Characteristics of gain in Ne-like Ar 69.8 nm laser pumped by capillary discharge based on double-pass amplification

Abstract

In this paper, a double-pass amplification experiment of a Ne-like Ar C line 69.8 nm laser is established. The 45-cmlong capillary is used as the discharge load to obtain a double-pass amplification output of a Ne-like Ar C line 69.8 nm laser. Under the same initial experimental conditions that the initial pressure is 15.4 Pa and the main pulse current amplitude is 13.5 kA, the laser pulse intensity and the full width at half maximum (FWHM) of the laser pulse of the single-pass amplification output and the double-pass amplification output are measured by a vacuum X-ray diode (XRD) behind a vacuum ultraviolet (VUV) monochromator (Acton VSN-515) which is used to disperse the extreme ultraviolet (EUV) emission. And then the laser beam divergence of single-pass amplification output and double-pass amplification output are also measured by a space-resolving flat-field EUV spectrograph combined with an EUV CCD (Andor Newton DO920P-BN). The amplitude of the double-pass amplification laser output is 9 times larger than that of single-pass amplification output, and the FWHM of the double-pass amplification laser pulse is nearly 2.4 ns. While the laser beam divergence angle of the double-pass amplification output is 6.6 times wider than that of single-pass amplification output. By comparing the single-pass amplification and double-pass amplification output experimental results, the gain duration of the gain medium in the double-pass amplification and the radial distribution characteristics of the gain medium are analyzed by using the calculation formula of the double-pass amplification laser intensity. The gain duration is more than 4 ns, during this time the gain coefficient decreases at 1.6 ns. And the gain coefficient is the smallest at 2.8 ns, meanwhile the intensity of the single-pass amplification laser is maximum, and the gain medium is in the gain saturation state. So this result indicates that the minimum gain coefficient at this moment is due to the gain saturation effect. Using a similar calculation method to analyze the spatial distribution of gain coefficients, the gain on the plasma axis is larger than that off the plasma axis. These results lay a foundation for the subsequent establishment of resonant cavity and the multi-pass amplification experiment of capillary discharge Ne-like Ar laser.