World Library  
Flag as Inappropriate
Email this Article

Field-emission electric propulsion

Article Id: WHEBN0000089918
Reproduction Date:

Title: Field-emission electric propulsion  
Author: World Heritage Encyclopedia
Language: English
Subject: Spacecraft propulsion, Martin Tajmar, Ion engines, Arcjet rocket, High Power Electric Propulsion
Collection: Ion Engines, Spacecraft Propulsion
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Field-emission electric propulsion

Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses liquid metal (usually either caesium, indium or mercury) as a propellant. A FEEP device consists of an emitter and an accelerator electrode. A potential difference of the order of 10 kV is applied between the two, which generates a strong electric field at the tip of the metal surface. The interplay of electric force and surface tension generates surface instabilities which give rise to Taylor cones on the liquid surface. At sufficiently high values of the applied field, ions are extracted from the cone tip by field evaporation or similar mechanisms, which then are accelerated to high velocities (typically 100 km/s or more).

A separate electron source is required to keep the spacecraft electrically neutral. Due to its very low thrust (in the micronewton to millinewton range), FEEP thrusters are primarily used for microradian, micronewton attitude control on spacecraft, such as in the ESA/NASA LISA Pathfinder scientific spacecraft.

Contents

  • The Field Emission Electric Propulsion Concept 1
  • The Slit Emitter 2
  • References 3
  • External links 4

The Field Emission Electric Propulsion Concept

Field Emission Electric Propulsion (FEEP) is an electrostatic propulsion concept based on field ionization of a liquid metal and subsequent acceleration of the ions by a strong electric field. FEEP is currently the object of interest in the scientific community, due to its unique features: sub-μN to mN thrust range, near instantaneous switch on/switch off capability, and high-resolution throttleability (better than one part in 104), which enables accurate thrust modulation in both continuous and pulsed modes.[1] Presently baseline for scientific missions onboard drag-free satellites, this propulsion system has been also proposed for attitude control and orbit maintenance on commercial small satellites and constellations.

This type of thruster can accelerate a large number of different liquid metals or alloys. The best performance (in terms of thrust efficiency and power-to-thrust ratio) can be obtained using high atomic weight alkali metals, such as cesium and rubidium (133 amu for Cs, 85.5 amu for Rb). These propellants have a low ionization potential (3.87 eV for Cs and 4.16 eV for Rb), low melting point (28.7 oC for Cs and 38.9 °C for Rb) and very good wetting capabilities. These features lead to low power losses due to ionization and heating and the capability to use capillary forces for feeding purposes (i.e. no pressurised tanks nor valves are required). Moreover, alkali metals have the lowest attitude to form ionized droplets or multiply-charged ions, thus leading to the best attainable mass efficiency. The actual thrust is produced by exhausting a beam of mainly singly-ionized cesium or rubidium atoms, produced by field evaporation at the tip of the emitter.

An accelerating electrode (accelerator) is placed directly in front of the emitter. This electrode consists of a metal (usually stainless steel) plate where two sharp blades are machined. When thrust is required, a strong electric field is generated by the application of a high voltage difference between the emitter and the accelerator. Under this condition, the free surface of the liquid metal enters a regime of local instability, due to the combined effects of the electrostatic force and the surface tension. A series of protruding cusps, or “Taylor cones” are thus created. When the electric field reaches a value in the order of 109 V/m, the atoms at the tip of the cusps spontaneously ionize and an ion jet is extracted by the electric field, while the electrons are rejected in the bulk of the liquid. An external source of electrons (neutralizer) provides negative charges to maintain global electrical neutrality of the thruster assembly.

The Slit Emitter

Liquid metal ion sources (LMIS) based on field ionization or field evaporation were introduced in the late ‘60s and quickly became widespread as simple, cheap ion sources for a number of applications. In particular, the use of LMIS operated on gallium, indium, alkali metals or alloys has been standard practice in secondary ion mass spectrometry (SIMS) since the ‘70s.

While there exist different field emitter configurations, such as the needle, the capillary and slit emitter types, the principle of operation is the same in all cases. In the slit emitter, for example, a liquid metal propellant is fed by capillary forces through a narrow channel. The emitter consists of two identical halves made from stainless steel, and clamped or screwed together. A nickel layer, sputter deposited onto one of the emitter halves, outlines the desired channel contour and determines channel height (a.k.a. slit height, typically 1 - 2 μm) and channel width (a.k.a. slit length, ranging from 1 mm up to about 7 cm).

The channel ends at the emitter tip, formed by sharp edges that are located opposite a negative, or accelerator, electrode, and separated by a small gap (about 0.6 mm) from the emitter tip. An extraction voltage is applied between the two electrodes. The emitter carries a positive potential while the accelerator is at negative potential. The electric field being generated between the emitter and accelerator now acts on the liquid metal propellant.

The narrow slit width not only enables the capillary feed, but, when combined with the sharp channel edges directly opposite the accelerator, also ensures that a high electric field strength is obtained near the slit exit. The liquid metal column, when subjected to this electric field, begins to deform, forming cusps (Taylor cones), which protrude from the surface of the liquid. As the liquid cusps form ever sharper cones due to the action of the electric field, the local electric field strength near these cusps intensifies. Once a local electric field strength of about 109 V/m is reached, electrons are ripped off the metal atoms. These electrons are collected through the liquid metal column by the channel walls, and the positive ions are accelerated away from the liquid through a gap in the negative accelerator electrode by the same electric field that created them.

Slit emitters had been developed to increase the emitting area of the thruster in order to yield higher thrust levels and to avoid the irregular behaviour observed for single emitters. The substantial advantage of slit emitters over stacked needles is in the self-adjusting mechanism governing the formation and redistribution of emission sites on the liquid metal surface according to the operating parameters; in a stacked-needle array, on the contrary, the Taylor cones can only exist on the fixed tips, which pre-configure a geometrical arrangement that can only be consistent with one particular operating condition.

Slit emitters with a wide variety of slit widths have been fabricated; currently, devices with slit widths between 2 mm and 7 cm are available. These devices, spanning a thrust range from 0.1 μN to 2 mN, are operated with cesium or rubidium.

References

  1. ^ Marcuccio, S., Genovese, A., Andrenucci, M., “Experimental Performance of Field Emission Microthrusters”, Journal of Propulsion and Power, Vol. 14, No. 5, September-October 1998, pp. 774-781.

External links

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.