A model for plasma formation in a reflective axicon and tube for a proposed Ne-like argon laser at 46.9 nm

摘要

We have sought ways to improve the coupling of the pump laser energy to a low density plasma suitable for collisional XUV lasing. At very high intensity, laser light can be coupled directly to a low density gas through multiphoton ionization. At the intensities of our pump, the radiation must be absorbed by inverse bremsstrahlung, which is inefficient at low density. To improve the overall efficiency, an optic was designed which integrates an axicon and a reflective tube such that unabsorbed pump radiation is repeatedly refocused on axis, permitting the creation of extended line foci in gases. Experimentally it was found that 6 cm sparks could be created in 200-760 torr argon with very high pump absorption, particularly on a second, delayed pulse. To investigate the coupling of pump laser energy into the plasma column, and subsequent plasma development, a computational model has been developed. Initial plasma formation occurs as a result of multiphoton ionization. Subsequent heating of the plasma occurs by inverse bremsstrahlung absorption, while higher stages of ionization are obtained through collisional ionization. A one-dimensional three-fluid hydrodynamic model, which incorporates collisions between charged particles and neutral atoms, is used to predict plasma expansion into the surrounding gas, while the coupling of absorbed energy to the plasma front is modeled by flux-limited electron thermal conduction.