Studies On 2.45 GHz Electron Cyclotron Resonance 5 Cm Ion Thruster With Cross Magnetic Field

Microwave electron cyclotron resonance（ECR）ion thruster,as one kind of electric thrusters,has advantages of both magnetic confinement and ECR heating,leading to a high ionization efficiency in a low-pressure small-size chamber.Therefore,microwave ECR ion thrusters have been attractive for attitude and orbit control of micro spacecrafts.Due to the limited power that can be allocated to ion thrusters of micro spacecrafts,it is of significance to study the discharge breakdown power of microwave ECR ion thrusters.However,existing investigations on microwave ECR discharge breakdown focus on gas phase processes but ignore surface ones.In magnetized plasmas of microwave ECR ion thrusters,different combinations of antenna and magnetic field affect microwave wave mode,propagation,cut-off and absorption,and then influence plasma characteristics and thruster performance.Simultaneously,they also give rise to physical problems to be studied.When ions are extracted from gridded ion sources,crossover and direct impingements occur in low and high plasma density regimes,respectively.However,reported investigations are limited to ion sputtering etching of accelerator grid,ignoring the secondary electron emission of accelerator grid.Aiming at the above problems,such as discharge breakdown,wave mode excitation,plasma characteristics,secondary electron emitted from acceleration grid and so on,further investigations are conducted in this thesis.The cross magnetic field,ion extraction grid and microwave antenna for the 2.45 GHz ECR 5 cm ion thruster were designed and optimized.Eight arc-shaped magnets were selected to form a closed magnet cylinder,each of which was magnetized in a parallel direction,generating a relatively smoother confinement magnetic field to avoid the weak magnetic field at the magnet gap.The ECR region near the end face was shielded by inserting a thin aluminum plate,so that the local discharge therein was suppressed.The double grid ion extraction system was designed and optimized by numerical calculation using ion optics software IBSimu.It was found that there is a common problem in the existing ion optics software:the breakdown voltage of the ion extraction system depends on parameters such as grid gap,but it can not be calculated by the program.Therefore,an optimization method of interactive iteration between calculation and experiment was proposed.In preliminary experiments on ECR discharges driven with rod antenna,arcing events were found to occur at the front end of the rod antenna as the microwave power exceeded a threshold in the case of ion extraction.After the triggering mechanism was revealed,the arcing was successfully suppressed by replacing the rod antenna with a disc one of a larger area.Under the circular plate antenna,plasma simulation was conducted by COMSOL to explain two opposite discharge profiles observed in experiments.In gas breakdown experiment of microwave ECR ion source with circular plate antenna,the microwave breakdown power(P_wb)was found to not only depend on gas pressure,but also varies with the time interval(t_in)between two successive breakdowns.The measured P_wbdropped rapidly from a high value at a short t_into a low level at a long t_in.The obtained dependence of P_wbon pressure（gas flow rate）exhibited distinct features:the normal monotonicity and abnormal non-monotonicity at the short and long t_invalues,respectively.The effective zone affecting the surface process was a 10 mm wide circular zone at the edge of the antenna,which was validated by:（1）masking the antenna with a film having a variable radius;（2）calculating the distribution of the vertical component of the microwave electric field with respect to the static magnetic field by using COMSOL;（3）imaging glows of transient breakdown discharges with a fast camera.The effective zone on the antenna’s surface was actually what can be bombarded by hot electrons heated in the ECR layer during the breakdown phase.On the basis of analysis of surface physical processes,the sub-monolayer gas adsorbed on the molybdenum antenna surface was assured to reduce the work function.The reduction in P_wbwas mainly attributed to the enhanced emission ofδ-electrons from the gas-adsorbed antenna under the bombardment of energetic electrons coming from the ECR layer.The correlation among the value of t_in,the temperature of antenna,the dynamic gas coverage and the emission coefficient ofδ-electron was established to understand the abnormal ECR breakdown features.Under the condition that the gas coverage on the antenna surface was saturated,the variation of P_wbwith the antenna radius was studied experimentally,and the optimal antenna radius for the lowest P_wbwas obtained.The principle of antenna optimization for reducing P_wbwas determined by comparing and analyzing of the antenna radii corresponding to the experimentally obtained lowest P_wb,the calculated minimum of 1/E_⊥and 1/（E_⊥/|dB/ds|）in the ECR layer,respectively.In ion extraction experiments with microwave ECR ion thruster excited by circular plate antenna,two unreported jumps in ion beam current（I_b）were observed in low and high microwave power regions,respectively.The conducted investigations are as follows.（1）The abrupt jump of I_b in the low microwave power region is always accompanied by a sudden radial-expansion of discharge glow.Based on analyses about radial gray profiles of end-view images captured from discharge glows under different microwave powers,influences of standing wave and the skin effect in ordinary-wave discharges are determined.The fast evolution of the radial plasma-profile is attributed to the counteraction of standing wave and the skin effect,the coincidence of enhanced microwave absorption in the electron plasma resonance layer,and the transverse magnetic confinement of electrons.The jump of I_band the sudden radial-expansion of discharge glow are confirmed to be originated from the extraordinary-wave discharge ignited at the ECR layer beyond the plate antenna when the skin effect is dominant.（2）The jump of I_b in high microwave power region was attributed to ion extraction,because there is no corresponding jump in microwave discharge without ion extraction.Energeticγ-electrons,emitted from the screen grid due to crossover ion impingement in the central region of a low plasma density induced by the skin effect of the ordinary wave,intrigue the jump of I_b via a positive feedback loop consisting of the subsequent processes,such as warmδ-electron emission from the antenna’s surface under bombardment of energeticγ-electrons,increments in the voltage drop across the two sheaths formed at the antenna and the screen grid,and the enhanced electrical confinement of cold electrons.Energeticγ-electrons from the accelerator grid and warmδ-electrons from the opposite antenna do not produce direct excitation and ionization,but they enhance the electrical confinement of cold electrons by elevating the voltage drop across the sheaths at the antenna and screen grid,thus leading to the jump of I_b.The presence of energetic secondaryγ-electrons was confirmed by the pattern appearing on the temperature sticker exposed to the discharge after the jump of I_b.The indirect evidence for the presence of warmδ-electrons and their influence on the screen grid sheath were provided by using ion energy analyzer to measure the plasma potential in the radially central region before and after the jump of I_b in the high discharge power region.The energeticγ-electrons-based model can explain not only the jump of I_b in this thesis,but also the abnormal results observed in other gridded ion sources.In the experimental optimization for circular plate antenna,the maximal ion beam current was attained in the medium radius region.The maximal I_b obtained at the optimized antenna is higher than those of the counterparts under the same condition.Comparing the optimized antenna radii for the lowest P_wband the highest I_b,it is known that the two optimizations can not be simultaneously realized at the same antenna radius,and a compromise has to be made depending on the specific operation condition.In summary,the cross magnetic field of the 2.45 GHz microwave ECR 5 cm ion thruster was optimized,and the effects of ordinary wave,extraordinary wave,sub-monolayer gas adsorbed on a solid surface,energeticγ-electrons from accelerator grid and antenna radii on discharge breakdown,maintenance in chamber and ion extraction were investigated in the thesis study.The results of the study deepen the understanding of microwave ECR ion thruster with cross magnetic field,in which the effects of energeticγ-electrons from accelerator grid on the discharge in chamber is common to gridded ion sources and contributes to the overall analysis of gridded ion source characteristics.