Picosecond laser krypton plasma emission in water window spectral range

Laser plasma created in a krypton gas puff target is studied as a source of radiation in the water window spectral range (λ = 2.3–4.4 nm). The spatial development of the plasma induced by a sub-nanosecond Nd:YAG laser pulse focused on the gas puff target is modeled using 2d RMHD code Z*. It is shown that the created plasma is quickly heated and the critical electron density is achieved at the very beginning of the laser pulse. Space-time distributions of plasma quantities, namely, electron temperature, electron density, mass density, and plasma expansion velocity were evaluated. Furthermore, the temporal dependences of plasma electron temperature and electron density in a selected point were introduced into the kinetic code FLYCHK. Instantaneous spectra during the laser pulse and during plasma decay period are calculated showing the intense spectral lines in the water window range at the laser peak and delayed up to 0.8 ns. Temporal evolutions of the krypton ions relative populations prove that ions from Kr21+ and Kr22+ are responsible for the dominant spectral intensity emitted at a wavelength around λ = 3 nm. Evaluated time resolved spectra are compared with the time integrated spectra obtained experimentally. The spatial distribution of the measured plasma luminosity is compared with the estimated area of plasma emission based on the evaluated distributions of plasma electron density and temperature.