| Ablative Pulsed Plasma Thruster(APPT) is one of the promising electric propulsion system, which has some unique features such as low power consumption, light mass, low impulse bit, high specific impulse and simple configuration. It may be a rather attractive propulsion option for orbit maintenance, constellation station-keeping, deorbit and drag compensation of micro-spacecraft.In order to improve propellant utilization efficiency, prolong service life, enhance thruster performance and reliability of APPT, and to explore the interaction mechanism between thruster itself and spacecraft, the operation process and plume characteristics of Pulsed Plasma Thruster was investigated emphatically by means of theoretical analysis, numerical simulation and experimental methods in this dissertation.Based on analysis of complex behaviors and phenomena of PTFE in ablation process, at the same time considering more some details including internal absorption of radiation, reflectivity of material and surface emission, a new one-dimensional ablation model was developed to gain greater insight into the process of propellant ablation. some influences of different discharge characteristics on the propellant ablation characteristic were studied. The results show that the propellant ablation presents different characteristics at different stages of APPT discharge. The discharge current oscillation result in increasing of APPT system energy loss and the amourphous PTFE thickness. Reducing discharge current oscillation extent can effectively decrease the propellant loss caused by particle emission.APPT ground experimental system and devices were built in the study. A kind of parallel-plate APPT was designed and manufactured. The ground power supply system and ignition system were improved. A novel laser-electromagnetic coupling Pulsed Plasma Thruster was designed and realized, whose mass consumptions of propellants per shot was determined by laser ablation. A variety of materials including metals can be used as operating substance of this kind of thruster. PTFE base doping modification propellant was sintered. The method of using pendulum balance to measure microimpulse-bit was improved, and the displacement signal of pendulum balance was measured by position-sensing detector(PSD). A real-time on-line non-contact electromagnetic calibration technique was proposed and used in thrust-stand calibration.Some experimental studies were carried out to investigate the performance of APPT and laser-electromagnetic coupling APPT. Based on theoretical analysis for APPT system performance, the concerned influences and effects of system parameters including initial energy and energy supply mode, electrode spacing, and electrode shape on APPT performance were systematically studied. By fitting and estimating the parameters of discharge loop and APPT performance, some internal mechanism of each parameter influences on thruster performance was found out. The results show that increasing capacitance to enhance the system initial energy, reducing the electrode spacing, increasing angle between electrodes in a certain angle range, and using tongue shape electrode under the same voltage condition, can effectively improve the thruster specific impulse and efficiency. The effects of electrode and propellant material on APPT performance were also invesitgated. Different electrode and propellant materials have certain influence on discharge characteristics and performance of APPT. The most erosion resistance material in all these tested materials turns out to be 70%Wu30%Cu. When 70%Wu30%Cu electrode is used, the impulse bit of APPT is the highest one. However, its specific impulse is the lowest one. When Al90%10% PTFE is used as propellant, its thruster impulse bit and efficiency is gotten to improve. Experimental study on the performance of Laser-electromagnetic coupling APPT shows that the specific impulse of laser-electromagnetic coupling APPT tested is 5970 s, and efficiency is 50.39% when Al is used as the propellant. obviously, the system performance has been improved when comparing with traditional APPT.APPT plume characteristics were measured and diagnosed. A current-mode triple Langmuir probe apparatus was developed and used to measure electron temperature and density in the plume region of APPT. By means of probe array, four quadrupole mass spectrometer and plasma emission spectra, the plasma velocity distribution characteristics and the plasma composition changes of APPT at different initial energy were studied. The results show that the electron density and electron temperature decrease with increasing distance from thruster exit and with increasing angle away from the thruster centerline in the APPT plume. In the perpendicular plane, plasma has a non- symmetrical distribution characteristics with the plume canted towards the cathode electrode, while in the plane parallel plate, plasma has a relative symmetrical distribution characteristics. With increasing of initial energy, relative content of plasma composition presents different trends in different energy range. With increasing of initial energy, relative content of neutral component is gradually reduced, while relative content of ionized component is increased.APPT plume contamination characteristics were studied experimentally. The morphology of films deposited were analyzed by scanning electron microscopy(SEM) and scanning probe microscope(SPM). The film structure and composition were investigated by Fourier transform infrared absorption spectroscopy(FTIR) and X-ray photoelectron spectroscopy(XPS). The optical properties of films were studied by Ultraviolet-visible light spectrophotometry. The effects of initial energy and axial distance on the contamination characteristics were analyzed. The results show that low fluorine-carbon ratio fluorocarbon films with different structural and optical properties were deposited by Pulsed Plasma Thruster. Some changes in density of the neutral particle and flux of ions that reach the surface of deposited substrates lead to different trends with respect to various structural units and furtherly result in different microstructure of the films deposited at different positions. Surface roughness and chemical structure influence the optical properties of films. Different transmittance and reflectance were observed at different positions. In the perpendicular plane, the deposited film microstructure and optical properties presented different trends in different regions with 30 degree angle as the boundary. The films deposited on the anode side with compared to the films deposited on the cathode side, have relatively small fluorine-carbon ratio, lower films transmittance and higher reflectance. The fluorine-carbon ratio and transmission of the films deposited in the parallel plane is relatively large with compared to the films deposited in the perpendicular plane. Due to the change of plasma characteristics and the content of ionization groups, the chemical structure and optical properties of these films deposited at different initial energy and different spatial location present different trends and characteristics. The change of optical properties has better reflect to the change of microstructure of films deposited under specific conditions such as different space orientation and different energy level. |
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