## 1 Introduction  
Microwave devices working in the space environment are constantly influenced by the irradiation from the universal rays and particles. The phenomenon of secondary electron emission can be induced when space particles or rays irradiate the material surface. The excited secondary electrons may undergo resonance avalanche multiplication under the action of high-power radio frequency (RF) fields, further inducing the secondary electron avalanche multiplication discharge effect, namely, multipactor [1]. The large number of electrons generated inside the device in a short duration is able to severely interfere with the normal running of the high-power microwave (HPM) systems, and may even cause discharge breakdown [1; 2; 3; 4]. Nowadays, multipactor becomes one of the severe bottleneck problems that obstruct the power increase in microwave devices [4]. Therefore, multipactor threshold is one of the key factors in evaluating the reliability of space HPM systems. There are three essential conditions for the multipactor occurrence, namely, the secondary electron emission yield (SEY) of the irradiated surfaces bigger than 1, the existence of RF fields, as well as, the resonation between secondary electrons and the RF field [1]. Generally speaking, destroying the resonation condition can be considered an effective method to mitigate multipactor [5]. In addition, lowering the SEY of the multipactor sensitive surface can decrease the risk of multipactor since secondary electron avalanche is the direct reason for the multipactor discharge [6; 7; 8; 9]. In other words, a higher SEY always results in a lower multipactor threshold. Currently, the common methods for lowering SEY include coating low-SEY films, such as TiN, TiZrV, and graphene [10; 11; 12; 13; 14; 15; 16], as well as designing surface microstructures to trap the secondary electrons [17; 18; 19; 20]. Alumina and polytetrafluoroethylene (PTFE) have been widely used in high-power microwave systems in space. The multipactor phenomenon for these two dielectric materials has been paid more attention to nowadays since many HPM devices have been developed with dielectric loaded [21; 22; 23]. In addition, the dielectric materials possess a relatively higher SEY [24; 25; 26], in order that the multipactor may be easier to be triggered. In recent years, the construction of periodic microstructures on the surface of dielectric materials has received widespread attention and discussion [5; 27; 28]. These surface microstructures are capable of blocking the movement of secondary electrons while not affecting the normal function of the device, allowing for fewer electrons to participate in the multiplication process. The current studies involve the multipactor of microstructured surfaces including grooves [29; 30], columnar arrays [31; 32; 33], and porous structures [17; 18; 19]. Typically, researchers use feature size parameters to quantify these microstructures for discussion. However, scholars are more concerned with a detailed discussion of the secondary electron emission process under microstructures and choose to apply the SEY to predict the ability of the material to suppress multipactor. The relationship between feature size parameters of the microstructure and multipactor threshold is not adequately studied, and the relevant mechanisms still require further discussion. Traditionally, the multipactor threshold relied on charts obtained through experimental studies. However, with the development of particle simulators, accurate predictions can be quickly and conveniently provided. In this paper, we focus on the groove structure and design a coaxial low-pass filter with dielectric sheets loaded as a test device for multipactor. The filter has an ideal flat gap and perpendicular RF electric field to the surfaces.