## 4 Conclusions  
The cusped field thruster is a candidate microthruster for the gravitational wave detection mission with advantages of low complexity, potential long life and large throttling ability on thrust. Besides the GW mission, the cusped field thruster also shows competitiveness for small satellite missions and space science missions demanding a drag-free system. This paper presents a brief review of the research on cusped field thrusters at Harbin Institute of Technology since 2012, including efforts on low power prototype iterations, structure configuration optimization, physical mechanism study, thruster downscaling and preliminary investigation of the microwave discharge cusped field thruster. Prototypes on thrust level from 100 mN to 7 mN are experimentally investigated, and the general large throttling ability is verified. A series of experiments and simulations are carried out to study the operation mechanism, the component effects and the optimization methods of the cusped field thruster. Results show that:  
1. A higher downstream magnet ratio leads to flatter exit magnetic separatrix and extension of the ionization region, and, as a result, more focused plume and better performance. 2. A higher magnetic field intensity gives rise to more effective confinement of electrons, and a reduction of propellent utilization due to ionization region narrowness. 3. The main potential drop locates near the channel exit in a cusped field thruster, which makes the exit magnetic separatrix a determinant factor of plume divergence. Thus, plume divergence reduction is achieved through optimization on it. 4. The variable cross-section is applied to change the mass flow flux distribution in the channel, and a spacer with appropriate location contributes to better total performance by enhancing the ionization in the dominant region. 5. Two electron conduction routes are found in the cusped field thruster. The competition between the two electron routes, as well as the two ionization regions, is the intrinsic reason for the mode transition phenomenon. Cusped field thruster downscaling works are carried out, including investigations on thruster geometry, expanding cross-section configuration and channel wall materials. Results show that:  
1. Size effects are significant in cusped field thrusters, but diameter reduction is still necessary for downscaling because of the significant mass flow flux difference. 2. An equilibrium on energy efficiency and propellant utilization should be considered to find out the optimal channel length for a cusped field thruster. 3. The equilibrium on electron temperature and electron loss should also be considered to find out the wall material with an appropriate SEE coefficient. 4. Expanding cross-section configuration contributes to a wider operation range and better mode stability. Figure 24: Thrust variation with anode voltage and mass flow rate (**a**), and thrust variation with micro-wave power and mass flow rate (**b**) in a MW coaxial resonant cusped field thruster. The DC discharge cusped field thruster is unlikely to meet the ultimate requirements of GW detection mission. A novel concept of the microwave discharge cusped field thruster is carried out and experimentally investigated. ECR is successfully induced in a low power cusped field thruster, and the ionization enhancement is verified on low mass flow rate conditions. Using a coaxial transmission line resonator (CTLR), the second-generation microwave discharge cusped field thruster performs a 1.9 to 30.8 uN thrust with an extremely low mass flow rate of 0.04 sccm. Further studies and efforts for optimization of the thruster is to be done in future works. Conceptualization, H.L. and M.Z. ; validation, H.L., M.Z., X.N., H.H. and D.Y. ; formal analysis, M.Z. ; writing--original draft preparation, M.Z. ; writing--review and editing, M.Z. ; visualization, M.Z. ; supervision, H.H. and D.Y. ; project administration, H.L. and D.Y. ; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.