XUV fluorescence in a vacuum arc discharge by resonant optical pumping

Abstract

A vacuum-arc plasma of molybdenum is investigated as a lasing medium for an extreme-ultraviolet laser at 645 Å. This thesis presents experiments to achieve resonant photo-excitation of the 4p⁵6s-4p ⁶ transition at 136.5 Å in Mo⁶⁺ ions by a closely matching 5s²S_1/2-4p²P_1/2 line of Mo¹¹⁺. The scheme was first proposed by Feldman and Reader. Studies by K. Ilcisin have shown the feasibility of realizing the resonant photopumping scheme in a set of two molybdenum plasmas produced by pulsed IR and visible lasers. This work describes a vacuum arc plasma pumped by a laser-produced plasma as a step towards a simpler and more compact XUV source. Increased fluorescence on several 6s-5p transitions was observed. Comparison with numerical modeling points to resonant photo-excitation as the cause for the increase, and indicates that a population inversion was achieved on the 645.4 Å transition. Several designs of the arc source and laser target were tested. The fluorescence is strongly affected by collisional interaction of the plasmas and the ablation of insulating material of the “sliding spark” discharge gap. A 1-D hydrodynamics code is used to model the plasma temperature, density and ionization stage populations. A detailed time-dependent collisional-radiative model of the lasing plasma was developed and used in conjunction with the hydrocode. While the model predicts gain of 3 cm ⁻¹ at 645 Å, in practice the interaction of the plasmas and the XUV absorption by the ablated impurities reduce radiation at potential lasing lines as well as other transitions in the XUV. Approaches to alleviating this problem, such as the use of different materials and geometries, are discussed. The XUV reflectivity of hydrogenated amorphous carbon has been measured and compared with the reflectivities of other potential cavity mirror materials.