| Traditional chemical propulsion systems have limited applications on micro satellites due to their low specific impulse, large mass and relatively complicated structures. Field Emission Electric Propulsion thruster (FEEP), a micro electrostatic thruster based on the technology of liquid metal ion sources (LMIS) with low thrust, high specific impulse and high efficiency, has been successfully applied on micro satellites for various missions such as attitude control, station keeping, spacecraft potential control, orbit distracting, correcting and elevating, drag compensating, relocation and disorbiting. A prototype of FEEP thruster was developed in this dissertation, and its operation principle and performance was studied both theoretically and experimentally.To study the operation process, a prototype of needle type indium FEEP (In-FEEP) thruster which consists of a control system, a power supply and the thruster body itself was designed and implemented The electrode distance and operation voltage of the prototype can be adjusted. As a core part of thruster, the emitter tip was fabricated with a tungsten rod by using the electrochemical etching method. The tip was then processed by a series of treatments, such as surface roughening treatment, ultrasonic and chemical cleaning and wetting with liquid indium. An experimental platform was established to measure the running parameters of the indium FEEP thruster, which include emission current, operation voltage and thrust. An indirect target measurement apparatus was established to gauge the thrust (<0.01mN) by measuring the deflection of a thin cantilever beam.The electric field strength near the emitter apex has been found to have effects on the performance of a FEEP. Based on a new shape model of the emitter, which used half ellipsoid rather than hemisphere as the apex model, the electric field strength between the emitter and extractor was calculated from the geometry of electrodes with a spherical coordinate with increasing mesh method. The results show that the radius of the emitter apex has greater influence on the electric field strength than the inner radius of the extractor and the distance between the extractor and emitter.The processes of the liquid metal emission and the liquid metal supply were theoretically modeled and described in detail. It was observed that a stable emission can be achieved when the rate of the liquid metal emission by field evaporation equals to the rate of the liquid metal supply by fluid flow. The thruster operates stably at the equilibrium between the emission and the supply of the liquid metal. The minimum emission current model is referred to as a state in which the emission occurs at the minimum current. To prolong the thruster lifetime, an effective method is to operate the FEEP thruster at the minimum emission current state. The estimation model of the minimum emission current was established based on the equilibrium between the emission and the supply of liquid metal.Experiments were conducted to test the prototype performance and verify the analysis presented in the dissertation. Effects of the design parameters such as operation voltage, emission current, emitting tip radius, distance between the emitter and extractor on the running parameter such as thrust, mass efficiency, specific impulse were investigated. The minimum emission current state of the FEEP thruster prototype was validated.This dissertation has attempted to focus on the operation process and emission mechanism of the FEEP thruster. The results showed that the needle type indium FEEP thruster achieved the design objective with the thrust less than 2?N, the specific impulse larger than 7500s and the mass efficiency up to 95%. The FEEP prototype operates stably when the emission and the supply of liquid indium are at equilibrium. The stable emission covers a range of emission current within 0.4~15?A at 200?. The existence of minimum emission current verified the stable emission mechanism proposed in this dissertation. |
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