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We have measured the time of lasing onset of a capillary z-pinch discharge used for Ne-like Ar soft x-ray laser generation. It was found that for a relatively long plasma, the relationship between the time of lasing onset and the filling pressure of the Ar gas agrees, in trend, with that predicted by simulation. An interesting result was observed: the time of lasing onset becomes delayed when the length of the capillary becomes shorter, which is considered to be caused by an end-effect of the capillary on the pinching process. This also implies that a relatively long capillary is favourable for obtaining a plasma column of uniformity, and thus for reducing the relative absorption of laser power.

The timing, intensity, and pulse width of lasing signals in fast capillary discharge plasma at a strong 2p 5 3p-2p 5 3s transition of Ne-like Ar are experimentally investigated. To clarify the lasing mechanism, the results are compared with those obtained by numerical calculation using a one-dimensional magneto-hydrodynamics (MHD) simulation code. The comparison indicates that the lasing occurs when a cylindrically imploding shock wave arrives at the axis of the capillary and it depends not only on the electron density and temperature but also strongly on the effects of time-varying refraction and the opacity of the pinching plasma.

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