The formation of a current-plasma shell is studied during the expansion of a laser-ignited low-power vacuum-discharge cathode plasma jet into the interelectrode gap. The shell geometry is found to be determined by the mode of laser-plasma expansion at the discharge ignition stage. It is shown that the increase in the laser-beam focal spot area on the cathode surface leads to the increase in the matter density and the decrease in the density gradient in the discharge gap and to transition from the spherical laser-plasma expansion mode to the jet mode. The latter considerably stabilizes the current transfer in the discharge plasma, even during the development of the hydrodynamic sausage instability in it.

Under typical experimental conditions related to the interaction of a short pulse laser with a nanometer foil target, the assumption of a target step-function number density profile ceases to be valid due to the existence of a nanosecond long amplified spontaneous emission pedestal prior to the arrival of the main pulse. As a consequence, the formation of a low density extended preplasma should be considered, making the achievement of high ion energy extremely challenging. In this work, a multiparametric study of various preplasma distributions is presented, obtained by combinations of the pedestal intensity, initial foil thickness, and main pulse intensity. Hydrodynamic simulations have been employed to find the target number density distribution prior to the arrival of the main laser pulse. The output of the hydrodynamic simulations is then combined with particle-in-cell simulations, providing a detailed understanding of the complete nanosecond-long laser-foil interaction. Once the laser pulse interacts with the preplasma, it deposits a fraction of its energy on the target, before it is either reflected from the critical density surface or transmitted through an underdense plasma channel. A fraction of hot electrons is ejected from the target, leaving the foil in a net positive potential, which in turn results in proton and heavy ion acceleration. The results of our multiparametric studies are important for forthcoming experiments on the ion acceleration with multipetawatt laser facilities.

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