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13Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2019
X-RAY DAIGNOSTICS OF HYDRODYNAMIC PHENOMENA IN LASER-INDUCED ASTROPHYSICALLY-RELEVANT PLASMA
E.D. Filippov1, I.Yu.Skobelev1,2, S.N. Ryazantsev1, G.Revet3,4, S.N. Chen5, J. Fuchs3,4 andS.A. Pikuz1,2
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia,
e-mail: edfilippov@ihed.ras.ru 2National Research Nuclear University MEPhI, Moscow, Russia,
3 '
LULI-CNRS, EcolePolytechnique, CEA: Université Paris-Saclay; Palaiseaucedex, France.
4Institute of Applied Physics, RAS, Nizhny Novgorod, Russia 5ELI-NP, "HoriaHulubei " National Institute for Physics and Nuclear Engineering, Bucharest-
Magurele, Romania
Nowadays plasma hydrodynamic phenomena such as supersonic jets of young stellar objects, accretion columns and coronal massejections are widely investigated in astrophysics. Actually, only way to research such processes and to improve their theoretical description - is simulation in laboratory conditions. Recently, laboratory laser-induced plasma flows were proved to be scaled to some astrophysical systems by matching dimensionless scaling parameters and thus providing the studies of astrophysical phenomena in controllable conditions. Today astrophysicists are interested in objects that require creating plasma streams with high aspect ratio, i.e. ratio of length to diameter, and supersonic speed of propagation in laboratory. One of the examples-young stellar objects (YSOs) that radiate plasma flows due to accretion processes - Herbig-Haro objects (HH). In laboratory, it was shown that the application of external magnetic field to plasma flows allows to collimate plasma and to observe stable plasma jets with large aspect ratio which are similar to HH. Therefore, the application of this stable plasma jets helps to study the dynamic of HH and other astrophysical objects varying conditions of plasma propagation in experiment, e.g. placing plasma jet in medium like gas, pre-plasma etc.
In the experiment the plasma jet is initiated by the interaction of 20-40 J laser pulses of 0.6 ns duration with a bulk CF2 targets and then immersed into 6-30 T poloidal B field. The combined x-ray spectroscopy method is developed to determine together electron temperature Te and density Ne profiles for plasma propagated along magnetic field lines. In approach, the plasma expanding from the laser irradiated target surface is considered as overcooled with non-stationary ionization state, and recombining plasma model is applied for spectra analysis giving consistent Te profiles in the range from 100 to 20 eV. The method also allowed to research influence of differently oriented magnetic field on collimation of plasma jet. It is demonstrated that collimation of jet is high enough up to 20 degrees.
Then, these plasma streams were used to simulate formation of magnetized accreting columns in young stars. Experiment was performed when expanding plasma stream impacts on the solid obstacle mimicking stellar photosphere. As a result, simultaneous measurements of plasma parameters by means of optical interferometry and x-ray spectroscopy allowed to discover formation of plasma shell with temperatures of about hundreds eV that envelops shocked core. The obtained parameters and complex spatial configuration of the plasma in accretion zone are used to analyze possible opacity effects on the emitted X-ray spectra in order to understand a systematic discrepancy between mass accretion rates derived from X-ray and optical astronomical observations.
Finally, the impact of magnetic field of a high strength (up to 30 T) on laboratory plasma flows propagating across the lines of magnetic field was investigated. External transverse magnetic field forms a shocked plasma region with increased values of electron temperature and density regarding the cases of free propagation and propagation in longitudinal magnetic field. The localization of this region depends on a magnetic field strength. Two-component model of plasma, where certain velocity of propagation corresponds to each fraction, is used to explain discrepancies between profiles of plasma parameters and intensities registered by x-ray spectrometers. This approach allowed to state that actually plasma emission localized far enough (L>>4 mm) from the target can be formed much earlier in time (up to 10 times in experiment) regarding the emission closer to the target. As a result, transverse magnetic field impacts on a laser plume as a separator in space, sorting ions with identical charge but different propagation velocity.
