A plasma production method using the irradiation of an array
of small spots has been investigated from the point of view
of soft X-ray laser generation in the recombining plasma scheme
pumped by a pulse-train laser. The expansion geometry of highly
ionized ions produced by the micro-dot array irradiation method
has been measured and compared with that by a simple line
irradiation. Spatial distribution of gain coefficients of the
Li-like Al ion transition lines have also been measured for
both irradiation methods. Highly ionized ions were observed
to spread wider in the micro-dot array irradiation method. It
is expected that rapid expansion and efficient cooling are achieved
in plasmas produced by the micro-dot array irradiation method,
which is consistent with the experimental results on the spatial
structure of the X-ray laser gain region.
Time- and space-integrated emission spectra measurements have been performed in plasma produced by 308 nm wavelength XeCl laser radiation (IL = (4–10)·1012 W/cm2, τ = 10 ns) and by 248 nm wavelength KrF laser pulse train radiation (IL = 5·1015 W/cm2, τ = 7 ps, 16 pulses in train) on CF2 plane target. Theoretical modelling of Lyman series and He-like ion resonance series of fluorine and its fit of experimental data show considerable differences in the absorption of laser radiation in the two plasmas.
Thermal load has been a haunting factor that undermines the brightness and coherence of the self-seeded X-ray free-electron laser. Different from uniformly pulsed mode, in pulse train mode a thermal quasi-steady state of the crystal monochromator may not be reached. This leads to a dynamic thermal distortion of the spectral transmission curves and seed quality degradation. In this paper, the pulse-to-pulse thermal load effects on the spectral transmission curves and seed quality are shown, and some instructive information for the tuning process is provided.
Production of highly ionized plasma by micro-dot array
irradiation and its application to compact X-ray lasers