A SIMPLE-MODEL OF REFRACTION IN THE NE-LIKE ZN, COLLISIONALLY PUMPED LASER

We investigate the behaviour of the J=2-1 and J=0-1 lasing lines of the collisionally pumped Ne-like Zn laser through a detailed, schematic representation of refraction and amplification in the plasma. The density profile in the coronal region is represented as exponential, thus enabling the effects of differing scale length density gradients to be modelled. Reasonable density dependencies for the gains are obtained from considerations of detailed atomic physics/hydrodynamic calculations in nearby elements. Some 10(5) ray packets are traced through the system, and used to form the near and far field output profiles of the laser. Whilst a broad, weakly amplified J=2-1 signal is calculated for all plasma scale lengths, the presence of high J=0-1 gain in a high density, narrow region of plasma leads to a narrowly divergent, strongly amplified beam at sufficiently large (> 100 mu m) scale lengths. This latter effect is observed experimentally when the target is prepulsed at a low level, supporting the generally accepted assumption that the presence of a prepulse reduces the damaging effects of refraction through a decrease in refractive index gradients in the plasma. The surprising result that the J=2-1 emission is more refracted than that of the J=0-1 (even though the former has the higher gain in low density plasma) is explained as arising from the need to simultaneously maximise both the gain length product and the source function.