Francisco Suzuki-Vidal

Episodic Magnetic Bubbles and Jets: Astrophysical Implications from Laboratory Experiments

Collimated outflows (jets) are ubiquitous in the universe, appearing around sources as diverse as protostars and extragalactic supermassive black holes. Jets are thought to be magnetically collimated, and launched from a magnetized accretion disk surrounding a compact gravitating object. We have developed the first laboratory experiments to address time-dependent, episodic phenomena relevant to the poorly understood jet acceleration and collimation region. The experimental results show the periodic ejections of magnetic bubbles naturally evolving into a heterogeneous jet propagating inside a channel made of self-collimated magnetic cavities. The results provide a unique view of the possible transition from a relatively steady-state jet launching to the observed highly structured outflows.

Generation of episodic magnetically driven plasma jets in a radial foil Z-pinch

We present experimental results of the formation of magnetically driven plasma jets, showing for the first time a way of producing episodic jet/ouflows in the laboratory. The jets are produced using a 6.5 μm thick aluminum disk (a radial foil), which is subjected to the 1 MA, 250 ns current pulse from the MAGPIE generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]. The early time motion of the foil is characterized by the bulk motion of the mass due to the magnetic pressure, together with the formation of a surface plasma following the direction of the J×B force. A low density plasma fills the region above the foil preceding the formation of subsequent magnetically driven jets on the axis of expanding magnetic bubbles. The outflows emerge in timescales of ∼30–40 ns and their episodic nature is the result of current reconnection in the foil, aided by the formation of current-driven instabilities in the jet and the distribution of mass available from the foil. The additional inductance due ...

Quantitative analysis of plasma ablation using inverse wire array Z pinches

An inverse (exploding) wire array configuration, in which the wires form a cylinder around a current carrying electrode on axis, was used to study the ablation phase of the wires. This configuration allows the parameters of the plasma from individual wires of the array to be measured as the ablated plasma streams propagate in the outward radial direction. The density distribution and the evolution of the natural mode of modulation of the ablation flow was measured with interferometry and soft x-ray imaging. Measurements of the voltage across the array, which in this configuration is determined by the private magnetic flux around the individual wires, allow information on the localization of the current to be obtained. Results are compared to three-dimensional magnetohydrodynamics simulations.

Formation of episodic magnetically driven radiatively cooled plasma jets in the laboratory

We report on experiments in which magnetically driven radiatively cooled plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed power facility. The jets were driven by the pressure of a toroidal magnetic field in a “magnetic tower” jet configuration. This scenario is characterized by the formation of a magnetically collimated plasma jet on the axis of a magnetic “bubble”, confined by the ambient medium. The use of a radial metallic foil instead of the radial wire arrays employed in our previous work allows for the generation of episodic magnetic tower outflows which emerge periodically on timescales of ∼30 ns. The subsequent magnetic bubbles propagate with velocities reaching ∼300 km/s and interact with previous eruptions leading to the formation of shocks.

Implosion and stagnation of wire array Z pinches

Detailed measurements of the dynamics of aluminum wire array Z pinches from immediately prior to implosion until stagnation and dissipation on axis are presented. Before implosion, the ∼0.5mm axial modulation seen in earlier laser probing images is observed as ablation on the surface of the wire cores facing away from the array axis. This results in the complete ablation of sections of the wire cores and a redistribution of current at the start of implosion. The dynamics of implosion are then strongly influenced by the number of wires in the array. With only eight wires, discrete snowplough bubbles expand from each wire toward the precursor. There is little, if any, correlation between the bubbles from adjacent wires, and a large temporal spread over which the bubbles arrive at the precursor is observed, along with a long rise time, low power soft x-ray pulse. With 32 or more wires, bubbles from adjacent wires merge close to the array edge to form an imploding sheath. The front edge of the sheath is well ...

Interaction of a supersonic, radiatively cooled plasma jet with an ambient medium

An experimental investigation into the interaction of a supersonic, radiatively cooled plasma jet with argon gas is presented. The jet is formed by ablation of an aluminum foil driven by a 1.4 MA, 250 ns current pulse in a radial foil Z-pinch configuration. The outflow consists of a supersonic (Mach number ∼3–5), dense (ion density ni ∼ 1018 cm−3), highly collimated (half-opening angle ∼2°−5°) jet surrounded by a lower density halo plasma moving with the same axial velocity as the jet. The addition of argon above the foil leads to the formation of a shock driven by the ablation of halo plasma, together with a bow-shock driven by the dense jet. Experimental data with and without the presence of argon are compared with three-dimensional, magneto-hydrodynamic simulations using the GORGON code.

Effect of Wire Diameter and Addition of an Axial Magnetic Field on the Dynamics of Radial Wire Array

The operation of radial wire array Z-pinches driven by a 1-MA 250-ns current pulse was studied. Variation in the cathode diameter and wire diameter does not affect the overall plasma dynamics but controls the time of wire breakage and the time of pinch formation. The measured times of full wire ablation at the cathode were used to determine the ablation velocity (V abl), and the results give a scaling V abl ~ (wire diameter)-0.46. Experiments with added axial magnetic field show an increase in the pinched plasma diameter, possibly due to the compression of the axial magnetic flux by the imploding plasma.

Z

The role of radiative cooling during the evolution of a bow shock was studied in laboratory-astrophysics experiments that are scalable to bow shocks present in jets from young stellar objects. The laboratory bow shock is formed during the collision of two counter-streaming, supersonic plasma jets produced by an opposing pair of radial foil Z-pinches driven by the current pulse from the MAGPIE pulsed-power generator. The jets have different flow velocities in the laboratory frame and the experiments are driven over many times the characteristic cooling time-scale. The initially smooth bow shock rapidly develops small-scale non-uniformities over temporal and spatial scales that are consistent with a thermal instability triggered by strong radiative cooling in the shock. The growth of these perturbations eventually results in a global fragmentation of the bow shock front. The formation of a thermal instability is supported by analysis of the plasma cooling function calculated for the experimental conditions with the radiative packages ABAKO/RAPCAL.

-Pinches

Preliminary results of the self-emission of charged particles from magnetically driven plasma jets has been investigated. The jets were launched and driven by a toroidal magnetic field generated by introducing a ???1.4?MA, 250?ns electrical current pulse from the MAGPIE generator into a radial wire array. This configuration has shown to reproduce some aspects of the astrophysical magnetic-tower jet launching model, in which a jet is collimated by a toroidal magnetic field inside a magnetic cavity. The emission of ions and protons from the plasma was recorded onto Columbia Resin 39 plates using time-integrated pinhole cameras. In addition a fly-eye camera, an array of 25?496 cylindrical apertures allowed estimating the location of the ion emitting source. The results show the ion emission comes from both the jet and its surrounding magnetic cavity, with the emission extending to a height of at least ???9?cm from the initial position of the wires. The emission of ions is consistent with the dynamics of the jet obtained from time-resolved imaging diagnostics, i.e.?optical laser probing and self-emission of the plasma in the extreme ultra-violet. These preliminary results suggest the ions are trapped inside the cavity due to the strong toroidal magnetic field which drives the jet. In addition these studies provide first estimates of the energy and fluence of protons for future laser-driven proton probing diagnostics aimed at measuring the magnetic field in these experiments.

BOW SHOCK FRAGMENTATION DRIVEN BY A THERMAL INSTABILITY IN LABORATORY ASTROPHYSICS EXPERIMENTS

Numerical simulations of laboratory astrophysics experiments on plasma flows require plasma microscopic properties that are obtained by means of an atomic kinetic model. This fact implies a careful choice of the most suitable model for the experiment under analysis. Otherwise, the calculations could lead to inaccurate results and inappropriate conclusions. First, a study of the validity of the local thermodynamic equilibrium in the calculation of the average ionization, mean radiative properties, and cooling times of argon plasmas in a range of plasma conditions of interest in laboratory astrophysics experiments on radiative shocks is performed in this work. In the second part, we have made an analysis of the influence of the atomic kinetic model used to calculate plasma microscopic properties of experiments carried out on magpie on radiative bow shocks propagating in argon. The models considered were developed assuming both local and nonlocal thermodynamic equilibrium and, for the latter situation, we have considered in the kinetic model different effects such as external radiation field and plasma mixture. The microscopic properties studied were the average ionization, the charge state distributions, the monochromatic opacities and emissivities, the Planck mean opacity, and the radiative power loss. The microscopic study was made as a postprocess of a radiative-hydrodynamic simulation of the experiment. We have also performed a theoretical analysis of the influence of these atomic kinetic models in the criteria for the onset possibility of thermal instabilities due to radiative cooling in those experiments in which small structures were experimentally observed in the bow shock that could be due to this kind of instability.