Katharine Smith

Electrospray Performance of Microfabricated Colloid Thruster Arrays

Microfabricated emitters have been produced by deep reactive ion etch technology. To demonstrate their suitability as components of integrated colloid thruster systems, we have evaluated the electrospray performance of individual and arrays of these microfabricated emitters and compared them to that of conventional stainless-steel emitters. We show that, after accounting for electrostatic differences caused by changes in physical geometry, the spray current dependence on flow rate for microemitters demonstrates similar scaling behavior to that of conventional single stainless-steel emitters. The spray current per nozzle is found to be independent of array size and the total spray current to depend simply on the number of nozzles in the emitter array. We have also found that in a triangular array of microemitters there is no significant geometry-induced shielding or field reduction between emitters. We report on the electrospray performance of the room- temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate with conventional emitters, which appears to be a promising new colloid thruster propellant.

The Sensitivity of Volumetric Flow Rate to Applied Voltage in Cone-Jet Mode Electrospray and the Influence of Solution Properties and Emitter Geometry

A high accuracy online flow rate measurement system has been used to demonstrate the effect of applied voltage, Vapp, on the volumetric flow rate, Q, through an electrospray system. Several solutions of the organic solvents ethylene and triethylene glycols doped with sodium iodide to give varying conductivities in the range of 0.0025–0.23S∕m have been sprayed. It was established for the first time that solution conductivity has no appreciable effect on the sensitivity of flow rate to applied voltage in the cone-jet mode of electrospray. However, it appears that even when the hydraulic resistance is taken into account, the sensitivity of flow rate as controlled by the applied voltage is additionally related to the emitter exit geometry. These findings are of particular importance to both spacecraft propulsion and electrospray mass spectrometry technologies and suggest careful emitter geometry design considerations will lead to greater control over electrospray properties.

Voltage-Modulated Flow Rate for Precise Thrust Control in Colloid Electrospray Propulsion

We describe a feature of electrospray operation that allows precise control of flow rate through an electrospray emitter by use of the extraction voltage. The effect of extractor voltage on the propellant flow rate through an electrospray emitter has been determined for triethylene glycol and ethylene glycol solutions doped with varying levels of sodium iodide and the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate using an in-line high-accuracy flow measurement system. In these experiments, a nominally fixed flow rate, obtained by providing a fixed supply pressure, is observed to be influenced by the applied voltage during stable cone-jet electrospray production. The relative sensitivity of flow rate to applied voltage was found to be higher as the nominal flow rate decreased. This method of flow rate control holds particular significance for colloidal electrospray thruster systems, which operate at or near minimum flow rate conditions.

Colloid Propulsion A Re-Evaluation, with an Integrated Design

A fully integrated micro-fabricated concept design of a colloid thruster is presented. A review of the possible mission applications for this thruster is provided, which concludes that the colloid thruster is versatile and shows good system advantages for missions as diverse as fundamental science missions, requiring high thrust stability, to low power consumption, high performance missions for small satellites.

The role of molar conductivity in electrospray cone-jet mode current scaling

A high accuracy online flow rate measurement system has been used to determine the current flow rate scaling relationships for solutions of organic solvents doped with sodium iodide and for the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate over a range of conductivities from 0.0025−1.3S∕m. The current flow rate trends for these solutions were found to exhibit a power law relationship similar to that described by previous researchers, where I (the electrospray current) is proportional to Q (the volumetric flow rate) to some power n. However, the exponent n of the current flow rate trends was found to differ from the theoretical predictions reported in the literature. A study including data from literature revealed the exponent of the current flow rate trends to be sensitive to the molar conductivity of the sprayed solution.A high accuracy online flow rate measurement system has been used to determine the current flow rate scaling relationships for solutions of organic solvents doped with sodium iodide and for the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate over a range of conductivities from 0.0025−1.3S∕m. The current flow rate trends for these solutions were found to exhibit a power law relationship similar to that described by previous researchers, where I (the electrospray current) is proportional to Q (the volumetric flow rate) to some power n. However, the exponent n of the current flow rate trends was found to differ from the theoretical predictions reported in the literature. A study including data from literature revealed the exponent of the current flow rate trends to be sensitive to the molar conductivity of the sprayed solution.

The flow rate sensitivity to voltage across four electrospray modes

The influence of potential difference on the emitted flow rate across four modes of electrospray is described for an unrestricted electrospray system. The modes are those most commonly occurring; enhanced dripping, pulsation, cone-jet, and multi-jet. It is demonstrated that within three of these modes, the effect of voltage on flow rate is generally linear, with similar magnitude of gradient across all. The effect is demonstrated to be calculable across these three modes. This finding highlights that in the absence of any flow control mechanism, the influence of electrostatic pressure in driving the flow is the key process in voltage-driven electrospray.

Inexpensive Optically Isolated Nanoammeter for Use with Micro-Newton Electric Propulsion Technology

An inexpensive optically isolated nanoammeter, which permits digital logging of dc currents from an electrospray colloid thruster operated in vacuum has been designed for using with micro-Newton electric propulsion technology. The two-stage optically isolated system is intended to safely measure and record a current generated at high voltage, facilitating data acquisition with a grounded logging device such as a personal computer. It operates by converting the current into a frequency in a battery-powered, floated, high-voltage stage. Agilent Technologies fiber-optic kits were selected for their low cost and reliability. The fiber-optic cable passes between the high voltage and the grounded receiver stage via grommeted holes drilled in the metal housing. The use this optically isolated system permits accurate monitoring of micro-Newton thruster currents directly rather than through inaccurate proxies, giving physical insight into the behavior of the system at high accuracy and low cost.

High Accuracy Measurements in Electrospray Source Relevant to Colloid Thrusters

Measurements are presented for a variety of electrospray conditions, which are relevant to the design of colloid thrusters. Broadly this new data is consistent within model predictions of previously published scaling law data, with however the notable exception that under vacuum conditions the spray properties are sensitive to electrostatic conditions. Data is also presented on the jet length formed in the cone-jet mode, as a function of both fluid conductivity and flow rate. It is found that the jet length is insensitive to the electrostatic conditions investigated.

Solar activity simulation and forecast with a flux-transport dynamo

We present the assessment of a diffusion-dominated mean field axisymmetric dynamo model in reproducing historical solar activity and forecast for solar cycle 25. Previous studies point to the Suns polar magnetic field as an important proxy for solar activity prediction. Extended research using this proxy has been impeded by reduced observational data record only available from 1976. However, there is a recognised need for a solar dynamo model with ample verification over various activity scenarios to improve theoretical standards. The present study aims to explore the use of helioseismology data and reconstructed solar polar magnetic field, to foster the development of robust solar activity forecasts. The research is based on observationally inferred differential rotation morphology, as well as observed and reconstructed polar field using artificial neural network methods via the hemispheric sunspot areas record. Results show consistent reproduction of historical solar activity trends with enhanced results by introducing a precursor rise time coefficient. A weak solar cycle 25, with slow rise time and maximum activity

Launch and deployment of distributed small satellite systems

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