Order unity reconnection rate scaling during anti-parallel magnetic reconnection on TREX

The Terrestrial Reconnection Experiment (TREX) is a device optimized to study the role of kinetic dynamics during collisionless magnetic reconnection\footnote{Olson, J., et al., Phys. Rev. Letters, \textbf{116}, 255001 (2016).}. In a recent experimental run consisting of $\sim900$ shots while varying certain experimental parameters we measured the reconnection rate using the Cassak-Shay scaling for asymmetric anti-parallel reconnection\footnote{Cassak, P.A., and Shay, M.A., Phys. of Plasmas, \textbf{14}, 102114 (2007).}. In this study, we observe that the absolute reconnection rate $E_{rec}$ is set by the applied drive voltage while being insensitive to the applied background field, ion species, or plasma density. However, for all epxerimental configurations the observed relative reconnection rate is $E_{rec}/(V_{A}B_{rec})\sim1$ instead of the expected rate of $E_{rec}/(V_{A}B_{rec})\sim0.1$. These experiments suggest that the reconnecting magnetic field self-regulates to match the externally applied drive in order to provide a self-consistent reconnection rate. This has important implications for determining the parameters of any given reconnection experiment while also challenging the ubiquity of the 0.1 rate scaling for fast magnetic reconnection.