Generation of a High-Density Recombining Plasma sheet with Inverted Population as a Medium for VUV Laser

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The mechanism of formation of a recombining plasma as a short-wavelength laser medium by gas-contact cooling of a high temperature plasma was experimentally studied by means of the spectroscopic measurement.1) Three kinds of helium plasmas with different initial electron temperatures (T_e) and densities (n_e) were produced by a Z-pinch gun, and each was injected into the hydrogen molecular gas. The time-resolved spectra and spatial distributions in emission lines of H. He and He_+ were observed by a spectroscopic system with a fast-gated multichannel detector.2)The homogeneous mixingne of the plasma with hydrogen gas was achieved when the density of hydrogen gas exceeded the initial n_e of each plasma : At that gas density, the Stark broadening in H beta and He II 468.6nm lines showed that the atomic hydrogen felt, the same electron density as the He^+ ion did, and the spatial distribution of H beta line was also similar with that of Hell468.6nm line.3)Cooling of electron temperature induced by the mixing was observed in each plasma. The final T_e after cooling strongly depended on the initial one.4)The plasma cooled down less than 2eV was recognized to be in a recombining phase from the remarkable enhancement in the population density of the excited states of He^+ ion.Thus, it was found that the formation of the recombining plasma was obviously dependent upon the initial plasma parameters, especially T_e. Further detailed investigation is to be proceeded in order to obtain the optimum plasma condition.