Simulation of Charge and Mass Distributions of Indium Droplets Created by Field Emission

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77396/1/AIAA-2006-3560-440.pd

Analysis of ion emitting jet structures during ionic liquid electrospraying

The ionic liquid [Bmim][DCA] is a propellant candidate for a standalone electrospray thruster or a dual-mode propulsion system. Characterization of positive polarity ions produced by [Bmim][DCA] capillary emitters with a nominal extraction voltage of 2.0 kV within a quadrupole and time-of-flight mass spectrometers is presented along with the predictions of propulsion performance. Flow rates from 0.05 to 2.18 nL/s are used to investigate the impact variations in the flow parameter have on the electrospray plume. The retarding potential analysis reveals ions emitted from the capillary are formed below the emitter potential of 500 eV. Angular distributions indicate broadening of both the beam current and mass distribution for increasing flow rates. Derived thrust and specific impulse change from 0.84 micro-Newtons and 200 s to 2.90 micro-Newtons and 80 s, respectively. Time-of-flight measurements delineate two distinct droplet distributions at approximately 2,000 to 40,000 amu/q and 50,000 to 300,000 amu/q. These multiply-charged droplet species, with wide mass-to-charge distributions, are due to the electric field conditions and associated charge range available within the parent jet. Additionally, the data show a transition to higher mass ions and droplets with increasing flow rate. The combination of two data sources allow for the assertion that ionic liquid electrospray from 0.05 to 2.18 nL/s does not conform to the traditional view of emission from the electrospray cone-jet. The mixed droplet and ion emission suggest that the primary jet disintegrates into secondary structures that are responsible for the emission species detected by the TOF instrument. --Abstract, page iv

Meniscus Modeling and Emission Studies of an Ionic Liquid Ferrofluid Electrospray Source Emitting from a Magneto-Electric Instability

This dissertation presents three studies on the electrospray of ionic liquid ferrofluid. Ionic liquid ferrofluids are electrically conductive super-paramagnetic fluids which respond strongly in the presence of electric and magnetic fields. When a small reservoir of ionic liquid ferrofluid is positioned within a magnetic field, magnetic stresses will deform the fluid interface into a peak. The addition of a strong electric field will further stress the fluid interface until a threshold stress is reached at which point the surface tension cannot contain the combined stresses and a spray of fluid or ions results at the apex. This process is termed electrospray, albeit a less understood form of electrospray owing to the addition of magnetic stresses which are not present in traditional electrospray. The first study included in this dissertation presents a computational fluid dynamics model of the combined electro-magnetic instability critical for electrospray. The developed model utilized the static formulation of the Maxwell equations to calculate the Maxwell stress tensor for an ionic liquid ferrofluid. When combined with the Stokes stress tensor, the duo of equations capture the fluid stresses present within the instability. The model was first employed to study the influence of a magnetic field on the onset potential of a capillary needle electrospray source. The simulation predicted onset potential agreed well with the experimentally captured onset under matching field conditions. The numerical tool was then utilized to study the dynamics of sessile ionic liquid ferrofluid droplets. The computational results were verified against laboratory images of sessile drops obtained under matching field conditions. The simulation performed exceptionally up until about 85% of the onset potential at which point the simulation began to over predict the apex height of the combined instability. The second portion of this dissertation consisted of long duration emission studies of an ionic liquid ferrofluid normal-field source. An operational procedure was developed which permitted a source consisting of a single emitter to operate with constant extraction potential for spans in extent of 15 hours. Time-lapse imagery of source enabled the mass flow rate to be approximated, permitting derived propulsion performance parameters to be obtained. Three different magnetic field strengths were investigated, and it was found that the magnetic field strength has no identifiable impact on propulsion performance. On average, the mass flow rate of the source was 28 ng/s (15.5 pL/s), with a specific impulse of 1385 s and a thrust of 0.380 µN per emitter. During the telemetry, the sensitivity of the source was analyzed and it was found that for moderate changes in extraction potential the source remained stable, but for increases on the order of 25-30% of the baseline voltage secondary emission sites were observed to form. The final set of studies included in this dissertation focuses investigated the angular divergence of ferrofluid electrospray emitting via the normal-field instability. The angular current density was measured through the use of a segmented Faraday probe and quantified in terms of an angular power utilization efficiency factor. For the source, the average power efficiency was found to be 94%. A strong correlation was found between increased emission current and increased mass flow rate and decreased power efficiency. Finally, a very small difference in efficiency was resolved between the positive and negative emission polarities. The last chapter of this dissertation models the magnitude of the Kelvin and Lorentz forces in the emission plume to determine their potential to influence particle trajectories. It was found that in the apex region, the Coulomb force dominates the Kelvin force by several orders of magnitude – indicating that the Kelvin force is unlikely to affect the trajectories of emitted magnetic particles. It was also found that the magnitude of the Lorentz force in the apex region was too small to influence particle trajectories for even the lightest ions expected

Progress in colloid propulsion

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2004.Includes bibliographical references (p. 61-68).In the early decades of the Space Age, a great deal of work was put into the development of the Colloid Thruster as an electric propulsion system for spacecraft. In spite of the effort by the end of the 70s the programs were stopped in the USA and Europe before any design had gotten to fly in space. An exhaustive study of the literature has been performed to identify what were the reasons behind the disappearance of Colloid Thrusters. Apart from programmatic reasons related to the introduction of the Space Shuttle, some technical reasons where identified. The technical difficulties had to do with the use of arrays of needles. Aiming at overcoming these difficulties, an alternative way to construct Colloid Thrusters has been proposed. Instead of needles, holes in Teflon where used. This has been tested both numerically and experimentally with positive results. This development may be useful not only for colloid propulsion but also for other technologies that require electrospray emission.by Jóse Mariano López Urdiales.S.M

Advanced propulsion for microsatellites

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2000.Includes bibliographical references (leaves 162-166).Microsatellites have become increasingly popular in recent years as they offer significant cost savings, higher reliability, and are generally more affordable for a large variety of commercial applications. Since many microsatellite missions require considerable propulsion capabilities, miniaturization of the propulsion subsystem is critical in the design of most miniature spacecraft. A broad range of existing propulsion technologies have been considered for the purpose of identifying those devices which maintain high performance at small scale. Scaling laws were developed for each of the selected devices so as to preserve, whenever possible, the basic non-dimensional quantities which ultimately determine the performance of the individual thrusters at small scale. Hall thrusters were initially identified as most promising. In an effort to miniaturize the Hall thruster, a number of complications have been encountered. Some of the most troublesome were higher magnetic field requirements, larger internal heat fluxes and temperatures, and difficulties associated with the manufacturing of the various miniaturized components. In order to validate the proposed scaling laws, a 50 Watt Hall thruster has been designed, manufactured, and tested in a vacuum tank. Results of the experimental testing indicate that, although the maximum thrust levels obtained were on the order of 1.8 mN, about two thirds of the nominal design value, the propellant utilization efficiencies were unexpectedly low at approximately 40%. Close examination of the magnetic assembly has shown that the tip of the iron center pole was overheating during operation due to the insufficient heat conduction. The tip temperatures were estimated to reach 900°C, exceeding the Curie point of iron. As a consequence of the change in the magnetic field profile and the resultant leakage of electrons, the observed ionization fraction and, therefore, the utilization efficiency were lower than expected. Despite the low efficiencies, which were most likely caused by the design imperfections rather than physical limitations, the effort to miniaturize a Hall thruster has provided a number of useful insights for any such attempts in the future. Most importantly, this work has highlighted the generic difficulty, common to all plasma thrusters, associated with the increase of the plasma density as the scale of the device is reduced. The consequences of strict scaling, most notably the higher particle fluxes which cause an increase in the erosion rates and significant loss of operating life at small scale, created a strong incentive to search for propulsion schemes which avoid ionization by electron bombardment. In the quest for a more durable device that could operate at low power, yet provide sufficient operating life to be of practical interest, colloidal thrusters were considered for miniaturization. These are representatives of a technology of electrostatic accelerators which does not rely on ionization in the gas phase and, hence, their operating life is not compromised at small scale. In addition to their intrinsically small dimensions and extremely low operating power levels, eliminating the need for further "miniaturization", colloidal thrusters possess a number of desirable characteristics which make them ideal for many microsatellite missions. Although the physics of electrospray emitters has been studied for decades, many of the mechanisms responsible for the formation of charged jets are still poorly understood. In order to gain further insight, a semi-analytical fluid model was developed to predict the effects of fluid's viscosity on the flow pattern. Results of the analysis indicate that over a broad range of operating conditions viscous shear flow is insignificant in the vicinity of the jet irrespective of the fluid's viscosity. In an attempt to further understand the physics of colloidal thrusters, specifically the effects of internal pressure, electrode geometry, and the internal electrostatic fields on the processes involved in the formation of charged jets, a detailed electrohydrodynamic model was formulated. A numerical scheme was developed to solve for the shape of the fluid meniscus given a prescribed set of operating conditions, fluid properties, and electrode configurations. Intermediate solutions for the conical region have already been obtained, however, convergence in the vicinity of the jet requires further studies. A fully developed model promises to provide valuable information and guidance in the design of colloidal thrusters.by Vadim Khayms.Ph.D

Numerical simulation of a single emitter colloid thruster in pure droplet cone-jet mode

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (leaves 112-117).New Scientific missions call for emerging propulsion technologies capable of fine tuning a satellite's relative position and cancelling small disturbances. One candidate technology that holds promise for these type of missions are colloidal thrusters. These thrusters are electrostatic accelerators which do not rely on gas ionization (plasma), are intrinsically small, and operate at low power levels, while having small plume divergence angles to avoid spacecraft (S/C) contamination problems. Colloid thrusters deliver low thrust (0.1 [mu]N/emitter) which can be multiplied many times over by integrating them in microfabricated arrays. An important advantage is that with proper choice of propellant and operating regime their specific impulse can be tailored from 500-7000 seconds making them viable candidates for a multitude of mission profiles. In the past many research groups have worked on developing colloid thrusters; however, their basic physics are still not completely understood. For this reason, we have undertaken the development of a numerical simulation of a colloid thruster to complement experimental and analytical research in the area. The goal of this project has been to create a flexible numerical tool to compute single-emitter current, droplet size, velocity, electric field strengths for a given geometry, fluid, flow rate, and voltage. We have approached this issue from the numerical perspective by developing a simulation of a colloid thruster operating in the single cone-jet pure droplet mode. Our numerical simulation models the cone-jet transition region of the colloid jet; starting from the needle up to the extractor grid, thus reproducing a typical electrospray experimental configuration.(cont.) The liquid is modelled as an incompressible viscous fluid with constant conductivity. Surface charge relaxation effects are included, and the potential and electric fields are solved for. The equations have been simplified by employing a slenderness approximation except for the free surface boundary conditions where the terms have been kept exact, in order to reproduce the Taylor cone shape correctly. Simulation results are shown to compare well to experimental data for a variety of liquids, and flow rates. In more detail, this thesis presents (a) results for a broader parametric range than previously published materials; (b) careful comparison to available experimental data and scalings proposed by several authors; (c) validation of several simple sub-models for the cone and jet sections; (d) analysis of length scales in the cone-jet transition region; (e) clarification of the role of relative permittivity for the range 7-110; (f) discussion of electrode geometry and counter-electrode effects; (g) preliminary results for the onset of ion evaporation.by Jorge Alejandro Carretero Benignos.Ph.D

Molecular dynamics modeling of ionic liquids in electrospray propulsion

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 119-124).Micro-propulsion has been studied for many years due to its applications in small-to-medium sized spacecraft for precise satellite attitude control. Electrospray thrusters are promising thrusters built upon the state of the art in micro-technology and with flexible performance in terms of their high efficiency and high specific impulse. One challenge is to investigate in detail the mechanism for ion emission to complement experimental results and understand better how emission occurs in the micro to nano scale. Thus, atomistic modeling is used to understand properties of emitted charged particles which determine how the thrusters perform. As a preliminary study of ion emission from Taylor cones, ion evaporation from 3 - 5 nm droplets was observed in molecular dynamics (MD) simulations to validate the atomistic modeling and to investigate activation energies. Ion emission was examined in terms of internal and external electric fields and the activation energies of each case were obtained using Schottky's model and direct energy calculation to compare with experimental values. Ion emission was mainly observed with electric field strengths between 1.2 -2.0 V/nm and the emitted species include both solvated and non-solvated ions. Propulsive properties from Taylor cones are examined using results from the analysis of electric current from ion emission. In addition to an observation of ion emission from liquid droplets, numerical simulations for interactions between a solid plate and liquid droplets were conducted with MD simulation. It was concluded that another selection of force field needs to be considered to pursue further details, such as electrochemical effects.by Nanako Takahashi.S.M

Analysis of an Electrospray Thruster with a Concave Propellant Meniscus

The low thrust, high specific impulse, and low mass of electrospray thrusters (ETs) make them ideal for maneuvering nanosatellites, especially with the new requirement to deorbit a satellite within five years of completing its mission. These innovative thrusters use electrohydrodynamic principles of electrospray (ES) to provide thrust. These principles have been subject to much research over the past decade, though much more research is needed to fully understand the underlying physics of these thrusters. The first part of this study establishes a procedure for analyzing the theoretical thrust performance of an ET, by using propellant properties and well-documented ES scaling laws to identify the ES mode and governing equations applicable to the ET of interest. Next, this procedure is demonstrated by analyzing a novel ET in development at the University of Tennessee Space Institute, which culminates in the comparison of three theoretical thrust density and specific impulse equations based on geometry, electric pressure, and Ohmic conduction. The second part of this study will focus on a numerical analysis of ES produced from a concave meniscus using ANSYS Fluent. The novel ET will use capillary action to drive propellant through its capillaries, unlike most ETs that use a pump. The capillary action will result in the propellant forming a concave meniscus at the end of the capillary. There is little research on whether a concave meniscus can produce ES because most studies assume the meniscus is initially flat or in the Taylor cone shape. This model will focus on ES on the microscale using a low-conductivity fluid flowing through a charged capillary. The numerical model used in this study is verified by comparing the jet diameter produced by the model with well-established microscale ES scaling laws for the jet diameter. The results of this numerical model show that it is possible to produce a Taylor cone and an unsteady jet from a concave meniscus. Finally, this study will make recommendations on future work for analyzing and understanding the underlying physics of ETs

Characterisation of electrospray properties in high vacuum with a view to application in colloid thruster technology

PhDThe operational environment of colloid thrusters is high vacuum (10-3 _ 10-6 mbar) however, much of the experimental data collected to date to identify parameter relationships in cone jet mode electrosprays (ES), such as current-volumetric flow rate scaling laws, has been conducted in atmospheric conditions. This highlights a need for electrospray data under high vacuum conditions. Electrospray experimental data was collected using medium conductivity solutions (0.0025 -0.0160 S/m) of TEG doped with sodium iodide in high vacuum. These sprays were obtained from a stainless steel capillary and a disk counter electrode with central aperture. An online flow measurement system is described, which has been developed during this research to measure the fluid volumetric flow rate, concurrently with applied voltage and spray properties such as spray current and cone, jet and spray geometry. This automated flow measurement system was used to measure flow rates as low as InUs with an absolute accuracy of 0.3nUs and a resolution of 0.03nus. It is identified that this system may be easily adapted for lower flow rates and higher resolutions. The ES data collected demonstrates, for the first time, the detailed dependence of volumetric flow rate upon the applied voltage. The sensitivity of nominal flow rate to applied voltage was found to be higher for lower nominal flow rates. For a volumetric flow rate -4nLIs a 25% a change in flow rate per kV was recorded over a cone-jet mode stability range spanning -1.5kV. This volumetric flow rate voltage sensitivity holds particular significance for colloid thruster systems, which operate at or near minimum flow rate conditions. The current was found to have a power law dependence on flow rate similar to the current scaling laws of F. de la Mora and Gahan-Calvo however the exponent of this power law differs significantly from these scaling laws. A study considering the effect of charge carrier mobility in simple 1: 1 electrolytes shows that the exponent of the power law current-flow rate scaling increased with increasing charge carrier mobility. Contrary to the various scaling laws the spray current was found to be dependent on electrostatic conditions. The sensitivity of the emitted current to the applied voltage was also found to increase with increasing nominal volumetric flow rate. The geometrical parameters of cone angle, spray angle and jet length were measured for varying TEG/Nal solution conductivity. Cone geometry was found to be relatively independent of conductivity in the range tested. Jet length was found to have an inverse relationship with solution conductivity

Studies on the ion-droplet mixed regime in colloid thrusters

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2003.Page 222 blank.Includes bibliographical references (p. 217-221).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Colloid thrusters working with mixtures of ions and droplets are gradually becoming an alternative technology for space micro-propulsion needs in missions requiring high position controllability, compactness and low power consumption. The mechanics of the colloid thruster emission process are discussed through a theoretical review of its general properties and by means of experimental characterization. Droplets are the most energetic particles in the beam, while ions are emitted with energies that overlap those of the droplets but extend down a few hundreds of volts in comparison. A small fraction of the ion current is emitted from the jet breakup region with considerably lower energies. This energy variety transforms the optical hardware elements into energy filters by taking advantage of the chromatic aberration property of electrostatic lenses. The relatively wide ion energy distribution is conceptually explained as a result of emission from different locations in the cone-jet structure where the normal electric field is most intense and where the convective current produces drastic changes in the local potential. The energy spread of purely ionic emission from EMI-BF4 is measured and is found to be of the order of a few tens of volts. A high-speed electron multiplier detector is used for the first time to analyze the ion component emitted directly from electrospray sources. Ion identification is performed and is found that the most probable degree of solvation is n = 5.1 for (CH3NO)nNa+ ions in formamide doped with NaI for a conductivity of 2.15 siemens per meter.(cont.) Two ions are observed for the ionic liquid EMI-BF4: EMI+ and (EMI-BF4)EMI+. It is found that these ions are emitted with a small energy differential. The use of 5 micron ID capillary emitters, working with flow rates close to 20 pico-liters per second, is successfully achieved. Under such conditions, highly charged droplets with specific charges in excess of 10 coulombs per gram are obtained, representing the highest charge state obtained so far in experiments of this kind. The applicability of colloid thrusters for space propulsion is discussed in terms of performance parameters in the ion-droplet mixed regime, along with other practical considerations, such as the problem of beam neutralization.by Paulo C. Lozano.Ph.D