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Initial Results from TREX

The Terrestrial Reconnection Experiment, TREX, is now operational at the Wisconsin Plasma Astrophysics Laboratory, WiPAL. The first data recorded in September 2014 documented an unprecedented low collisional regime where magnetic reconnection is dominated by kinetic particle dynamics. These kinetic effects are important to reconnection in nature but have not been accessible to any previous reconnection experiment. TREX has since been updated to better study this collisionless reconnection regime. Below are highlights from the recent TREX results.



Early Results

The first implementation of TREX demonstrated the ability to drive magnetic reconnection using the existing Madison Plasma Dynamo eXperiment vacuum vessel and plasma generation. These events exhibit collisionless, two-fluid reconnection with plasma parameters of Te~20 eV and ne~10^18 1/m^3. In the figure below, the out-of-plane Hall magnetic fields are clearly present. These occur when the ions demagnetize and decouple from the electron fluid motion. This decoupling supports in-plane current structures that induce the quadrupolar Hall fields. The out-of-plane current density is very strong in the center X-line region. Strong currents can also be seen along the separators, which has never before been observed on an experiment even though these currents have long been known to exist in simulations.

TREX Magnetic Data

Figure 7: The measured magnetic data from a single point in time during reconnection on TREX. left) The in-plane magnetic field lines superimposed over the out-of-plane current density. The current sheet is visible across the X line, as well as along the separators. right) The out-of-plane Hall magnetic fields shown with the characteristic quadrupole structure, indicating TREX is in a regime collisionless enough such that the electron and ion fluids decouple from the magnetic field.





Fall 2015 Highlights



TREX Schematic

Figure 8: Schematic of the most recent implementation of TREX with a cutout showing the internal drive coils and magnetic diagnostic arrays. After initiating a plasma, two internal coils are pulsed opposite to a steady state magnetic field provided by the external Helmholtz coil. A projection of the vacuum fields during a pulse is provided with an illustration of the reconnection layer inflows and outflows. As the current in the internal coils is increased, the opposing field lines begin to reconnect, forming a thin current sheet, which then migrates toward the center of the vessel. The primary diagnostic is a 144 probe magnetic flux array, indicated in grey. Reconnection characteristics are easily computed using the flux probe array.



The updates to TREX include the new, robust hardware shown in the figure above to handle more powerful electronic drive circuity, as well as an improved diagnostic array. With these upgrades, TREX was able to run in the Fall of 2015 to study, in more depth, reconnection in the collisionless regime. Below are movies that show some of the characteristic reconnection events during this run campaign.

1. This first movie shows a typical reconnection event as measured by the flux probe array, including profiles of the loop voltage, reconnecting field, and current density. The solid lines are the contours of the magnetic flux function, indicating the projection of the magnetic field lines.

Collisionless Reconnection

2. Some events exhibit the formation of plasmoids (closed magnetic islands) inside the current layer. These are explosive events that greatly influence the reconnection dynamics. Plasmoids have been extensively studied and are thought to contribute to the onset of solar flares as well as particle heating during reconnection.

TREX Plasmoids


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