Barry Kent

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.

Fabrication and Operation of Microfabricated Emitters as Components for a Colloid Thruster

An integrated design for a colloid thruster, using microfabricated components, is described. This thruster is suitable as a propulsion unit for both a small satellite and a conventional satellite. One key component of thethruster is a nano-emitter array. A description of the fabrication processes that are used for the manufacture of these nano-emitters is provided. We report the first quantitative results for the electrospray current as a function of both flow rate and applied voltage for these emitters. Electrospray test results comparing the performance of these new emitters with conventional stainless-steel capillaries reveal that when allowance is made for the electrostatic configuration differences the spray current demonstrates qualitatively similar flow-rate dependence. Importantly, our electrospray current measurements from differently sized emitters show, for the first time, that this current is sensitive to the geometry of the emitter itself. This result is used to highlight the role of emitter design in the efficiency of a colloid thruster.

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.

Optimization of silicon field-emission arrays fabrication for space applications

Optimization of fabrication of silicon field-emission arrays has been carried out on 4 in. silicon wafers for field-emission electric propulsion neutralizer application. A mixture of SF6, O2, and Ar for silicon tip etches has been optimized to improve the uniformity and process repeatability. A thin aluminum nitride (AlN) film has been coated on gated emitter arrays to enhance the field at the tip and to protect tips from ion bombardment. A statistical analysis of emission characteristic shows a narrow distribution of the turn-on voltage from array to array, which makes it possible to achieve a few milliamperes current emission by connecting several arrays in parallel. Lifetime tests over 1000 h have been carried out on single arrays, producing in excess of 10 μA of emission current under continuous mode of operation.

Variability in long-duration operation of silicon tip field emission devices

Development of reliable field emission devices requires detailed characterization of variability over the device lifetime. Silicon tip manufacturing processes and test equipment limitations are theoretically predicted to cause ∼5% variability in field emission currents. Long-duration emission current measurements at both constant voltage and constant current, controlled with a software feedback loop, were made. Power spectra from the two operational modes were significantly different. The software feedback loop response was determined; it is expected to contribute ∼1% of the standard deviation in emission current when used to control emission by modulating the gate voltage. Convolution of constant voltage time series with the software feedback loop response reproduced the spectral characteristics of constant current time series, indicating that many of the differences between the two operating modes were caused by the operating mode itself rather than the field emitters tested. This has implications for t...

Investigation of fabrication uniformity and emission reliability of silicon field emitters for use in space

A silicon field emitter neutralizer is under development at the Rutherford Appleton Laboratory for the LISA Pathfinder mission [B.J. Kent et al., Class Quantum Grav. 22, S483 (2005)]. A summary of this project from the fabrication point of view is presented in this article. An investigation of the effect of fabrication uniformity on emission characteristics showed that the geometrical nonuniformity, including tip height and gate diameter, across a 4-in. wafer is below 15%. This variation had only a small effect (∼10%) on the field-emission characteristics. In order to improve the reliability of the silicon field emitters for the space environment operation, a thin aluminum nitride (AlN) film is coated on the silicon emitters, and the chromium gate electrode is oxidized in furnace. The effects of adding the layer of AlN and thermal oxidation of the chromium gate electrode on field emission and lifetime were investigated in this article. A fabrication yield analysis estimated that 50% of arrays on the origi...

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.

An Experimental Colloid Thruster Package for Nano Satellites

The advent of Micro and Nano satellites coupled with the maturity of micro fabrication technology has lead to a reappraisal of the potential for colloid thrusters. When produced as micro fabricated arrays, colloid thruster emitter heads enable new mission opportunities to be explored. We outline here the implications and system advantages of using micro fabricated colloid thrusters to implement a project in which a nano satellite co-flies with a micro satellite. We show that the mass of the propulsion elements of the selected mission, which has been previously demonstrated using cold gas thruster, could be reduced by about a factor 2 using colloids.