, respect to the control, was greater in the acid soil than in the calcareous. The Zn concentrations in plant indicated that in the calcareous soil, new applications of Zn are desirable in subsequent crops to maintain suitable concentrations in plant. 15 The highest concentrations of Zn in navy bean plant, performed under moisture conditions above the field capacity, were obtained with the residual effect of Zn-HEDTA at the dose of 10 mg Zn kg-1 (280.87 mg Zn kg-1) in the acid soil, and with the residual effect of Zn- DTPA-HEDTA-EDTA at a dose of 10 mg Zn kg-1 (49.89 mg Zn kg-1) in the calcareous soil. In the flax crop, performed under moisture conditions below field capacity, the highest Zn concentrations in plant were obtained with the residual effect of Zn-AML at the dose of 10 mg Zn kg-1 (224.75 Zn mg kg-1) and with the residual effect of Zn-EDTA at a dose of 10 mg Zn kg-1 (99.83 mg Zn kg-1) in the calcareous soil. The Zn uptake was determined as a combination of yield and Zn concentration in plant. Under moisture conditions above field capacity, with leaching, Zn uptake by navy bean decreased in the current crop, respect to the previous crop. However, in the flax crop, under moisture conditions below field capacity, Zn uptake was higher in the current crop than in the previous. This trend is also observed in both cases, with the percentage of Zn used by the plant

An experimental study on the effect of rolling shutters on the field measurements of airborne sound insulation of façades

The façade is the visible part of a building, and generally consists of various different constructive systems. The sound reduction index of the closing elements for the openings on a room’s façade is a determining factor in the sound insulation from airborne noise inside the space. Windows are the transparent part of the façade, and to improve their thermal behaviour and control solar radiation, they are often fitted with a series of external and internal protections such as shutters, slats and blinds. This work contains a summary of studies carried out using field measurements of airborne sound insulation on façades in rooms, in application of the standard UNE-EN ISO 140-5:1999. In all the rooms the windows were fitted with shutter boxes and rolling shutters, and the acoustic tests were made with the shutter in two positions (extended and fully retracted). The results were analysed considering the window opening system (openable or sliding) and the type of glass pane (monolithic or insulating glass unit, IGU). In the case of sliding windows, the airborne sound insulation of façades is greater when the shutter is extended than when it is retracted, and this should be taken into account when applying the aforementioned standard.

Magnetic self-field effects on current collection by an ionospheric bare tether

It was recently suggested that the magnetic field created by the current of a bare tether strongly reduces its own electron-collection capability when a magnetic separatrix disconnecting ambient magnetized plasma from tether extends beyond its electric sheath. It is here shown that current reduction by the self-field depends on the ratio meterizing bias and current profiles along the tether (Lt tether length, characteristic length gauging ohmic effects) and on a new dimensionless number Ks involving ambient and tether parameters. Current reduction is weaker the lower Ks and L*/ Lt, which depend critically on the type of cross section: Ks varies as R5/3, h2/3R, and h2/3 1/4 width for wires, round tethers conductive only in a thin layer, and thin tapes, respectively; L* varies as R2/3 for wires and as h2/3 for tapes and round tethers conductive in a layer (R radius, h thickness). Self-field effects are fully negligible for the last two types of cross sections whatever the mode of operation. In practical efficient tether systems having L*/Lt low, maximum current reduction in case of wires is again negligible for power generation; for deorbiting, reduction is <1% for a 10 km tether and 15% for a 20 km tether. In the reboost mode there are no effects for Ks below some threshold; moderate effects may occur in practical but heavy reboost-wire systems that need no dedicated solar power.

Magnetic self-field effects on bare-tether current collection

It has been recently suggested that the magnetic field created by the current in a bare tether could sensibly reduce its electron collection capability in the magnetised ionosphere, a region of closed magnetic surfaces disconnecting the cylinder from infinity. In this paper, the ohmic voltage drop along the tether is taken into account in considering self-field effects. Separate analyses are carried out for the thrust and power generation and drag modes of operation, which are affected in different ways. In the power generation and drag modes, bias decreases as current increases along the tether, starting at the anodic, positively-biased end (upper end in the usual, eastward-flying spacecraft); in the thrust mode of operation, bias increases as current increases along the tether, starting at the lower end. When the ohmic voltage drop is considered, self-field effects are shown to be weak, in all cases, for tape tethers, and for circular cross-section tethers just conductive in a thin outer layer. Self-field effects might become important, in the drag case only, for tethers with fully conductive cross sections that are unrealistically heavy.

Effect of strain and magnetic field on the critical current and electric resistance of the joints between HTS coated conductors

Engineering of devices and systems such as magnets, fault current limiters or cables, based on High Temperature Superconducting wires requires a deep characterization of the possible degradation of their properties by handling at room temperature as well as during the service life thus establishing the limits for building up functional devices and systems. In the present work we report our study regarding the mechanical behavior of spliced joints between commercial HTS coated conductors based on YBCO at room temperature and service temperature, 77 K. Tensile tests under axial stress and the evolution of the critical current and the electric resistance of the joints have been measured. The complete strain contour for the tape and the joint has been obtained by using Digital Image Correlation. Also, tensile tests under external magnetic field have been performed and the effect of the applied field on the critical current and the electric resistance of the joints has been studied. Finally, a preliminary numerical study by means of Finite Element Method (FEM) of the mechanical behavior of the joints between commercial HTS is presented.

Effect of strain and magnetic field on the critical current and electric resistance of the joints between HTS coated conductors

Engineering of devices and systems such as magnets, fault current limiters or cables, based on High Temperature Superconducting wires requires a deep characterization of the possible degradation of their properties by handling at room temperature as well as during the service life thus establishing the limits for building up functional devices and systems. In the present work we report our study regarding the mechanical behavior of spliced joints between commercial HTS coated conductors based on YBCO at room temperature and service temperature, 77 K. Tensile tests under axial stress and the evolution of the critical current and the electric resistance of the joints have been measured. The complete strain contour for the tape and the joints has been obtained by using Digital Image Correlation. Also, tensile tests under external magnetic field have been performed and the effect of the applied field on the critical current and the electric resistance of the joints has been studied. Additionally, fatigue tests under constant cyclic stress and loading-unloading ramps have been carried out in order to evaluate the electromechanical behavior of the joints and the effect of maximum applied stress on the critical current. Finally, a preliminary numerical study by means of the Finite Element Method (FEM) of the electromechanical behavior of the joints between commercial HTS is presented.

Drifting plasma collection by a positive biased tether wire in LEO-like plasma conditions: current measurement and plasma diagnostic

BETs is a three-year project financed by the Space Program of the European Commission, aimed at developing an efficient deorbit system that could be carried on board any future satellite launched into Low Earth Orbit (LEO). The operational system involves a conductive tape-tether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. As a part of this project, we are carrying out both numerical and experimental approaches to estimate the collected current by the positive part of the tether. This paper deals with experimental measurements performed in the IONospheric Atmosphere Simulator (JONAS) plasma chamber of the Onera-Space Environment Department. The JONAS facility is a 9- m3 vacuum chamber equipped with a plasma source providing drifting plasma simulating LEO conditions in terms of density and temperature. A thin metallic cylinder, simulating the tether, is set inside the chamber and polarized up to 1000 V. The Earth's magnetic field is neutralized inside the chamber. In a first time, tether collected current versus tether polarization is measured for different plasma source energies and densities. In complement, several types of Langmuir probes are used at the same location to allow the extraction of both ion densities and electron parameters by computer modeling (classical Langmuir probe characteristics are not accurate enough in the present situation). These two measurements permit estimation of the discrepancies between the theoretical collection laws, orbital motion limited law in particular, and the experimental data in LEO-like conditions without magnetic fields. In a second time, the spatial variations and the time evolutions of the plasma properties around the tether are investigated. Spherical and emissive Langmuir probes are also used for a more extensive characterization of the plasma in space and time dependent analysis. Results show the ion depletion because of the wake effect and the accumulation of- ions upstream of the tether. In some regimes (at large positive potential), oscillations are observed on the tether collected current and on Langmuir probe collected current in specific sites.

Effect of field damping layer on two step absorption of quantum dots solar cells

Multi-stacked InAs/AlGaAs quantum dot solar cells (QDSCs) introduced with field damping layers (FDL) which sustain the junction built-in potential have been studied. Without an external bias condition, the external quantum efficiency (EQE) of QD layers are reduced by introducing the thick FDL, because the carrier escape due to built-in electric field was suppressed. On the other hand, the photocurrent production due to two-step absorption is increased by the formation of flat-band QD structure for QDSC with thick FDL.

Low Work-Function Thermionic Emission and Orbital-Motion-Limited Ion Collection at Bare-Tether Cathodic Contact

Una amarra electrodinámica (electrodynamic tether) opera sobre principios electromagnéticos intercambiando momento con la magnetosfera planetaria e interactuando con su ionosfera. Es un subsistema pasivo fiable para desorbitar etapas de cohetes agotadas y satélites al final de su misión, mitigando el crecimiento de la basura espacial. Una amarra sin aislamiento captura electrones del plasma ambiente a lo largo de su segmento polarizado positivamente, el cual puede alcanzar varios kilómetros de longitud, mientras que emite electrones de vuelta al plasma mediante un contactor de plasma activo de baja impedancia en su extremo catódico, tal como un cátodo hueco (hollow cathode). En ausencia de un contactor catódico activo, la corriente que circula por una amarra desnuda en órbita es nula en ambos extremos de la amarra y se dice que ésta está flotando eléctricamente. Para emisión termoiónica despreciable y captura de corriente en condiciones limitadas por movimiento orbital (orbital-motion-limited, OML), el cociente entre las longitudes de los segmentos anódico y catódico es muy pequeño debido a la disparidad de masas entre iones y electrones. Tal modo de operación resulta en una corriente media y fuerza de Lorentz bajas en la amarra, la cual es poco eficiente como dispositivo para desorbitar. El electride C12A7 : e−, que podría presentar una función de trabajo (work function) tan baja como W = 0.6 eV y un comportamiento estable a temperaturas relativamente altas, ha sido propuesto como recubrimiento para amarras desnudas. La emisión termoiónica a lo largo de un segmento así recubierto y bajo el calentamiento de la operación espacial, puede ser más eficiente que la captura iónica. En el modo más simple de fuerza de frenado, podría eliminar la necesidad de un contactor catódico activo y su correspondientes requisitos de alimentación de gas y subsistema de potencia, lo que resultaría en un sistema real de amarra “sin combustible”. Con este recubrimiento de bajo W, cada segmento elemental del segmento catódico de una amarra desnuda de kilómetros de longitud emitiría corriente como si fuese parte de una sonda cilíndrica, caliente y uniformemente polarizada al potencial local de la amarra. La operación es similar a la de una sonda de Langmuir 2D tanto en los segmentos catódico como anódico. Sin embargo, en presencia de emisión, los electrones emitidos resultan en carga espacial (space charge) negativa, la cual reduce el campo eléctrico que los acelera hacia fuera, o incluso puede desacelerarlos y hacerlos volver a la sonda. Se forma una doble vainas (double sheath) estable con electrones emitidos desde la sonda e iones provenientes del plasma ambiente. La densidad de corriente termoiónica, variando a lo largo del segmento catódico, podría seguir dos leyes distintas bajo diferentes condiciones: (i) la ley de corriente limitada por la carga espacial (space-charge-limited, SCL) o (ii) la ley de Richardson-Dushman (RDS). Se presenta un estudio preliminar sobre la corriente SCL frente a una sonda emisora usando la teoría de vainas (sheath) formada por la captura iónica en condiciones OML, y la corriente electrónica SCL entre los electrodos cilíndricos según Langmuir. El modelo, que incluye efectos óhmicos y el efecto de transición de emisión SCL a emisión RDS, proporciona los perfiles de corriente y potencial a lo largo de la longitud completa de la amarra. El análisis muestra que en el modo más simple de fuerza de frenado, bajo condiciones orbitales y de amarras típicas, la emisión termoiónica proporciona un contacto catódico eficiente y resulta en una sección catódica pequeña. En el análisis anterior, tanto la transición de emisión SCL a RD como la propia ley de emisión SCL consiste en un modelo muy simplificado. Por ello, a continuación se ha estudiado con detalle la solución de vaina estacionaria de una sonda con emisión termoiónica polarizada negativamente respecto a un plasma isotrópico, no colisional y sin campo magnético. La existencia de posibles partículas atrapadas ha sido ignorada y el estudio incluye tanto un estudio semi-analítico mediante técnica asintóticas como soluciones numéricas completas del problema. Bajo las tres condiciones (i) alto potencial, (ii) R = Rmax para la validez de la captura iónica OML, y (iii) potencial monotónico, se desarrolla un análisis asintótico auto-consistente para la estructura de plasma compleja que contiene las tres especies de cargas (electrones e iones del plasma, electrones emitidos), y cuatro regiones espaciales distintas, utilizando teorías de movimiento orbital y modelos cinéticos de las especies. Aunque los electrones emitidos presentan carga espacial despreciable muy lejos de la sonda, su efecto no se puede despreciar en el análisis global de la estructura de la vaina y de dos capas finas entre la vaina y la región cuasi-neutra. El análisis proporciona las condiciones paramétricas para que la corriente sea SCL. También muestra que la emisión termoiónica aumenta el radio máximo de la sonda para operar dentro del régimen OML y que la emisión de electrones es mucho más eficiente que la captura iónica para el segmento catódico de la amarra. En el código numérico, los movimientos orbitales de las tres especies son modelados para potenciales tanto monotónico como no-monotónico, y sonda de radio R arbitrario (dentro o más allá del régimen de OML para la captura iónica). Aprovechando la existencia de dos invariante, el sistema de ecuaciones Poisson-Vlasov se escribe como una ecuación integro-diferencial, la cual se discretiza mediante un método de diferencias finitas. El sistema de ecuaciones algebraicas no lineal resultante se ha resuelto de con un método Newton-Raphson paralelizado. Los resultados, comparados satisfactoriamente con el análisis analítico, proporcionan la emisión de corriente y la estructura del plasma y del potencial electrostático. ABSTRACT An electrodynamic tether operates on electromagnetic principles and exchanges momentum through the planetary magnetosphere, by continuously interacting with the ionosphere. It is a reliable passive subsystem to deorbit spent rocket stages and satellites at its end of mission, mitigating the growth of orbital debris. A tether left bare of insulation collects electrons by its own uninsulated and positively biased segment with kilometer range, while electrons are emitted by a low-impedance active device at the cathodic end, such as a hollow cathode, to emit the full electron current. In the absence of an active cathodic device, the current flowing along an orbiting bare tether vanishes at both ends and the tether is said to be electrically floating. For negligible thermionic emission and orbital-motion-limited (OML) collection throughout the entire tether (electron/ion collection at anodic/cathodic segment, respectively), the anodic-to-cathodic length ratio is very small due to ions being much heavier, which results in low average current and Lorentz drag. The electride C12A7 : e−, which might present a possible work function as low as W = 0.6 eV and moderately high temperature stability, has been proposed as coating for floating bare tethers. Thermionic emission along a thus coated cathodic segment, under heating in space operation, can be more efficient than ion collection and, in the simplest drag mode, may eliminate the need for an active cathodic device and its corresponding gas-feed requirements and power subsystem, which would result in a truly “propellant-less” tether system. With this low-W coating, each elemental segment on the cathodic segment of a kilometers-long floating bare-tether would emit current as if it were part of a hot cylindrical probe uniformly polarized at the local tether bias, under 2D probe conditions that are also applied to the anodic-segment analysis. In the presence of emission, emitted electrons result in negative space charge, which decreases the electric field that accelerates them outwards, or even reverses it, decelerating electrons near the emitting probe. A double sheath would be established with electrons being emitted from the probe and ions coming from the ambient plasma. The thermionic current density, varying along the cathodic segment, might follow two distinct laws under different con ditions: i) space-charge-limited (SCL) emission or ii) full Richardson-Dushman (RDS) emission. A preliminary study on the SCL current in front of an emissive probe is presented using the orbital-motion-limited (OML) ion-collection sheath and Langmuir’s SCL electron current between cylindrical electrodes. A detailed calculation of current and bias profiles along the entire tether length is carried out with ohmic effects considered and the transition from SCL to full RDS emission is included. Analysis shows that in the simplest drag mode, under typical orbital and tether conditions, thermionic emission provides efficient cathodic contact and leads to a short cathodic section. In the previous analysis, both the transition between SCL and RDS emission and the current law for SCL condition have used a very simple model. To continue, considering an isotropic, unmagnetized, colissionless plasma and a stationary sheath, the probe-plasma contact is studied in detail for a negatively biased probe with thermionic emission. The possible trapped particles are ignored and this study includes both semianalytical solutions using asymptotic analysis and complete numerical solutions. Under conditions of i) high bias, ii) R = Rmax for ion OML collection validity, and iii) monotonic potential, a self-consistent asymptotic analysis is carried out for the complex plasma structure involving all three charge species (plasma electrons and ions, and emitted electrons) and four distinct spatial regions using orbital motion theories and kinetic modeling of the species. Although emitted electrons present negligible space charge far away from the probe, their effect cannot be neglected in the global analysis for the sheath structure and two thin layers in between the sheath and the quasineutral region. The parametric conditions for the current to be space-chargelimited are obtained. It is found that thermionic emission increases the range of probe radius for OML validity and is greatly more effective than ion collection for cathodic contact of tethers. In the numerical code, the orbital motions of all three species are modeled for both monotonic and non-monotonic potential, and for any probe radius R (within or beyond OML regime for ion collection). Taking advantage of two constants of motion (energy and angular momentum), the Poisson-Vlasov equation is described by an integro differential equation, which is discretized using finite difference method. The non-linear algebraic equations are solved using a parallel implementation of the Newton-Raphson method. The results, which show good agreement with the analytical results, provide the results for thermionic current, the sheath structure, and the electrostatic potential.

Low work-function thermionic emission and orbital-motion-limited ion collection at bare-tether cathodic contact

With a thin coating of low-work-function material, thermionic emission in the cathodic segment of bare tethers might be much greater than orbital-motion-limited (OML) ion collection current. The space charge of the emitted electrons decreases the electric field that accelerates them outwards, and could even reverse it for high enough emission, producing a potential hollow. In this work, at the conditions of high bias and relatively low emission that make the potential monotonic, an asymptotic analysis is carried out, extending the OML ion-collection analysis to investigate the probe response due to electrons emitted by the negatively biased cylindrical probe. At given emission, the space charge effect from emitted electrons increases with decreasing magnitude of negative probe bias. Although emitted electrons present negligible space charge far away from the probe, their effect cannot be neglected in the global analysis for the sheath structure and two thin layers in between sheath and the quasineutral region. The space-charge-limited condition is located. It is found that thermionic emission increases the range of probe radius for OML validity and is greatly more effective than ion collection for cathodic contact of tethers.