The advances in the area of High-Energy-Density Physics (HEDP) have allowed the extreme physical conditions relevant to astrophysical processes to be recreated in the laboratory. The scaling of astrophysical systems, which are characterized by spatial and temporal scales that are 15-20 orders of magnitude larger than in laboratory experiments, has been demonstrated to be possible under a series of constraints in the physical parameters that characterize the plasma, paving the way for the field of Laboratory Astrophysics.
Laboratory astrophysics experiments present a unique approach to producing and accurately studying a number of astrophysical scenarios. Experiment data allows computer codes to be verified, and, in addition, they are a discovery tool providing insights into the behaviour of complex highly non-linear systems.
On MAGPIE, we study a range of astrophysical systems, including jets from young stars, radiative shocks, magnetic reconnection, bow shocks and accretion disks.
The typical evolution of a MAGPIE experiment, with typical spatial scales up to a few cm and characteristic timescales of tens to hundreds of ns, can be a scaled version of large-scale astrophysical phenomena. One example is jets from young stars, with typical lengths of thousands of astronomical units 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 and the cooling parameter, among others.
Plasma Jet Experiments
In MAGPIE it is possible to create plasma jets with similar dimensionless parameters to those from young stellar jets. The current pulse is introduced into different configurations for instance an array of metallic wires (a conical or radial wire-array Z-pinch) or a continuous metallic disc (a radial foil). In all cases, highly supersonic, highly collimated, radiatively cooled plasma jets can be produced. Experiments allow complex astrophysical phenomena to be studied in detail in a repeatable, accessible, and controllable manner. Most importantly, experiments allow the initial conditions of the physical parameters of the plasma o be modified, and can provide insights of the complex 3-D geometry of jets.
Supersonic jets from conical wire arrays
A conical wire array is a modification of a standard cylindrical wire array Z-pinch. In a conical wire array, the wires in the cylinder are tilted to form an inverted cone shape. As the current passes through each one of the wires, they are rapidly heated and converted to plasma. The global toroidal magnetic field generated by the current passing through the array surrounds the entire cone, producing a net force that makes the plasma converge towards the axis of the array. The inclination of the wires leads to the formation of highly supersonic jet of plasma which propagates axially at velocities ~100 km/s. By changing the wire material it is possible to control the jet dynamics particularly due to radiative losses through a process known as radiative cooling.
Figure 1 shows an example of a wire array in MAGPIE. The figure shows numerical simulations done with the MHD code GORGON. The results show the dense wires at the bottom and a jet on the axis of the array, surrounded by low-density plasma. Superimposed are three radial profiles at different heights above the array, showing the degree of collimation of the jet. The panel on the right shows experimental results focused on the jet region. The results were obtained with optical laser interferometry and show the high degree of jet collimation. The displacement of the fringes is proportional to the electron density in the plasma.
When oppositely directed magnetic field lines meet inside a plasma, they annihilate and 'reconnect', forming new field lines that connect different regions of plasma than the lines which formed them. Along with this reconfiguration, the energy stores in the magnetic field is released, causing the plasma to heat up and accelerate outwards at high velocities. This process is fundamental to understanding phenomena such as solar flares, the Aurora and exotic astrophysical phenomena such as magnetar flares and Gamma-ray bursts.
In MAGPIE, we collide fast moving jets of plasma which carry oppositely directed magnetic fields. These fields reconnect, and we observe how the magnetic field is destroyed, leading to plasma heating and the formation of `plasmoids', fast moving blobs of plasma that violently tear the reconnection process apart.