Dynamics of laser-produced annular plasmas and laboratory astrophysics experiments with magnetically-driven rotating plasmas
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2018
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Abstract
This thesis is separated in two different parts, in which two different plasma experiments are studied. In Part I we studied the dynamics of a plasma plume produced by focusing ring-like laser beams produced by a Nd:YAG 3.5 ns FWHM, 1064 nm wave length laser beam focused using the combination of an axicon prism and a focusing lens onto a solid target. The experiments were carried in two different regimes of background gas pressure, ring radius and materials. Additionally the effects of a static, external magnetic field produced by NeFeB permanent magnets on the propagation of the plume were studied. Firstly, titanium plasmas in 80 mTorr argon (Ar) background were studied using a 50 ns time resolution intensified CCD capturing the optical self-emission of the plasma. Resulting from the expansion of the annular plume a central stagnation column was observed to form in ∼ 80 ns, propagating on axis at characteristic velocity ∼ 105−6cm/s. Following the propagation of the jet, the formation of a planar shock produced by the interaction of the plasma with the background gas was observed. This configuration changes in characteristic timescales ∼ 500 ns, once the central column can no longer be observed and the planar shock transitions into a bow shock. The velocity of the plume is not considerably modified by the presence of the field, however the shape of theplume and persistence in time of the radiance of the plasma was affected. These results are ascribed by the effects of magnetic confinement and single particle trajectories, respectively. Additionally, gradients in radiance along the shock suggest temperature gradients perpendicular to the density gradients imposed by the shock. Secondly, carbon in ∼ 0.1−1.0 atm Ar experiments were carried. A neutral shock produced by the rapid expansion of the plasma is observed. Interpreted as the action of the stagnation column within the blast wave, it is observed a bubble leading the axial propagation of the shock wave, thusforming an asymmetric shock. It propagates at characteristic velocity ∼ 105cm/s and observedto be dominated by the presence of neutrals rather than plasma. However, strong electron density gradients are observed within the downstream. Part II is focused on magnetically-driven rotating plasmas. We employed a modified Z pinch configuration consisting of a cylindrical 16 mm radius, 10 mm height arrangement of 8, 40 µm diameter aluminium wires, which are angularly perturbed by 8 1 mm diameter steel rods used asreturn posts, as a load of the MAGPIE pulsed power generator (1.2 MA, 240 ns) based at Imperial College London. As a consequence of the off-axis implosion ablation stream, a seemingly rotating Z pinch the subsequent emission of a jet. The rotation is indirectly evidenced by difference in the expected diffusion times required to fill the hollow centre of the rotating plasma estimated ∼ 25ns and the life time of such structure > 100 ns. The density of this plasma is estimated ∼ 1018−21cm−3 and it has characteristic circulation velocity ∼ 106−8cm/s. The jet is estimated to have anaxial velocity ∼ 106−7cm/s. It has a high-degree of collimation and reaches a length ≥ 1.6 cm.This experiment is framed within the field of Laboratory Plasma Astrophysics, which allow usto reproduce and isolate features of astrophysical systems in table-top experiments. A rigorous derivation of the similarity criteria to scale one environment to the other, namely laboratory to astrophysical, based upon the single-fluid magneto hydrodynamic formalism is given, obtaining the dimensionless parameters of the systems and their explicit relation with the dynamical evolution of them. Further more, a novel method to estimate error bars in such parameters is presented. According to the estimations benchmarked by the data, the plasma flows within this experiment sare scale invariants, therefore they are comparable to other scale invariant systems, which is the case for astrophysical environments, as dissipation is dynamically negligible.
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Tesis (Master of Science in Physics)--Pontificia Universidad Católica de Chile, 2018