Alternative Fabrication Techniques For and Emitter Densification of Ion Electrospray Propulsion Systems
Natalya Brikner, Paulo Lozano (Associate Professor)
Objective of the Research
At 10e9 V/m, ionic liquid ion source (ILIS) thrusters extract ions from ionic liquids—room temperature molten salts—that are passively supplied. Fields of this order are achieved at the tips of Taylor cones—electrostatically stressed menisci that result from the balance of electric stress and surface tension on the liquid propellant—which form on the ends of sharp structures, known as emitters. ILIS electrospray thrusters operate at modest potentials around 1.5 kV, require less than 0.65 W of operating power, and can produce microNewtons of thrust and a specific impulse of 2000-3000 seconds
|Figure 1. Ion emission from Taylor cone [Courtney, 2010]|
This work aims to address the following challenges:
1. Substrate selection and propellant transport – A substrate that lends itself to mature fabrication techniques and can support a passive propellant supply system (eg. wettable, porous) is desired
2. Electrochemistry – Emitters can be plated with or fabricated from inert materials to provide a benign electrochemical environment for improved thruster lifetime
3. Beam divergence – Ion emission can occur from Taylor cones forming at locations other than the emitter tip. Fabrication methods that allow better control over pore size can concentrate emission sites and decrease beam divergence
4. Densification – Techniques for fabricating emitters on the order of single µm’s can help realize thrust densities up to four orders of magnitude greater than the current state of the art
Approach / Tools
The approaches to alternative fabrication techniques and emitter densification can be divided into two categories: surface modifications and electroforming.
Surface modifications are made to existing thruster designs to achieve better spatial control of propellants and to increase thruster lifetime by mitigating electrochemical reactions. The surface modifications will include micro- and nanostructuring, chemical patterning, and electroplating with inert materials. An example of the setup for a platinum or nickel electroplating experiment is shown in Fig. 2.
|Figure 2. Electroplating experiment setup|
Electroforming techniques will be used to create emitter structures by adding material rather than etching. This method will allow better control over emitter pore size, and will hopefully lead to processes for fabricating tips on the order of single microns.
Courtney D.G. Ionic Liquid Ion Source Emitter Arrays Fabricated on Bulk Porous Substrates for Spacecraft Propulsion. PhD thesis, Massachusetts Institute of Technology, 2011.
Courtney D.G., Li H., Macqueo P.D.G., Fedkiw T.P., and Lozano P. On the validation of porous nickel as substrate material for electrospray ion propulsion. In 46th Joint Propulsion Conference and Exhibit, 2010. AIAA-2010-7020.
Gassend B., Velasquez-Garcia L.F., Akinwande A.I., and Martinez-Sanchez M. A microfabricated planar electrospray array ionic liquid ion source with integrated extractor. Journal of microelectromechanical systems, 18(3), June 2009.
Lozano P. and Martinez-Sanchez M. Ionic liquid ion sources: suppression of electrochemical reactions using voltage alternation. Journal of Colloid and Interface Science, 280, 2004.
Garza T.C. Optimizing wettability of externally wetted microfabricated silicon electrospray thrusters. Master's thesis, Massachusetts Institute of Technology, 2007.