| The metal plasma jet is a kind of directionally ejected plasma beam generated by ablating metal electrode materials in vacuum,which can be widely used in industrial applications,such as plasma propulsion.However,due to the lack of understanding of basic physical processes of formation and propagation of metal plasma jet,ejection performance of the produced plasma jet is poor,including low production of metal ions,low directional propagation energy and poor directional ejection performance,leading to an application limitation for the metal plasma jet.In this thesis,I carried out a fundamental research on the formation characteristic and ejection performance of metal plasma jet in a low-current（less than 200 A）pulsed vacuum discharge by combining the research methods of simulation,experimental exploration,theoretical analysis and experimental verification.Firstly,a segmented exposed anode electrode structure was proposed.By using this electrode structure,under the condition of the same initial field for electron emission,effects of electron absorption duration t1and absorption amount by anode on discharge characteristics,generation and ejection performance of the metal plasma jet were investigated.Study results showed that in a vacuum discharge,the breakdown between cathode and anode could be triggered only if the electrons were absorbed by the anode.And ejection performance of a plasma jet could be optimized by adjusting amplitude of electron absorption duration t1.Additionally,it was found that when an exposed anode structure（EAS）was used,a large number of charged particles entered the anode,leading to a lost in system energy,and ejection performance of plasma jet could be improved by blocking charged particles from entering anode.Based on the EAS,two methods were proposed to hinder charged particles from entering the anode:1)to reduce exposed area of the anode;2)to connect a resistor in series to the anode side of discharge electrode.In order to further prevent charged particles from entering anode,an insulated anode structure（IAS）,in which the electrical channel of charged particles into the anode during discharge was blocked,was raised.Results revealed that compared with the EAS,amount of plasma ejected from the electrode was increased,but it became more difficult to trigger the electrodes.The amplitude of cathode current was decreased,and total production of plasma generated from cathode was reduced.Furthermore,an insulated anode structure with a micropore（IASM）was put forward.Compared with the EAS and the IAS,ejection performance of plasma jet ejected from electrode was effectively improved without affecting trigger by using the IASM.Secondly,based on the IASM,the phenomenon of"double plasma jets",where one formed at the nozzle of insulating sleeve,and the other formed at the micropore nozzle,in a low-current pulsed vacuum discharge was observed for the first time.It made a breakthrough in people’s understanding of basic physical processes in vacuum discharge through this special vacuum phenomenon,and it also indirectly proved that in a low-current vacuum discharge,the anode not only passively collected various particles from the cathode,but also can accelerated plasma near the anode to form a plasma jet.Combining with methods of simulation,theoretical analysis and experimental verification,a"Double Hump Peaks"model which described the potential distribution between cathode and anode was proposed,reasonably explaining the"double plasma jet"phenomenon.According to the"Double Hump Peaks"model,the potential distribution between cathode and anode presents“double peaks”in a vacuum discharge,and the double plasma jets near the cathode and the anode are formed by the acceleration of potential peak near the cathode（Hump C）and that near the anode（Hump A）respectively.Micro-cathode arc thruster（μCAT）is a kind of electromagnetic thruster that utilizes a directionally ejected plasma jet to produce thrust.It has compact structure,small power consumption,and can produce a high specific impulse and precisely controllable thrust,making it an attractive propulsion system for the micro-satellites.In this thesis,based on the fundamental research results,some kinds of innovativeμCAT structures with better propulsion performance were put forward step by step,including aμCAT using a trumpet-shaped insulated anode with double micropores,a double-ejectionμCAT using double insulating sleeves,and aμCAT using a segmented insulated anode with a slit,et al.Eventually,peak thrust and thrust-to-power ratio of aμCAT were increased by one order of magnitude respectively.Study results of this thesis will provide important theoretical and technical support for further understanding of basic physical processes in a vacuum discharge,generation of high-performance metal plasma jet and its practical application in industrial fields,such as micro cathode arc thruster,et al.This thesis includes 132 figures,26 tables and 202 references. |
