## 3 Results and Discussion
### Processing Patterns  
The SEY characterizations of untreated Mo (green curve), laser engineered Mo samples #1 (light blue curve), #2 (orange curve), #3 (crimson curve), #4 (pink curve), #5 (purple curve), #6 (dark blue curve) and #7 (fluorescent green curve) are shown in Figure 2. The \(\delta_{\mathrm{max}}\) of untreated Mo is 1.46 in this study, while the one of Mo samples reported by other researchers varied between 1.0-1.3 [19; 20; 21; 22]. The SEY difference was associated with surface chemical states, primary electron doses, and so on. Samples #1 and #3 were processed by the laser with the same laser parameters except for the hatched pattern and scanning speed. The cross hatched pattern was used for the laser processing of sample #1 and #2, while the line hatched pattern was for samples #3 and #5. The \(\delta_{\mathrm{max}}\) of sample #1, #2, #3 and #5 decreased from 1.46 to 1.13, 1.11, 0.96 and 1.35, compared with the one of non-treated Mo sample. At a low scanning speed (100 mm s\({}^{-1}\)), the \(\delta_{\mathrm{max}}\) of sample #1 with the cross hatched pattern was higher than that of sample #3 with the line hatched pattern when the primary energy (\(E_{\mathrm{p}}\)) ranged from 100 eV and 3000 eV. While, at a high scanning speed (1000 mm s\({}^{-1}\)), the \(\delta_{\mathrm{max}}\) of sample #2 with cross hatched pattern was lower than that of sample #5 with the line hatched pattern at \(E_{\mathrm{p}}\leq 1800\) eV \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Hatched** & **Average** & **Spot** & **Pitch** & **Scanning** & **\multirow{2}{*}{**\(\delta\)max**} & **Emax** \\  & **Pattern** & **Power/W** & **/\(\mu\)m** & **Spacing/\(\mu\)m** & **Speed/mm s\({}^{-1}\)** & & **/eV** \\ \hline \#1 & Cross & 10 & 15 & 15 & 100 & 1.13 & 400 \\ \#2 & Cross & 10 & 15 & 15 & 1000 & 1.11 & 2400 \\ \#3 & Line & 10 & 15 & 15 & 100 & 0.96 & 1500 \\ \#4 & Line & 10 & 15 & 20 & 100 & 0.95 & 2000 \\ \#5 & Line & 10 & 15 & 15 & 1000 & 1.35 & 300 \\ \#6 & Line & 13.3 & 15 & 15 & 100 & 0.82 & 1200 \\ \#7 & Line & 13.3 & 15 & 15 & 1000 & 1.07 & 300 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Laser parameters of laser treated Mo samples. Here, \(\delta_{\mathrm{max}}\) is the maximum SEY within the certain primary electron energy range investigated (100–3000 eV in this study) and \(E_{\mathrm{max}}\) is the primary electron energy corresponding to the maximum SEY. and higher at 1800 eV \(\leq E_{\rm P}\leq 3000\) eV. Therefore, with the SEY less than one at 100 eV \(\leq E_{\rm P}\leq 3000\) eV, the laser parameters of sample #3 were appropriate for secondary electron suppression. The surface topography images of samples #1, #2, #3 and #5 are shown in Figure 3. Figure 3(a2-g2) suggest that the special topographies of laser treated Mo were similar to the flower of allium giganteum regel. Because the repetition rate was 20 kHz and the scan speed was 1000 mm s\({}^{-1}\), the distance between two spots was 50 \(\upmu\)m. Therefore, the distance of samples #2, #5 and #7 in the SEM micrographs was much larger than those of the others. Comparing to the \(\delta_{\rm max}\) of untreated Mo, the ones of samples #1, #2, #3 and #5 were lower. It was speculated that this kind of micro size allium giganteum regel like structures could to some extent capture the secondary electrons based on the geometric effect. However, the area of this special structure was part of the whole sample surface. Different laser parameters produced different area ratios of this allium giganteum regel like structure. Subsequently, the effect of this structure on SEY reduction varied with the laser ablation parameters. Sample #1 was processed by a laser with a cross pattern, while the SEM micrograph of this sample in Figure 3(a1) shows the traces of the line hatched pattern. This can be explained by the finding that transverse processing traces were overlapped by the longitudinal traces at a relatively high average power and a low scanning speed.