Three-Dimensional MHD Simulation of the Caltech Plasma Jet Experiment: First Results

Abstract

Magnetic fields are believed to play an essential role in astrophysical jets with observations suggesting the presence of helical magnetic fields. Here, we present three-dimensional (3D) ideal MHD simulationsof the Caltech plasma jet experiment using a magnetic tower scenario as the baseline model. Magnetic fields consist of an initially localized dipole-like poloidal component and a toroidal component that is continuously being injected into the domain. This flux injection mimics the poloidal currents driven by the anode-cathode voltage drop in the experiment. The injected toroidal field stretches the poloidal fields to large distances, while forming a collimated jet along with several other key features. Detailed comparisons between 3D MHD simulations and experimental measurements provide a comprehensive description of the interplay among magnetic force, pressure and flow effects. In particular, we delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms. With suitably chosen parameters that are derived from experiments, the jet in the simulation agrees quantitatively with the experimental jet in terms of magnetic/kinetic/inertial energy, total poloidal current, voltage, jet radius, and jet propagation velocity. Specifically, the jet velocity in the simulation is proportional to the poloidal current divided by the square root of the jet density, in agreement with both the experiment and analytical theory. This work provides a new and quantitative method for relating experiments, numerical simulations and astrophysical observation, and demonstrates the possibility of using terrestrial laboratory experiments to study astrophysical jets.

A hybrid Rayleigh-Taylor-current-driven coupled instability in a magnetohydrodynamically collimated cylindrical plasma with lateral gravity

“…This is satisfied by the MHD induction equation (Eqs. (19)- (21)) plus continuity at the plasma interface. The latter condition requires that the velocity component orthogonal to the interface must be continuous across the interface, i.e.,…”

“…The cylindrical jet carries a strong axial magnetic field of 0.2-0.6 T and a large axial current of 50-150 kA, so the magnetic field is in a typical flux rope configuration. 19,21,22 When the jet is sufficiently long, it undergoes an m ¼ À1 kink instability. 23 The kinked plasma grows nonlinearly fast and accelerates laterally away from the original central jet axis.…”

“…Magnetic field measurements show that actual radii of the jets (flux ropes) are 3-5 cm, larger than shown in the visible images. 19 use linear stability analysis to develop an ideal MHD theory of Rayleigh-Taylor instability occurring on the surface of a magnetically collimated plasma-filled flux rope in vacuum with the presence of a gravity perpendicular to the plasma axis. We linearly perturb an equilibrium flux rope and decompose the perturbation into a summation over all azimuthal modes.…”

Extreme ultra-violet burst, particle heating, and whistler wave emission in fast magnetic reconnection induced by kink-driven Rayleigh-Taylor instability

“…4, 10, and 11 in Ref. 38). This agreement is consistent with the MHD concept that magnetic field is frozen into the plasma and indicates that the images show the magnetic field configuration.…”

Spheromaks and how plasmas may explain the ultra high energy cosmic ray mystery

“…Experiments on acceleration need only include this central column, resembling jets produced in spheromak experiments during the early stages of plasma gun injection, before helicity propagation begins to fill in the closed flux in figure 1 (Hooper et al 2012). Evidence that spheromak 'flux cores' (their central columns) remain collimated during this formation phase, as astrophysical jets do, is discussed in paper II, confirmed by MHD simulations of experiments at Cal Tech (Zhai et al 2014). Collimation by pinch forces persists the full length of the jet, to a length limited only by the available power and the elapsed time.…”