SPACECRAFT PROPULSION - KTH

- 45 -To the above described category of ion thrusters, theStationary Plasma Thruster (SPT), which belongs to thecategory of Hall-effect Thrusters, uses an applied magneticfield to control electrons in a quasi-neutral plasmadischarge.+_Power SupplyElectron EmitterPositiv Ion BeamPropellant SupplyIoniserIoniser Exit GridAccelerator GridFigure 15: Schematic of Ion ThrusterFinally, as an example of liquid propellants for electrostaticelectric propulsion, Caesium (Cs) is the propellant of choicewith a melting point of 29°C. Caesium, as a liquid metal, isalso desirable because it has a high atomic mass andeffectively wets metal surfaces. It is used for field emissionthrusters, or Field Emission Electric Propulsion (FEEP)devices.In a FEEP thruster, a strong electric field is established atthe tip (tailored cones) of a pair of closely spaced electrodes,which even form a capillary propellant feed system for theliquid caesium. See also the schematic of the FEEP thrusterwhich is shown below.