Advanced Propellants for Scalable, Multipurpose Electrospray Ion Thrusters

Currently, there are a limited number of propulsion technologies suitable for use on microsatellites and CubeSats. As a result, most small spacecraft have flown without propulsion systems [1]. Without on-board propulsion, CubeSats are unable to control their orbit and attitude, which strongly limits their lifetime and mission potential. This motivates the development of micropropulsion technologies that are compatible with the size and power requirements of microspacecraft. With micropropulsion, CubeSats would be able to compensate for atmospheric drag, point instruments such as cameras, fly in formation, and perhaps even explore the Solar System. Micropropulsion could also be used for precision attitude control on large spacecraft to support leading-edge scientific missions such as gravity wave detection and exoplanet imaging. Ion electrospray propulsion is a new type of micropropulsion technology being developed at the MIT Space Propulsion Lab (SPL). Ion electrospray thrusters use novel substances called ionic liquids as propellant [2]. Ionic liquids are molten salts at room temperature that consist of positive and negative ions. Strong electric fields can be used to evaporate ions from the liquid and accelerate them to high velocities, producing thrust [3]. The SPL ion electrospray thruster architecture consists of a 1 cm2, 1 mm thick chip of porous material that is laser etched to create 480 emitter tips [2]. A micro-fabricated extractor plate with one aperture for each tip is placed on top of the emitter array. Once the ionic liquid permeates the porous emitter array, approximately 1 kV of electric potential of either polarity can be applied between the emitter array and the extractor plate. This creates strong electric fields at the tip of each emitter, allowing for the evaporation of ions from each tip. A single thruster produces 480 ion beams, which results in approximately 10 microNewtons of thrust [2]. This research is focused on improving the understanding of ion electrospray phenomena that affect performance. The performance of ion electrospray thrusters is significantly affected by the composition of the ion beam [4]. Ion electrospray thruster ion beams consist of single ions as well as clusters of ions. In this work, single ions are referred to as monomers. Dimers are single ions, or monomers, attached to a cation-anion pair, which is referred to as a neutral. Trimers are single ions attached to two neutrals. Larger ion clusters such as tetramers, pentamers, etc. are also possible but are rarely detected in significant quantities [5]. The monomers and ion clusters are accelerated through the same electric potential, but, since they have different masses, they exit the thruster at different speeds. This negatively impacts thruster efficiency and specific impulse because it is more propulsively efficient to have all the ions leave at the same speed with the same mass [6]. The beam composition varies with the thruster operating conditions, particularly the type of ionic liquid used [5], and cannot be easily controlled. Ion clusters, such as dimers and trimers, are not stable and can break apart after emission from the tip [7]. In this work, this phenomenon is referred to as fragmentation. Ion clusters break into an ion, called a broken ion, and a neutral cluster. For example, a dimer will break into a monomer and a neutral. Ion cluster fragmentation that occurs before the ions exit the thruster has a significant impact on the performance [6]. The broken ion is accelerated to a final kinetic energy less than the electric potential applied to the thruster, also referred to as the source potential. The neutral cannot be accelerated by the electric field within the thruster and exits at a relatively slow velocity. Both of these effects combine to reduce thruster efficiency and specific impulse [6]. Despite the performance losses, fragmentation does play an essential role in spacecraft neutralization since the low-energy broken ions can return to the spacecraft to offset charge build-up [8]. It has been hypothesized that the amount of fragmentation is related to the ionic liquid molecular composition [6]. The amount of ion cluster fragmentation has been measured for several ionic liquids in past experiments with each ionic liquid showing a different amount of fragmentation [6,7,9]. The ionic liquids with the most complex molecular ions, meaning molecules with many atoms, tend to have the least amount of fragmentation. This observation comes from data that were taken using different experimental set-ups operated under different conditions, so strong conclusions cannot be drawn. However, this hypothesis does make physical sense. Ion clusters are imparted with energy during the evaporation process that can drive the individual ions in the cluster apart [6]. Molecular ions with many degrees of freedom can absorb some of this energy throughout their many bonds, which lowers the probability of fragmentation [6]. The hypothesis that clusters composed of complex molecular ions tend to be more stable is also supported by recent molecular dynamics simulations [6]. Fragmentation of ion clusters in ion electrospray thrusters is not a well-understood phenomenon which requires further study. Past experiments have observed and even measured the amount of fragmentation that occurs both before the ions leave the thruster and after the thruster exit [6,7,9,10]. However, the rate of ion cluster fragmentation in ion electrospray thrusters has never been measured before. The effect of an external electric field, such as the field within the thruster, on the fragmentation rates is also unknown. The findings of past experiments have also been insufficient to evaluate the hypothesis that clusters composed of complex ions tend to be more stable. Additionally, the effect of ionic liquid temperature on fragmentation has not been investigated. A better understanding of fragmentation physics is necessary for the improvement of thruster design and performance, which is the motivation of this research. The main goal of this work is to characterize the fragmentation of ion clusters in ion electrospray thrusters more thoroughly than ever before. A major objective is to measure the rates of ion cluster fragmentation in ion electrospray thrusters for the first time. The rate measurements will provide a way to compare the stability of ion clusters from different ionic liquids, which should allow for a more conclusive evaluation of the hypothesis that complex ion clusters are more stable. Additionally, the rates could be used to estimate the initial internal temperature of the ion clusters for the first time. Another objective is to investigate how the electric field within the thruster modifies the fragmentation rates. The effect of ionic liquid temperature on the fragmentation rates is also an important area of study. Finally, the experimental measurements will be used to develop analytical models that provide a deeper understanding of fragmentation physics. To better characterize ion cluster fragmentation, a suite of sensitive instrumentation designed for this particular application is required. Instruments used in past experiments [im,bmii,bf4] were not designed to study ion cluster fragmentation in the full ion beam, which is why there is limited understanding of this phenomenon. In this work, a combined full-beam spherical retarding potential analyzer (RPA) and Channeltron electron multiplier time of flight (TOF) mass spectrometer were designed, built, and tested. The RPA and TOF detectors were specifically designed to be sensitive to ion cluster fragmentation. The RPA measures the energy distribution of the full ion beam and provides the amount of fragmentation within the thruster and after the thruster exit. The TOF detector measures the beam composition of a small sample of the ion beam and can also detect fragmentation inside the thruster. A single emitter ion electrospray thruster is used as the ion source and is mounted on rotational stages to point the ion beam. The ion source is also mounted on a linear stage that can be used to modify the distance between the ion source and the RPA detector. The RPA, in conjunction with a linear stage, is used to measure the fragmentation rates for the first time. Four ionic liquids were characterized using the experimental set-up at various ionic liquid temperatures. The ionic liquids are BMI-I, EMI-BF4, EMI-Im, and EMI-FAP. The following is a list of the major results of the experiments and subsequent analysis: 1. The rates of dimer fragmentation after the thruster exit were measured for the first time for several ionic liquids. The RPA was used to measure the amount of fragmentation after the thruster exit as a function of the distance between the source and detector. The shorter the distance, the less time the ions spend outside the thruster and therefore less fragmentation should be detected. Similarly, the longer the distance, the longer the time that ions spend outside the thruster and therefore the more fragmentation should be detected. A fit of the amount of fragmentation versus time ion clusters spent outside the thruster yields the fragmentation rates. It was determined that fragmentation outside of the thruster, where there is no electric field, can be modeled according to a constant rate equation. The dimer mean lifetimes across all of the ionic liquids ranged from 1-6 microseconds, which indicates that the ion clusters are very short-lived. Within 10-20 cm of the thruster exit, 99% of the dimers will have broken up. 2. The initial internal temperature of dimers for EMI-Im and EMI-FAP were estimated for the first time. The activation energy and rate constant for EMI-Im and EMI-FAP dimers were recently measured by Vidal [11]. Note that their dimers were prepared using a diluted ionic liquid solution in a capillary electrospray source, which is very different from an ion electrospray thruster. In this work, the dimer temperatures were estimated using the measured mean lifetimes and the reported activation energy from Vidal. The temperatures range from 525 - 800K, which is higher than the ionic liquid temperature, indicating that the ions are heated during the evaporation process. However these temperatures are lower than expected given that several electron volts of energy are lost during evaporation [7], which suggests that much of the energy is dissipated into the ionic liquid meniscus. The effect of ionic liquid temperature on the fragmentation rates was also investigated although no strong conclusions were drawn. 3. An analytical model was developed to investigate the effect of an external electric field on the fragmentation rates. Using the measured fragmentation rates outside the thruster and the reported activation energy and rate coefficient from Vidal, the rate of fragmentation was calculated as a function of the electric field within the thruster. It was estimated that the fragmentation rates close to the emitter tip, where the electric field is highest, were approximately one thousand times higher than those closer to the thruster exit, where the electric field is weakest. This suggests that significant ion cluster fragmentation may occur immediately after the ions are emitted from the tips. Such a scenario would create many low speed neutrals, which could negatively impact thruster efficiency. It was also observed that throughout much of the acceleration region (e.g. the region between the emitter tips and extractor plate in which ions are accelerated), where the electric field is more modest, the rate of fragmentation is relatively constant. This provides a physical explanation for why the measured amount of fragmentation is uniform with respect to acceleration potential, which was observed in the RPA measurements for all of the ionic liquids. 4. The rates of fragmentation both within the acceleration region and outside of the thruster were compared for the four ionic liquids tested. The fragmentation rates inside the thruster strongly support the hypothesis that clusters composed of complex ions tend to be more stable. The fragmentation rates outside the thruster also support the hypothesis. Note that there are challenges associated with fairly comparing different ionic liquids so this hypothesis should continue to be investigated, particularly using analytical and numerical models. Overall, this research supports the hypothesis that clusters composed of complex ions are more stable. In summary, this work has provided a deeper understanding of ion cluster fragmentation physics. The rates of ion cluster fragmentation were measured for the first time, revealing that the ion clusters are very short-lived. It was determined that fragmentation could be modeled as an activated process, which allows for the estimation of the initial internal temperature of dimers. Analytical models were developed to study how the electric field within the thruster modifies the fragmentation rates. Finally the experimental measurements are in support of the hypothesis that clusters composed of complex ions tend to be more stable. These findings are essential for modeling fragmentation in ion electrospray thrusters and for accurately estimating the effect fragmentation has on thruster performance. This work also opens to door to future investigations of the relationship between ionic liquid composition and thruster performance. With this research, it is possible that one day ionic liquid propellants could be designed for use in a variety of microsatellite space missions.