Laboratory Astrophysics

Astrophysical Scaling

The evolution of a laboratory plasma jet, with typical spatial scales of a few cm and characteristic timescales of hundreds of ns, can be a scaled version of large-scale jets from young stars (typically thousands of astronomical units in length and evolving in timescales of many years). In order for this scaling to be valid, both the laboratory and astrophysical jets must have similar dimensionless parameters such as the Mach number, Reynolds number, Peclet number, cooling parameter, among others.

Plasma Jet Experiments

Using the current from the MAGPIE generator (1.4MA in 240ns), it is possible to create plasma jets with similar dimensionless parameters to those from protostellar jets. The high-current pulse is introduced into an array of metallic wires (a conical or radial wire-array Z-pinch) or metallic disc (a radial foil). In all cases, highly supersonic, highly collimated, radiatively cooled plasma jets can be produced in our laboratory.

Superonic jets from conical wire arrays

The image shows a schematic setup of a conical wire array, in which the ablation of plasma from each one of the um-thick metallic wires produces a jet on the axis.

The image shows a 3D MHD simulation of a jet from a conical wire array together with a comparison with experimental results obtained with optical laser interferometry.

Magnetically-Driven Jets from radial wire arrays/radial foils

One of our aims is to produce a jet which can be driven by a predominant toroidal (azimuthal) magnetic field. The high-current pulse is introduced into an array of radial wires, or alternatively a continuous radial foil. In both setups the load is extended radially between two concentric electrodes.

A plasma jet is driven by the pressure of a toroidal magnetic field in a configuration similar to the "magnetic tower" model of astrophysical jets. The jets in the experiments are supersonic and can reach speeds in the orders of ~100-400 km/s.

In a radial wire array, the ablation of plasma from the net JxB force sorrounds the region above the wires until the wires are fully ablated near the cathode. Plasma is then driven by the toroidal magnetic field creating a "magnetic bubble" which confines plasma on its axis as jet.

These experiments have been extended to include the generation of episodic jets, observed when replacing the wires by a continuous metallic foil. Several magnetic tower jet eruptions can be created in a single experiment on MAGPIE.

The figure above shows an XUV image of an episodic jet propagating through ambient argon gas. The cavity is formed by coalescence of the bow shocks from several early jet episodes, and inside of this cavity the next tower jet with a well collimated magnetically confined core is seen. Experiments suggest that this scenario could be relevant to variability seen in astrophysical jets. Computer modelling of these experiments using 3-D laboratory plasma and astrophysical codes is in progress.

Jet-Ambient Interaction

Our latest efforts have have focused in studying experimentally the interaction of plasma jets with neutral gases (e.g. argon, helium or xenon). The results are characterised by the formations of several shock features, opening new possibilities for the study of laboratory astrophysics.

The figure above shows results from the interaction of an aluminium jet from a radial foil with an argon ambient, characterised by the formation of several shock features.