## 1 Introduction  
Traveling wave tube (TWT), a vital vacuum electronic device, has been widely used in satellite communications, radar, etc. [1; 2; 3; 4; 5]. The collector efficiency and beam wave interaction efficiency are the two major reasons for the efficiency enhancement of the TWT. The beam wave interaction efficiency is difficult to further enhance since it has reached a certain threshold value. Accordingly, improving collector efficiency will be the essential way for the efficiency improvement of the TWT. The multistage depressed collector, with the function of sorting and collecting interacted electrons based on the electron velocity, is commonly used to improve the collector efficiency. Secondary electron (SE) emission imposes on the working performance of multistage depressed collectors (MDCs), such as the change of collector efficiency, back-streaming rate, etc. Furthermore, the SEs will induce the heat dissipation power, additional noise power and the possible failure of the TWTs.  
Previous studies demonstrated that MDC efficiency can be improved significantly with the secondary electron yield (SEY) of the inner surface of MDCs less than one [3; 6]. In the earlier study [6], it was found that the maximum SEY (\(\delta_{\mathbf{max}}\)) of molybdenum masked ion-textured OFHC (oxygen-free high-conductivity copper) prepared by different ion-texturing parameters varied between 0.30 and 0.55 with the primary electron energy range of 200-2000 eV. [PERSON] et al. [7] demonstrated that the \(\delta_{\mathbf{max}}\) of ion modified OFHC surface changed between 0.64 and 1.00 with the primary electron energy range of 100-1000 eV.  
To improve the service behavior of MDCs, the conventional methods aim to optimize the electrode structure and the potential [8; 9; 10; 11]. For example, [PERSON] et al. proposed a novel non-axisymmetric structure MDC to achieve a low back-streaming rate and high collection efficiency with the advantage of easy processing and simple structure [12].  
Besides, the suppression of secondary electron emission can also enhance the efficiency of the MDCs of vacuum electron devices. In order to reduce the emission of oxygen-free high-conductivity copper (OFHC), the most commonly used material for MDC electrodes, the ion texturing method [3; 6; 13] and various low SEY film coatings such as diamond-like carbon (DLC) [4] and TiC film coatings [14] were employed to obtain a low SEY OFHC surface. [PERSON] et al. studied the preparation and characterization of the molybdenum films deposition on MDC surfaces to reduce SEY of the MDCs surface and improve the total efficiency of TWTs [13]. After Mo deposited on the surface of the copper MDC substrate, the rough structures were formed with the low SEY property [13; 14].  
The laser induced nano- and micro-structures surface technique is an effective way to obtain a low SEY surface [15; 16; 17], with the advantage of ease of processing and excellent stability. Therefore, we proposed that laser treated Mo and copper with low SEY can serve as collector electrodes. The SEY properties of the laser treated OFHC have been studied in the previous literature [16]. Molybdenum was selected because of its higher melting temperature than that of copper and being compatible with furnace-brazing procedures commonly used in the fabrication of MDC and TWT. In this paper, the SEY, surface topography and surface composition of laser treated Mo were characterized and the correlation mechanism of SEY and surface topography were analyzed. Moreover, allium giganteum flower-like biomimetic structures with the property of capturing the secondary electrons based on the geometric effect were found to mainly contribute for the SEY decrease of laser processed Mo.