A COMPUTATIONAL INVESTIGATION OF THE NEON-LIKE GERMANIUM COLLISIONALLY PUMPED LASER

The complex problem of a collisionally pumped Ne-like geranium laser is examined through several detailed models. The central model is EHYBRID; a 1 1/2D fluid code which self consistently treats the plasma expansion with the atomic physics of the Ne-like ion for 124 excited levels through a collisional radiative treatment. The output of EHYBRID is used as data for ray-tracing and saturation codes which generate experimental observables. A detailed description of the models is given. The atomic physics is investigated through a three-level approximation, a steady state collisional radiative treatment and a time-dependent solution within the fluid model. The accurate calculation of the non-steady state ionization balance is identified as a key issue. Time resolved and time integrated output profiles are generated for various experimental configurations, and the effects of saturation and pin narrowing are examined. The agreement with experiment is excellent in virtually every respect. The principal anomaly which we cannot definitely account for is the high predicted gain on the 196 angstrom J = 0 --> 1 transition. The magnitudes of gain and the deflected angle suffered by the output beam as calculated by 2D ray-tracing are both too large. However, a 3D ray-tracing treatment which includes the effects of transverse refraction totally resolves the latter of these difficulties and partially resolves the former. The slightly high gain coefficient (approximately 30% with 3D ray-tracing) and the slightly low beam divergence are the only difficulties at present, although plasma inhomogeneities am likely to be responsible for these small discrepancies. All other quantities we have examined, such as those associated with the atomic physics, output beam characteristics and saturation are very well reproduced.