## 3 Low Power Cusped Field Thruster Development at HIT
### The Microwave Discharge Cusped Field Thruster  
Significant negative size-effect has been confirmed during downscaling, including performance degradation and operation range limitation [73]. Our previous works show that the size effect is a bottleneck problem that is hard to solve by component geometry and configuration optimization. The ionization is inefficient due to limited space and an enlarged surface-to-volume ratio in a miniaturized cusped field thruster. These facts mean that external methods are necessary to improve the low mass flow rate performance in a \(\upmu\)N-level cusped field thruster. In respect to a \(\upmu\)N-level cusped field thruster, a non-uniform magnetic field is produced in a narrow room, and electron cyclotron resonance (ECR) is considered a competent tool for discharge enhancement [74]. After this, a microwave discharge cusped field thruster was released which satisfied the requirements of gravitational wave detection missions. ECR ion source is an effective low-pressure microwave plasma generator with exceptional stability and repeatability [75]. The first-generation microwave discharge cusped field thruster was designed with a cylindrical antenna located in the near-anode region. Through this antenna, microwave radiation at 2.45 GHz is transmitted into the 6 mm-diameter thruster channel in TEM coaxial mode. Electrons are heated to a high temperature and inelastic collisions are induced. In a cusped field thruster, several ECR surfaces are formed near the magnetic cusps, as shown in Figure 19.  
Figure 19: First-generation microwave discharge cusped field thruster, with several ECR surfaces marked out. Figure 18: (**a**) The \(\upmu\)N-level cusped field thruster in HIT, and its performance on (**b**) thrust operation range and (**c**) thrust noise PSD. The microwave discharge cusped field thruster is able to ignite by both the DC power supply and microwave (MW) power supply. Three operating modes are achieved: ECR&CFT combined mode with both DC and MW on, CFT mode with only DC on, and ECR mode with only MW on, as shown in Figure 20.  
The ECR mode performs a single peak plume with relatively low ion energy from 12 eV to 18 eV, and ions accelerate in the exit magnetic nozzle. Low ion energy makes the ECR mode not effective in generating considerable thrust. In contrast, experimental results show that the combined mode shows similar ion energy to the CFT mode. Significant ionization enhancement is found in the combined mode. With the introduction of microwaves, propellant utilization and thrust are raised several times higher in low mass flow rate conditions, but this improvement tends to decrease remarkably as the mass flow rate increases, as Figure 21 shows. This phenomenon is supposed to result from microwave reflection as the plasma density rises as the mass flow rate increases. When the plasma angular frequency \(\omega_{p}\approx 17.96\tau\sqrt{n_{e}[m^{-3}]}\) is higher than the microwave angular frequency, the microwave is reflected. The reflection lowers the microwave power feeding efficiency in high mass flow rate conditions. Though the feasibility of microwave enhancement in a cusped field thruster is experimentally validated, the first-generation microwave discharge cusped field thruster operates with relatively low performance [76]. The cylindrical antenna takes up the position of the inner electron conduction route, which makes it hard to form a DC discharge. Besides, this configuration makes it hard to insulate the microwave-feeding structure from the anode. However, an improvement idea to make use of a coaxial transmission line resonator (CTLR) [77] was formulated to solve the problem. Figure 21: (**a**) Thrust curves of the three modes in a microwave discharge cusped field thruster. (**b**) Thrust difference, propellant utilization difference, and energy efficiency difference between combined mode and CFT mode. Figure 20: Three operating modes of the microwave discharge cusped field thruster.