## 1 Introduction  
The space-borne gravitational wave detection mission requires several satellites which form several interferometer arms in space. The satellites are linked by bidirectional laser interferometers to measure the micro-position variations between the test masses (TMs) in the satellites [1]. In order to meet the required measurement resolution, the non-conservative forces from the space environment must be shielded to realize the free-flying of the TMs; therefore, the drag-free control system is required [2]. The drag-free control system compensates for the non-conservative disturbance with the micro propulsion system, to make the satellite track the TM in real time and minimize the relative displacement between them [3]. As shown in Figure 1, inertial sensors are applied to measure the real-time relative displacement between the satellite and the TM, and feedback the parameter signal to the controller [4]. The controller then calculates the required thrust and sends control demands to the micro propulsion system. The microthruster generates fast and accurate thrust to compensate the non-conservative force and eliminate the relative displacement. Since the microthruster is the actuator of the drag-free control system, the performance of the control system is limited by the ability of the thruster [5]. In that case, the microthruster is a key technology of the drag-free control system [6]. Figure 1: Drag-free control system. The gravitational wave detection satellites are affected by 2-30 \(\upmu\)N level disturbing influence [7] like fluctuating solar radiation pressure, solar wind and photon pressure. During the mission, the variation would be as small as 0.1 \(\upmu\)N with the change of the satellites' motion and attitude. In addition, the requirements of the laser instruments demand a thruster noise of no more than 0.1 \(\upmu\)N. The low-frequency gravitational wave detection mission demands long timescale (1-10,000 s) measurements, which leads to a strong requirement on the thruster lifetime [8]. For example, the LISA (Laser Interferometer Space Antenna) mission [9], put forward by European Space Agency, requires the microthruster to provide 5-30 \(\upmu\)N thrust with resolution better than 0.1 \(\upmu\)N, a thrust noise below 0.1 \(\upmu\)N/Hz\({}^{1/2}\) and a lifetime more than 35,000 h [10]. The microthruster development began in the 1960s [11], when research was mostly about the colloid thruster (also called the electrospray thruster) [12] and the pulsed plasma thruster (PPT) [13]. Several microthruster types were successfully applied on small satellites, which majorly operated the attitude control system. In 1964, the drag-free control system concept was raised, which made it possible to conduct high-precision science measurements on orbit [14]. The requirements of high accuracy, low noise, fast response, and wide range thrust were also raised on the microthrusters. With the LISA started in 1993 [15], and the TianQin mission [16] and Taiji mission [17] started in 2014 and 2016, respectively, selections of different microthruster types were ongoing. After the cold gas thruster [18] and the colloid thruster [19] operated successfully on the LISA Pathfinder in 2017 [20; 21], LISA issued four candidate microthruster types, which could possibly be used in the gravitational wave detection mission. These were the cold gas thruster, the colloid thruster [22], the radio frequency ion thruster [23] and the cusped field thruster (HEMPT, High Efficiency Multistage Plasma Thruster) [24], as shown in Figure 2 and Table 1[25]. The cusped field thruster is a novel concept of electric propulsion device [26] with the advantages of an extremely large thrust throttle ability (more than three orders of magnitude), low complexity, strong robustness and a long lifetime (more than 18,750 h) [27].