Experimental study on the effect of capillary inner diameter on 69.8 nm laser generation and analysis of the Z-pinch plasma state

This study investigates the influence of capillary inner diameter on the relative intensity of 69.8 nm laser emission under a main pulse current of 19 kA. Laser output at 69.8 nm was achieved for capillaries with inner diameters of 2.5, 3.0, and 3.2 mm, with the strongest emission observed at 3.0 mm. This work represents the first demonstration of a 69.8 nm laser generated by capillary discharge at a main pulse current other than 12 kA. The relationship between initial gas pressure and laser intensity was experimentally determined for each capillary diameter. The optimal initial pressures for 2.5, 3.0, and 3.2 mm capillaries were 21, 18, and 16 Pa, respectively. As the inner diameter increased, the pressure range for laser generation shifted toward lower values, with the widest operating range observed for the 3.0 mm capillary. No 69.8 nm laser emission was detected for capillaries with inner diameters of 3.5-5.6 mm, indicating that larger diameters are unfavorable for lasing. Theoretical analysis, consistent with experimental conditions, suggests that effective 69.8 nm laser generation occurs at electron temperatures of 113-145 eV and electron densities of (4.4 × 10¹⁷) - (1.3 × 10¹⁸) cm^-3. These plasma parameter ranges provide valuable guidance for realizing 69.8 nm laser emission in future experiments.