Experimental Study of Soft X-ray Emission from Pd-Like Xe Pumped by Capillary Discharge

Abstract

Under different pressures, the temporal evolution and spectra of soft X-ray emission from a capillary discharge xenon plasma were measured with an X-ray diode and spectrometer, respectively. Transition lines from Xe 8+ to Xe 11+ were clearly observed and showed up as small spikes in the temporal profi le. Within the 5-20 nm wavelength region, the spectra originating from transitions in the Xe 8+ to Xe 11+ ion species were observed bet ween 9 and 19 Pa. Among these spectral lines, the 16.47 nm line arises from the transition between the 4d 9 5p 1 P 1 level and the ground level of Pd-like xenon (Xe 8+ ). The 4d 9 5p 1 P 1 level is the lower level of the 41.8 nm laser transition of 4d 9 5d 1 S 0 -4d 9 5p 1 P 1 . However, lasing output at 41.8 nm was not observed. According to our analysis, in order to realize the 41.8 nm lasing output, it is necessary to increase the electron temperature and the abundance of Pd-like Xe ions in the capillary discharge plasma.Introduction. Soft X-ray lasers pumped by capillary discharge have been demonstrated to be a promising scheme that meets practical requirements in applications [1]. Ever since the fi rst demonstration of a discharge pumped table-top soft X-ray laser by J. J. Rocca in 1994, there have been realized Ne-like Ar 46.9 [2] and 69.8 nm [3], Ne-like S 60.8 nm [4], and Ne-like Cl 52.9 nm [5] soft X-ray lasers pumped by capillary discharge. The Ne-like Ar 46.9 nm soft X-ray laser was amplifi ed in a highly saturated regime, and its average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz [6]. Ne-like ions have a stable closed shell confi guration, which is favorable to maintain the gain time of the laser.Despite the early success of Ne-like Ar 46.9 nm soft X-ray laser, no soft X-ray laser with wavelength shorter than 46.9 nm has been realized by capillary discharge based on the electron collisional excitation scheme. A shorter wavelength laser has many advantages such as diagnostics of higher density plasmas, higher resolution metrology, and so on. Therefore, it is of signifi cant importance to realize shorter wavelength lasers with the electron collisional excitation scheme in a capillary discharge plasma. Because of this, we are focusing on soft X-ray lasers at 41.8 nm in Pd-like Xe (Xe 8+ ), which have a stable closed shell confi guration. Based on the collisionally pumped optical-fi eld-ionization (OFI) scheme, many groups have obtained Pd-like Xe 41.8 nm amplifi cation pumped by a femtosecond laser system [7-10]. The saturated amplifi cation of Pd-like Xe 41.8 nm lasers has been realized with the OFI scheme [8]. Compared to the OFI scheme, the capillary discharge scheme has the advantage of longer plasma length, lower cost, and simpler operation. It is believed that a Pd-like Xe 41.8 nm soft X-ray laser can be achieved with higer energy, longer gain lifetime, better beam quality, longer gain length, and higher energy conversion effi ciency.In this paper, by discharging gaseous xenon in a capillary to Z-pinch th...