Modeling of High-Pressure Rare Gas Lasers: Kinetics and Plasma Chemistry.

A computer model has been developed to investigate the excitation and deexcitation mechanisms and to optimize the laser performance over the wide range of operating parameters. Three rare gas lasers have been computationally investigated: the Xe laser with Ar, He/Ar and Ne/Ar buffer gases, the Ne laser with He/Ar buffer gases and the Ar laser with He buffer gases. The infrared atomic Xe laser (5d to 6p) is an attractive candidate for fission-fragment excitation which provides low-power deposition (1-100 W cm^{-3}), long pulse lengths (1-10 ms), and high energy deposition (100s J l ^{-1}). Optical gain at 1.73 and 2.03 μm has recently been measured in a reactor-excited Xe laser yielding values exceeding 0.03-0.05 cm^{-1} at power depositions of less than 10s W cm^{ -3}. Gain was also found to rapidly terminate before the peak of the pump pulse under some experimental conditions. A computer model has been developed to predict gain in fission-fragment-excited Xe lasers, and these experiments have been analyzed. It is found that the termination of gain is most likely attributable to gas heating which increases the electron density, leading to electron collision quenching. The specific dependence of gain on pump rate suggests that a reduced rate of recombination of molecular ions with increasing gas temperature is partly responsible for this behavior. The high pressure atomic Ne laser operates on four visible transitions between the 3p and 3s manifolds. Oscillation at 585 nm (3p' (1/2) _0 to 3s' (1/2) _1) with efficiency of >1% has been demonstrated by others. The upper laser level is believed to be populated by dissociative recombination of Ne_2^+, while state-selective Penning reactions relax the lower laser levels. To investigate these pumping mechanisms, experimental and modeling studies have been performed on a short pulse e-beam excited Ne laser, using He/Ne/Ar mixtures. The high pressure (>=0.5 atm) atomic Ar laser (3d to 4p) oscillates on four infrared transitions (1.27-2.4 mu m). Quasi-continuous oscillation on the 1.79 μm transition has been obtained using electron -beam and fission-fragment excitations over a wide range of power deposition and gas pressure. In this regard, a computer model has been developed to investigate excitation mechanisms of the Ar laser. Results from the model suggest that the upper laser level of the 1.79 mu m transition (Ar(3d (l/2) _1)) is dominantly populated by dissociative recombination of HeAr^+. In contrast, the dissociative recombination of Ar_2^+ is believed to predominantly produce Ar(4s) states. Electro -ionization from Ar metastables at moderate to high pump rates is responsible for the high efficiency of the Ar laser. Gain and laser oscillation are discussed and compared to measurements made for He/Ar gas mixtures which use various Ar mole fractions and total pressures. (Abstract shortened by UMI.).