## 3 Results and Discussion
### Average Power  
The SEY characterization of the laser processed Mo samples #3 (crimson curve), #5 (purple curve), #6 (dark blue curve) and #7 (fluorescent green curve) with the line hatched pattern are shown in Figure 2. Samples #3 and #6 were processed by laser with the same laser parameters excluding the  
Figure 3: SEM images of untreated and laser processed Mo samples. (**a1**,**a2**) sample #1, (**b1**,**b2**) sample #2, (**c1**,**c2**) sample #3, (**d1**,**d2**) sample #4, (**e1**,**e2**) sample #5, (**f1**,**f2**) sample #6, (**g1**,**g2**) sample #7 and (**h1**,**h2**) untreated Mo samples. Samples #1 (cross hatched pattern), #3 (line hatched pattern) and #6 (line hatched pattern) were processed with the same scanning speed of 100 mm s\({}^{-1}\). Samples #2 (cross hatched pattern), #5 (line hatched pattern) and #7 (line hatched pattern) were processed by the same scanning speed of 1000 mm s\({}^{-1}\). Samples #1, #2, #3, #4 and #5 were treated with the same laser power of 10 W, while sample #6 and sample #7 were processed by the same laser power of 13.3 W. The pitch spacings of samples #1, #2, #3, #5, #6 and #7 were all 15 \(\upmu\)m and the one of sample #4 was 20 \(\upmu\)m. The other laser ablation parameters of these seven laser treated samples are shown in Table 1.  
average power. The \(\delta_{\rm max}\) of samples #3, #5, #6 and #7 decreased from 1.46 to 0.96, 1.35, 0.82 and 1.07, compared to that of the non-treated Mo sample. When the scanning speeds of samples #3 and #6 were 100 mm s\({}^{-1}\), the shapes of the SEY curves for samples #3 with the average laser power of 10 W and #6 of 13.3 W were very similar. This case happened for sample #5 of 10 W and #7 of 13.3 W with the same laser scanning speed of 1000 mm s\({}^{-1}\). For samples #3 and #6, the SEYs of these two curves increased linearly at \(E_{\rm P}\leq 400\) eV and then flattened off at \(500\leq E_{\rm P}\leq 3000\) eV gradually, as shown in Figure 2. While, the curves of samples #5 and #7 increased linearly at \(E_{\rm P}\leq 3000\) eV and then decreased at \(300\leq E_{\rm P}\leq 3000\) eV. The SEY curves of samples #3, #5, #6 and #7 suggested that the SEY curves of samples #3 and #6 decreased gradually and that of samples #5 and #7 declined to less than 0.98 or even below 0.5 at \(E_{\rm P}\geq 3000\) eV.  
The surface topography images of samples #3, #5, #6 and #7 are shown in Figure 3. It can be seen that the surface topographies of samples #3 and #6 were all covered by sphere/column-like structures. However, the densities of sphere/column-like structures of samples #5 and #7 were lower than that of samples #3 and #6. This may be the major reason why the SEYs of samples #5 and #7 were higher than the ones of samples #3 and #6 in the perspective of the geometrical effect.  
In the operation of a multistage depressed collector, the trajectories of the true secondaries and reflected primaries were significantly different. The true secondary electrons were suppressed by the depressed collector fields, while the reflected primary electrons streamed back to the tube and slow wave circuit. The secondary electron current collected here included the true secondary electrons (typically less than 50 eV) and reflected primary electrons which were inelastic and elastically scattered (typically larger than 50 eV) from the surface with the same energy as the primaries [21; 23]. As indicated in Reference [23], the backscattering coefficient was the ratio of the number of inelastic and elastically scattered electrons with energy typically larger than 50 eV to the number of PEs. In most cases, the backscattering coefficient flattened off with the increase of PE energy for outgassed metals, such as Ni, Pt, Ta, etc., at \(400\leq E_{\rm P}\). While the true secondary electrons yield decreased with the increase of PE energy, at \(400\leq E_{\rm P}\).