Spectroscopic characterization of ion collisions and trapping at laser-irradiated double-foil targets

Precise X-ray spectroscopic investigation of the colliding plasmas produced at single-side laser-irradiated double-foil Al/Mg targets is reported. The spatially resolved, time-integrated line spectra of the Al Lyα group were measured using a high-dispersion vertical-geometry Johann spectrometer. The X-ray spectra were evaluated with the multilevel collisional-radiative code MARIA. The complex satellite-rich structure observed in the Al Lyα emission indicates a presence of relatively cold dense plasma close to the Al foil surface, hot plasma between both foils, and Al ion trapping close to the Mg foil. An unusual formation of the Al spectra at the Mg foil surface is discussed in terms of interaction of counter-propagating plasmas, in particular trapping, deceleration and thermalization of the Al ions in the colliding plasmas. The qualitative analysis of the observed spectral emission is supported by 2D hydrodynamic modeling and by time-resolved imaging of the expanding plasma. Prospective use of the J-satellite line shifts in diagnosis of colliding laser-produced plasmas is discussed.