Study On Capillary Discharge Pumped Pd-like Xe Soft X-ray Emission Spectra

As one of the most promising schemes for realizing table-top x-ray laser, capillarydischarge pumped soft x-ray laser scheme has realized Ne-like Ar46.9nm soft x-raylasing in highly saturated regime. However, capillary discharge scheme has neverrealized x-ray lasing shorter than46.9nm. Lasing at41.8nm in Pd-like Xe ions (Xe⁸⁺)has demonstrated by other groups in the world, it’s driven by femtosecond laser basedon Optical-Field-Ionization （OFI） scheme. Herein the possibility of realizing Pd-like Xe41.8nm lasing by capillary discharge scheme is researched both in theory andexperiment.In theory, the physical mechanisms of capillary discharge pumped Pd-like Xe41.8nmlasing are introduced, including Z-pinch, electron collisional excitation and metastablestate monopole electron excitation. Then Xe⁸⁺energy level parameters relative to4d95d1S0—4d95p1P141.8nm laser transitions are calculated with Cowan physical code, andthe partial energy level diagram is drawn. Particularly，spontaneous radioactive decayrate of16.47nm transmission line from4d95p1P1level and ground level of Pd-likexenon ions is very large. The4d95p1P1level is the lower level of41.8nm lasertransition4d⁹5d¹S₀—4d⁹5p¹P₁. Finally, based on local thermodynamic equilibrium（LTE） model, the SAHA equations are calculated, and the distributions of both theabundance of ions and electron density with electron temperature are obtained. It isfound that the production of Pd-like Xe ions requires relatively low electrontemperature.In experiment, on the capillary discharge device, the temporal characteristic of Xeplasma emission is measured with x-ray diode. With main current pulse of amplitude15kA and rise-time³0ns, the temporal evolution curve of emission between8Pa⁶2Pais measured. A little spike in the curve is observed between8Pa¹4Pa, and the variationlaw of this spike with pressure is concluded. Under the same discharge conditions, thespectra of Xe plasma emission in the range8Pa⁴0Pa and10nm⁷0nm are measuredwith grating spectrometer, and identified. The spectra intensity in10nm²0nm is muchlarger than that in other ranges, the transmission lines in10nm²0nm mainly originatefrom transmission in Xe⁸⁺、Xe⁹⁺and Xe¹⁰⁺ions. The intensity of16.47nm transitionline, between lower level and ground level of Pd-like xenon ions41.8nm laser, isrelatively large, which agrees well with theory results. These prove that Z-pinch produces certain abundance of Pd-like Xe ions, and during the pinch the partial Pd-likeXe ions are pumped to41.8nm laser lower level4d95p1P1through electron collision.The Pd-like Xe41.8nm transition line was observed under pressure17.3Pa and currentamplitude15kA, nevertheless, no lasing was observed. The analyses indicate that theelectron energy is not sufficient to excite enough Pd-like Xe ions from ground state toupper state.In order to explore the possibility of realizing Pd-like Xe41.8nm lasing by capillarydischarge scheme, under the same discharge conditions, the variation laws of theabundance of Xe¹⁰⁺, Xe⁹⁺and Xe⁸⁺with pressure are concluded, the optimum pressureranges for these ions’ generation are obtained. Moreover, under the same pressureconditions, the variation laws of the abundance of Xe¹⁰⁺, Xe⁹⁺and Xe⁸⁺with currentamplitude are concluded. The analyses of both the theoretical and experimental results,indicate that the Pd-like Xe ions are over-ionized easily and the plasma electrontemperature is too low, that’s the reason why the Pd-like Xe41.8nm lasing was notrealized.