## 4 Experimental Results and Their Discussions  
Based on the definition of far-field spot size with 86.5% surrounding energy, the far-field divergence angle \(\theta\) can be written as \(\theta\) = 2 \(r/f\). Here \(r\) is radius of the circle containing 86.5% of the total energy, and \(f\) is the focal length of the focusing mirror. There are two different reference standards for the reference value of total laser energy in the focal plane: one with the aperture 8 fully opened (\(\varphi\) = 27.5 mm) and the other with the aperture 8 removed. Then, we gradually reduced the aperture size, measured the laser energy, and calculated the percentage compared with the reference value. We performed comparative measurements of radiation divergence for these two types at \(M\) = 1.65 and the results are presented in Figure 6.  
Due to the existence of stray light in the cavity, the total energy of aperture fully opened was slightly lower than that without the aperture, and the difference was between 2% and 7%. Because of the large defecting angle, this stray light is ignored in far-field applications. In the following parts for \(M\) = 1.85, 2.05, and 2.25, we just consider the type with aperture fully opened when measuring the total energy. The far-field divergence angle for these two types at \(M\) = 1.65 was 1.08 and 1.19 mrad, respectively, and the energy distribution in the experiment was similar to that of the calculation. The maximum energy at 1 m distance from the laser exit was 2.76 J. Figure 7 shows the near-field spots at 1, 6, and 10 m distance from the laser exit and far-field spot at the focal plane recorded by the heat-sensitive paper and organic glass. As we can see from Figure 7, the near-field spots had good symmetry in both the horizontal and vertical directions. The near-field spots had a round shape in the vertical direction and straight edges in the horizontal direction. The size of the ring-shaped near-field spot at 1 m was about 46.5 \(\times\) 48.1 mm in the horizontal and vertical directions, and the radius of the center hole was about 30.2 mm, which was the same as the size of the convex mirror. The straight edges in the horizontal direction are caused by the interception of light beam by electrodes. The size of the back mirror of the unstable cavity was 50 mm, while  
Figure 6: Normalized energy distribution of laser emission as a function of the angle embraced by the aperture. the distance between discharge electrodes was 40 mm, so light beams in the horizontal direction were partly cut by the electrode edges. A straight-edge diffraction appeared in the near-field spots at 1 m, which is also caused by the electrode diffraction at the edges. As the distance increased, the ring hole diffraction pattern became more obvious. Because the power intensity decreases with the propagation distance, the near field spot at 10 m had poor clarity compared with spots at 1 m and 6 m. The far-field ablation spot distribution on the heat-sensitive paper was relatively uniform at center, but the vertical size was larger than the horizontal size at the external circle. This can be eliminated by applying a back mirror with a diameter smaller than the electrode gap, which would decrease the total output energy. In practical applications, the energy and beam quality should be weighed according to the requirements of applications. The beam divergence and output energy were compared between different magnifications under the same gas parameters, which were SF6: D2 = 8:1 and \(P_{\text{total}}\) = 9.5 kPa. Figure 8 shows the dependence of output energy on the charge voltage under different magnifications. The output energy increases with the charge voltage, and the increase rate at lower voltages is larger than that at high voltages. As we know, for a certain gas pressure, there exists an optimum charge voltage for the active medium to be adequately excited. At lower voltages, too little excitation energy makes the generated excited state DF molecules too small, and thus the laser energy is low.