Browsing by Author "Lebedev, S.V."

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    Cooling and instabilities in colliding flows
    (Oxford University Press, 2021) Markwick, R.N.; Frank, A.; Carroll-Nellenback, J.; Liu, B.; Blackman, E.G.; Lebedev, S.V.; Hartigan, P.M.
    Collisional self-interactions occurring in protostellar jets give rise to strong shocks, the structure of which can be affected by radiative cooling within the flow. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in both one and three dimensions with a power-law cooling function. The characteristic length and time-scales for cooling are temperature dependent and thus may vary as shocked gas cools. When the cooling length decreases sufficiently and rapidly, the system becomes unstable to the radiative shock instability, which produces oscillations in the position of the shock front; these oscillations can be seen in both the one- and three-dimensional cases. Our simulations show no evidence of the density clumping characteristic of a thermal instability, even when the cooling function meets the expected criteria. In the three-dimensional case, the nonlinear thin shell instability (NTSI) is found to dominate when the cooling length is sufficiently small. When the flows are subjected to the radiative shock instability, oscillations in the size of the cooling region allow NTSI to occur at larger cooling lengths, though larger cooling lengths delay the onset of NTSI by increasing the oscillation period.
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    ON THE STRUCTURE AND STABILITY OF MAGNETIC TOWER JETS
    (The American Astronomical Society, 2012) Huarte-Espinosa, M.; Blackman, E.G.; Ciardi, A.; Hartigan, P.; Lebedev, S.V.; Chittenden, J.P.
    Modern theoretical models of astrophysical jets combine accretion, rotation, and magnetic fields to launch and collimate supersonic flows from a central source. Near the source, magnetic field strengths must be large enough to collimate the jet requiring that the Poynting flux exceeds the kinetic energy flux. The extent to which the Poynting flux dominates kinetic energy flux at large distances from the engine distinguishes two classes of models. In magneto-centrifugal launch models, magnetic fields dominate only at scales 100 engine radii, after which the jets become hydrodynamically dominated (HD). By contrast, in Poynting flux dominated (PFD) magnetic tower models, the field dominates even out to much larger scales. To compare the large distance propagation differences of these two paradigms, we perform three-dimensional ideal magnetohydrodynamic adaptive mesh refinement simulations of both HD and PFD stellar jets formed via the same energy flux.We also compare how thermal energy losses and rotation of the jet base affects the stability in these jets. For the conditions described, we show that PFD and HD exhibit observationally distinguishable features: PFD jets are lighter, slower, and less stable than HD jets. Unlike HD jets, PFD jets develop current-driven instabilities that are exacerbated as cooling and rotation increase, resulting in jets that are clumpier than those in the HD limit. Our PFD jet simulations also resemble the magnetic towers that have been recently created in laboratory astrophysical jet experiments.
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    Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet
    (Springer Nature, 2016) Li, C.K.; Tzeferacos, P.; Lamb, D.; Gregori, G.; Norreys, P.A.; Rosenberg, M.J.; Follett, R.K.; Froula, D.H.; Koenig, M.; Seguin, F.H.; Frenje, J.A.; Rinderknecht, H.G.; Sio, H.; Zylstra, A.B.; Petrasso, R.D.; Amendt, P.A.; Park, H.S.; Remington, B.A.; Ryutov, D.D.; Wilks, S.C.; Betti, R.; Frank, A.; Hu, S.X.; Sangster, T.C.; Hartigan, P.; Drake, R.P.; Kuranz, C.C.; Lebedev, S.V.; Woolsey, N.C.
    The remarkable discovery by the Chandra X-ray observatory that the Crab nebulaﾒs jet periodically changes direction provides a challenge to our understanding of astrophysical jet dynamics. It has been suggested that this phenomenon may be the consequence of magnetic fields and magnetohydrodynamic instabilities, but experimental demonstration in a controlled laboratory environment has remained elusive. Here we report experiments that use high-power lasers to create a plasma jet that can be directly compared with the Crab jet through well-defined physical scaling laws. The jet generates its own embedded toroidal magnetic fields; as it moves, plasma instabilities result in multiple deflections of the propagation direction, mimicking the kink behaviour of the Crab jet. The experiment is modelled with three-dimensional numerical simulations that show exactly how the instability develops and results in changes of direction of the jet.