Charles Ryan

Microfabricated electrospray emitter arrays with integrated extractor and accelerator electrodes for the propulsion of small spacecraft

Microfabricated electrospray thrusters could revolutionize the spacecraft industry by providing efficient propulsion capabilities to micro and nano satellites (1–100 kg). We present the modeling, design, fabrication and characterization of a new generation of devices, for the first time integrating in the fabrication process individual accelerator electrodes capable of focusing and accelerating the emitted sprays. Integrating these electrodes is a key milestone in the development of this technology; in addition to increasing the critical performance metrics of thrust, specific impulse and propulsive efficiency, the accelerators enable a number of new system features such as power tuning and thrust vectoring and balancing. Through microfabrication, we produced high density arrays (213 emitters cm−2) of capillary emitters, assembling them at wafer-level with an extractor/accelerator electrode pair separated by micro-sandblasted glass. Through IV measurements, we could confirm that acceleration could be decoupled from the extraction of the spray—an important element towards the flexibility of this technology. We present the largest reported internally fed microfabricated arrays operation, with 127 emitters spraying in parallel, for a total beam of 10–30 µA composed by 95% of ions. Effective beam focusing was also demonstrated, with plume half-angles being reduced from approximately 30° to 15° with 2000 V acceleration. Based on these results, we predict, with 3000 V acceleration, thrust per emitter of 38.4 nN, specific impulse of 1103 s and a propulsive efficiency of 22% with <1 mW/emitter power consumption.

Effect of emitter geometry on flow rate sensitivity to voltage in cone jet mode electrospray

The effect of voltage on flow rate within cone jet mode electrospraying has been investigated, with particular emphasis on the effect of emitter geometry. A set of experiments investigated the effect of the outer and inner diameters on the flow rate relationship to voltage, in cone jet mode electrospray. This was accomplished by the use of a high fidelity flow meter, capable of measuring changes in flow rate to a fraction of a nanolitre per second.It has been previously demonstrated that there are two separate parameters that influence the flow rate sensitivity to voltage; the hydraulic resistance of the flow system, and the outer diameter of the emitter. By a simple derivation, the second of these two is explained by the variation of theoretical electric pressure with voltage, as the outer diameter is varied.Good agreement is found between experimental and theoretical results, suggesting the simple theory reasonably explains the physics of the situation.As well as elucidating the physics involved in electrospray—suggesting the electric field is an important controlling parameter within cone jet mode electrospray—the theoretical and experimental agreement has important implications for variable throttling of thrust in colloid thrusters, and could bring about better optimization of performance in other electrospray-employing fields.

The influence of geometry on the flow rate sensitivity to applied voltage within cone-jet mode electrospray

This work investigates in greater detail than in previous studies the effect of geometry on the relationship between emitted flow rate and applied potential difference in cone-jet mode electrospray systems. The magnitude of the flow rate to voltage relationship is demonstrated to be sensitive to numerous geometric parameters. An explanation of this variation is offered; it is demonstrated that in the cone-jet mode of operation the change of flow rate with the applied extraction voltage is due to the change in electric field at the tip of the emitter. By a finite element method simulation of the assumed electrostatic process the analysis is further extended to include all geometric parameters. The results outlined show the change of flow rate with applied voltage in cone-jet mode electrospray can be significant. This dependence will, under some conditions, have a considerable effect on the electrospray flow rate, and consequently current and droplet size. This has implications on electrospray applications ...

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.

Progress Towards a Miniaturized Electrospray Thruster for Propulsion of Small Spacecraft

Miniaturized electrical thrusters based on electrospray (or colloid) emitters could rev- olutionize the spacecraft industry by providing efficient propulsion capabilities to micro and nano satellites (1-100 kg). We report on our recent advances in the development of this technology within the MicroThrust (www.microthrust.eu) European consortium. We present the design and operation of the currently fabricated and next generation emitter arrays, describing their microfabrication process and measured performance. The emitters are out-of-plane internally fed capillaries micromachined in monolithic silicon. They are 100 µm tall and have an inner diameter of 5-10 µm. We operate the devices in both unipolar and bi-polar modes and find that the latest devices operate in a mixed regime, with the emitted spray a composition of ions and droplets. Their specific impulse is consequently in the few hundred seconds, highlighting the need for higher impedance, smaller emitters. Onset voltages are of than 800-850V for 200 µm inner diameter extractors, current levels for 19 emitter arrays of 2-3 µA. Preliminary analysis hints to plume half-angles of 35-40◦, although these values depend on the operation mode and beam composition.