Ion deceleration in interpenetrating plasma jets

Complete text of publication follows. Inertial and magnetic confinement fusion schemes involve collisions of high-temperature plasma jets and their interaction with solid surfaces (the so-called plasma-wall interaction, PWI). A fundamental understanding of the PWI effects requires a detailed characterization of the transient collisional phenomena occurring in the interaction region. In this paper we discuss a PWI experiment with double-foil Al/Mg targets fielded at the PALS laser system. An energetic plasma jet was created at the rear (non-irradiated) side of the 0.8-μm-thick Al foil exploded by the main laser beam (50-200 J, 0.44/1.315 μm, 0.25-0.3 ns, < 1 x 1016 W/cm2). This plasma jet streamed towards the 2-μm-thick Mg foil, where the secondary plasma was created either by an auxiliary 5 J laser beam or by the main laser radiation transmitted through the Al foil, together with radiation and particles emitted from the Al foil. The environmental conditions in the plasma were diagnosed by means of high-resolution x-ray spectroscopy and temporally-resolved x-ray imaging. For the first time, the deceleration of the incident Al ions in the near-wall region was directly observed and quantitatively characterized from the Doppler shift of the J-satellite from the Al Lyα spectral group. The interaction scenario was numerically modeled by two concerted codes, namely, i) the Prague Arbitrary Lagrangian Eulerian 2-D code PALE, which solves the Lagrangian mesh distortions by smoothing and conservative remapping of conserved quantities, and ii) the multispecies 1.5-D code MULTIF which models the hydrodynamics of an arbitrary number of interpenetrating ion species in a single space dimension while assuming self-similar plasma expansion in the other directions, and taking into account detailed Coulombian collisional processes. PALE was used to model two counter-streaming Al/Mg plasma plumes until the beginning of their interaction, and the resulting plasma state was then used as an initial condition for the simulation of the subsequent plasma interpenetration by MULTIF. The fine structure in the Al ion velocity profile discovered near the Mg wall is discussed with respect to the numerical description of the interpenetration and stagnation of the counter-propagating plasmas, in particular in terms of the trapping and thermalization of the Al plasma jet close to the Mg target. To conclude, a combination of high-resolution x-ray spectroscopy with advanced plasma simulations contributes to a development of new diagnostics for investigation of PWI effects. The reported results provide novel information on interaction of high-temperature plasma jets with walls and more specifically, on the ions deceleration in the near-wall plasma region. Acknowledgements. The help of M. Smid in experimental data reconstruction is acknowledged. This research was supported by the Czech Science Foundation Grant P205/10/0814, the CNRS PICS project No. 4343 and the Czech Ministry of Education, Youth, and Sports projects No. MSM 6840770022 and LC528.